G
0 ALL II N
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a
VV~TI
EYNAUA L A T 1 0
Gordon 0, MoOutohan
a thesis
in partial fulfillment of
the requiremente for the
MASTER OP ARCHITWTURE
degree.....September 1950
Massaohusetts Institute of Technology. Oambridge
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This following pages within this thesis are
unumbered.
to Margaret
285 Westgate West
Cambridge 39, Mass.
September 1, 1950
Professor Lawrence B. Anderson, Head
Department of Architecture
Massachusetts Institute of Technology
Canbridge 39, Massachusetts
Dear Professor Anderson:
This thesis entitled "Cooling by Natural Ventilation" is
respectfully submitted in partial fulfillment of requirements for the "Master of Architecture" degree.
Sine'rely yours,
Gordon C. McCutchan
TABLE OF CONTENTS
INTBONOT ION
PART I
AIR MOVEMENT AND BODY COQLING
Heat Loss from the Body
Conduction
Convection
Radiation
Evaporation
Relationships under Varying Conditions
The Ideal Climate
Comfort Chart, ASHVE
Comfort Zone and Air Movement
PART II
BEHAVIOR OF MOVING AIR
Air and Temperature
Velocity and Altitude
Changes in Velocity and Direction
Summary
PART III
AIRFLOW AROUND OBSTRUCTIONS
Bernoulli and Venturi
Streamlines
Simple Geometric Forms
Wind Around Fences
Around Buildings
Two and Three Dimensions
Pressure Distribution
Change in Shape
Change in Wind Angle
The Sheltering Effect
Summary
PART IV -
AIRFLOW THROUGH BUILDINGS
Objectives (1) Currents at Body Level
(2) Reduction of Radiant Temperatures
(3) Maintenance of Air Temperature
Temperature Difference Forces
Wind Forces
Shape of the Building
The Air Wash
The Unknowns
BI BLIOGRAPHY
IliTROIJCTION
With the remarkable present-day air conditioning equipment it is possible to create almost any thermal environment desired by man.
However,
the cost of installing and operating the necessary equipment prohibits
its use in a great many buildings.
Those of us who have experienced
the long hot summers of the Southwest are probably more acutely aware
of the desirability of any successful cooling method, however crude or
inefficient.
Realizing that some cooling can be had without recourse to mechanical
deivces, architects have provided greater comfort at lower cost by
experimenting with various shapes and materials.
But in contrast to
the work of air conditioning engineers, architects have had to content
themselves with trial-and-error methods.
This has been due to either
an absence of information or to the fact that existing information has
been scattered and fragmentary.
In designing to take maximum advantage
of the climate, architects need more precise information in more useable
form.
This report is a first step in providing such information.
that--a first step.
It is only
It will give some of the required information, but
in the great majority of cases it will only point a direction.
This
can be understood from the fact that few other studies have attempted
to bring together the many bits of information from the several broad
fields that can contribute to the subject.
Henry J. Kaiser, the fabulous industrialist, was quoted in a recent
periodical as saying that he had no problems.
One of the "secrets
of his success," he said, was the substitution of the word "opportunity" for the word "problem."
The big opportunity of this study was
to become acquainted with the literature of five relatvely unfamiliar
fields.
Physiology was delved into rather deeply, for a layman, in
the investigation of the effects of air movement on the human body.
In dealing with the behavior of moving air, meteorology was most helpful.
Aero-dynamics entered both the section on natural air currents
and airflow around obstructions.
Civil engineering offered the in-
formation about pressure distribution on buildings.
And of course,
mechanical engineering (heating, ventilating and air conditioning) was
a thread through the whole discussion.
It appeared that the general subject of cooling by natural ventilation
fell naturally into four divisions.
Since the object we are trying to
cool is the human body, it is first necessary to determine how air
movement effects the body.
Since the medium for this cooling is the
air that nature provides, something should be known of the properties
of this air and the manner in which it is delivered.
Part III, airflow
around obstructions, was a necessary preliminary to Part IV, airflow
through buildings.
The principle objectives of this thesis are to define the problems
(i.e. opportunities) of the architect in designing for natural air
movement and to give some specific criteria with which to work.
Therefore, emphasis is given to the first three sections.
Some ex-
1-111'..----
amples of application are given in the last section, but no attempt
was made to recommend architectural solutions.
It was felt best to
devote the limited time to supplying the elements that would enter
such decisions.
THE
IPORTANCE OF COOLING
In a certain sense, it would be a waste of time to dwell on the importance
of more cooling in hot climates.
Certainly the individual requires no
charts or formulas to tell him when he is uncomfortably hot;
he senses
this without instruction and, in the extreme cases at least, instruction
will not change his opinion.
Neither are statistics necessary to show
that people desire a cooler environment than nature offers them in most
of the United States during the summer months.
Despite these obvious facts, there has not been sufficient compulsion to
cool adequately a large majority of work places.
And only a very small
percentage of residences can boast of comfortable conditions during the
warmest days.
This neglect can be attributed partly to ignorance of the
price paid for inadequate cooling;
a price paid in unhappiness, ill-health
and inefficiency.
Happiness is difficult to measure, and the relationship of this complicated
subject to the thermal environment cannot be included in this study.
However, there is little doubt that a sumnary of all the little irritations
caused by hot weather would show significant correlation between heat and
conduct.
On the other hand,
cents, and it
is
efficiency can be measured in dollars and
believed that proof of increased efficiency will do more
toi bring about adequate cooling than any similar facts regarding personal
pleasure.
There is much work to be done in this field also, but there
are some studies which show definite results.
There is
some evidence to support the widely-held belief that climate
influences health.
Winslow and Herington devote a full chapter to the
subject of Climate, Season, and Health in their book on "Temperature and
Human Life".
After analysis of a wealth of data (cited in the book),
their general conclusions are as follows:
(1) Even the extremes of heat
and cold in the United States may increase mortality rates as much as
double their normal volume;
(2) Minimum mortality rates occur with a
daily mean temperature of 600 to 700 F with a mean daily relative humidity
of 60 to 80 per cent;
(3) Achievement is highest in climates where the
above conditions prevail, with a stimulus given by moderate variations
toward the cooler side of optimum;
rhthms of physiological activity;
(4) The human body has regular annual
(5)
Seasonal and climatic variations
in morbidity from communicable diseases are particularly significant, and
always tend to show an increase in
intestinal infections under hot con-
ditions.
Figures for accident frequency in coal mines show a minimum at a point near
600
.
For semi-skilled labor in a munitions factory, accident frequency
is at a minimum near 670 with an increase in frequency of accidents on both
sides of this figure.
Recent information from the Royal Navy shows that
mental tasks are affected as well.
Their data show that in radio code
reception errors are more frequent under conditions of thermal stress.
But it is quite possible that heat, like sound and light, can do damage
when the individual is not conscious of being uncomfortable.
On same of
the very hottest days in Washington, D.C. some officials have seen fit to
stop the work of hundreds of office workers,
that they might seek more com-
fortable surroundings anywhere they could find then.
Undoubtedly the
efficiency of these workers was not at a peak during the hour or two it
took someone in charge to make such a big decision;
and in all probability
there were many days before and after the unexpected vacation when climatic
conditions were very similar.
climatic fatigue is
a ditch.
Dr. L.P. Herrington has said, "what we call
not comparable to the fatigue a man gets from digging
It is worse, because it frequently involves the sense of tired-
ness without the reward of work accomplished".
Having been convinced that a hot environment is
for you",
neither pleasant nor "good
there are some rather striking contrasts between the amount of
effort put into winter heating and the amount of effort given to summer
cooling.
Look at the average man in winter.
He sleeps comfortably,
because his house is heated or because he uses such accessories as a hot
water bottle or electric blanket.
He rides to work comfortably because his
automobile, bus or subway is heated.
He works comfortably because his
office or shop has an expensive heating plant.
His wife at home can also
go about her work with a high degree of thermal comfort.
this same man sleeps in
In the summer
a pool of his own sweat, arising unrested to
start the day.
His conveyance to and from work offers him little
from the heat.
At his place of business he is uncomfortable both because
relief
the air and walls are too hot and also because he is wearing clothes illsuited to the climate.
His wife has the choice of either becoming
extremely uncomfortable at her work or of simply not doing it.
Another example comes to mind;
perhaps extreme,
but true.
Take the case
of the backward child who must go to sumer school to keep up with his
classmates.
In the first
place he doesn't learn as fast,
even under
ideal conditions.
Secondly, he would much rather be out playing with
the other children.
environment.
Finally, he must attempt to learn in an uncomfortable
The poor kid has three strikes against him before he enters
the door.
But the illustration of the school child need not be so extreme to illustrate a need for adequate cooling.
west it
is
In many parts of the South and South-
very hot during the regular school session.
And even the
average or brilliant child deserves an environment conducive to his best
efforts.
It
is
little
consolation to say, "well, he has only one strike
against him".
Thinking beyond the individual, the widespread practice of summer cooling
might influence the development of whole world regions.
Climatologists
and historians have shown that the highest civilizations have developed
in either a naturally favorable climate, or have developed with the advent
of efficient heating.
In the past century the powerful and efficient
nat ons have been those of Northwestern Europe and Northern North America.
In these climates the temperature rarely goes above 750 and winter conditions are controlled by adequate heating.
With as efficient and wide-
spread utilization of summer cooling, there is reason to believe that the
tropic and sub-tropic areas might develop to the same high degree.
PART
I
Air Movement and Body Cooling
Since our primary concern is
consider first
it
seems wise to
the process of heat production by the human mechanism.
Winslow and Herrington put it,
combustion".
with cooling the human body,
"the whole life process is
The "fuel" df food and oxygen is
of tissues, for work,
As
a form of slow
utilized for the regeneration
and for heat.
Physiologists use the term metabolism to mean the quantitative relationship
between the intake of this fuel and the resultant work and heat produced
by the body.
Metabolism is influenced by such factors as individual body
build, age, sex, and the amount of muscular work performed.
Therefore any
reference to metabolism should allude to the special conditions which produced this metabolism.
Most of us are familiar with the term, if not the meaning, of basal
metabolism.
This concept was introduced to eliminate the many variables
connected with metabolism alone.
As defined by Winslow and Herrington, it
is "the level of metabolic activity displayed by a subject at rest at an
air temperature of about 700 F and at a period long enough after a meal to
avoid the specific dynamic action of food".
After looking at a number of
references on body temperature with a laynan's point of view, other nonphysiologists are cautioned to be aware of the important difference between
metabolism and basal metabolism.
basal metabolism is
For persons of average weight and build,
roughly 60 calories per hour.
Women have slightly less metabolism than men, and in both sexes the maximum
rate is achieved at about 10 years of age, dropping off thereafter.
For
reasons which appear later, metabolism is
body.
related to surface area of the
The surface area of men varies from approximately *91 square meters
to.2.1 square meters.
Most people have a metabolic rate of within 101/1. of
39.7 calories per square meter.
The variance with age and sex is
indicated in
the following chart:
METABOLISM PER SQ. METER (CALORIES PER HOUR)
Male Female
AGE
49I~
35.5
14-16
70-80
43.0
33.0
These figures refer to basal metabolism.
While they may have some value
in the design of buildings, (for example, the contrast between bedrooms
designed for a boy's school as against those designed for an old ladies'
hame) by far the most significant variable is muscular work.
is
The range
from about 60 calories per man hour when sleeping, up to a maximum of
around 1200 calories per man per hour when performing extremely strenuous
tasks such as rowing.
The following chart gives some indication of the metabolic rates for a
man of average weight and stature when involved in various activities:
Occupation
Sleeping
Sitting at Rest
Typewriting rapidly
Walking 2.6 m.p.h.
t
3.75m.p.h.
Stone working
Swimming
Walking upstairs
Calories per
man per hour
65
100
140
200
300
400
500
1100
It
is
is
attributed to the effort required to overcome gravity.
interesting to note the very high figure for walking upstairs.
remembered,
This
It should be
however, that a persan can't continue to walk upstairs all day.
I
-
--
Only a figure of between 500 to 1000 calories can be maintained for as
long as an hour.
to most people.
The 2.6 mph figure for walking is
a rate about normal
Military marches are continued at approximately this
rate for many hours a day, with only very short rest periods.
The daily totals for various activities vary from about 2000 to 5000
calories.
A normal day for a carpenter would run something like this:
8 hours sleep @ 65 calories per hour
6
2
8
"
520 calories
sitting at rest 6 100 cal. per hr.
light exercise @ 170 "
"t
carpentry work 0 240
"
"
"
"
24 hour total
600
340
1920
3380 calories
The influence of atmospheric conditions on metabolism is very strange
indeed.
Within the comfort range, metabolism seems to be unaffected
by the environment, but outside this range the metabolic rates increase.
Below the comfort level (cold weather) the increase is clearly an adaptive
reaction, useful in maintaining body temperature.
(hot weather4
Above the comfort level
the increase in metabolism is not an adaptive process, but
is a vicious circle which may be detrimental to health.
To quote Winslow
3'&
and Herrington, "under extreme conditions of heat, the warming up of body
tissues was accompanied by an increase in metabolism, which, in turn,
accentuated heating up.
The body temperature rose to 1050 -1100 F and
death promptly ensued".
It appears that the whole purpose of temperature regulation by the body is
to maintain a stable deep-body temperature.
The control mechanism (both
thermostat and thermometer) is in the brain, and seems to get its
instructions from the temperature of the blood which peruses it.
Under
all normal conditions the body produces more heat than it
7
regeneration of tissues and for work.
can use for the
Excessheat is stored in the tissues
for gradual discharge over a period of hours.
This heat must be eliminated
at approximately the same rate at which it is produced if the temperature
of the tissues is to remain at a level which affords comfort.
We see that the problem of the themal environment is
loss from the body,
to control the heat
so that the rate of loss is approximately equal to
the rate of production.
In winter the natural environment will drain
heat fram the body at a faster rate than the body can produce it.
summer the difficulty with the natural environment is
two-fold.
In
Not
only does the atmosphere keep body-heat-loss below the optimmn rate, in
sane cases it
is
adding heat to the body, thus causing the vicious circle
mentioned above.
HEAT LOSS FROM THE BODY
There are four methods of heat transfer utilized by the body.
convection,
conduction,
radiation,
and evaporation.
These are:
Of course the four
avenues of heat emission are connected in a rather complex way.
all four are put into play at once.
At other times the heqt loss from the
body is channeled through only one of these methods.
one separately, then in
At times
Let's consider each
combination, and finally analyse in detail only
those concerned with air movement.
Conduction
Conduction is
the transfer of heat between two surfaces of unequal
temperature by actual contact of the two.
example of this is
taking a bath.
the skin gives up heat to the water;
If
Probably the most conmon
the water is
if
cooler than the skin,
the water is warmer, then the skin
is heated.
The reason conduction is considered here first is that this
method of heat transfer is the one least used;
that is, the periods of
time when conduction is used make up such a small percentage of a person's
Activities that this method is not usually shown as a separate percentage
of total heat loss.
In most discussions, conduction losses are included
in the figures shown for convection,
vection is
or the total for conduction and con-
indicated as one sum.
For el1 general purposes this seens a very reasonable way of handling conduction.
However, it
should be borne in mind that this method is
rapid of all, and would probably play a significant role in
conditions.
It
the mzst
very extreme
also appears possible to make more use of the principles of
conduction in the design of furniture and gadgets for human confort.
may not be too fantastic to consider,
for example,
It
a mattress cooled by some
mechanical means which would do the same thing for the sleeper in summer
that the electric blanket does for him in the winter - or even circulating
cool water thru the tubular frame of a metal lawn chair.
Convection
Here we are concerned with the transfer of heat between the body and the
air.
Convection, like conduction, can work both ways:
either the body
gives up heat to the air or receives heat from it, depending upon the
relative tenperatures of the two.
There are two components to the measurement of heat interchange due to
convection.
The minor component is the amount of heat required to warm
the air we breathe.
The major component is
the heat transfer between
the surface of the body and the air which circulates by it.
Heat loss by convection is ddpendent upon four variables:
(1) the mean
temperature of the body surface,
vective heat loss,
(3)
(2)
the area of surface exposed to con-
the mean dry bulb temperature of the air,
and (4)
the rate of air movement.
Radiation
Heat transfer by radiation does not depend upon contact with the air or any
other material.
Radiant heat waves are transmitted thru the air from one
surface to another without directly warming the air in between.
The
amount of this transfer depends upon (1) the relative surface temperatures
of the two bodies, (2) the distance between them, (3) the amount of surface
area exposed to transmit and receive radiant energy, and (4) the radiant
properties of each surface.
The so-called steam radiator actually warms people less by radiation than
it
does by convection.
the individual,
It
exposes a relatively small amount of surface to
generally has a poor surface for transmitting radiant heat
(especially when painted a dark color) and does most of its
raising the temperature of the air.
heating by
The fireplace and the electric
radiant heater are our most familiar exanples of a primary radiant source.
Of course nearly every true radiant source also heats the air indirectly
by first
heating the objects which intercept its
The human skin is
remarkably efficient when it
rays.
comes to this method of
heat transfer, being almost 99 per cent amnissive and receptive to the
infra-red (heat) rays.
Evaporation
When a wetted surface is
surrounded by air that is
not saturated,
it
con-
tinually gives off moisture to the air.
In this process it is also
Variables in this process are:
transferring heat to the atmosphere.
(1) air temperature, (2) air movement, (3) relative hunidity of the air,
(3) area of evaporative surface, and (4) the available moisture for
evaporation.
Everyday examples of the evaporative process are hard to find.
An
interesting use of this method was found, however, by troops in the
tropics during the last war.
If
you put a can of beer in
a helmet with
some gasoline and force a jet of compressed air thru the gasoline, you
have to be careful to stop soon enough or you'll have frozen beer.
Unlike the preceding three methods, heat transfer by evaporation from the
hunan body is always cooling.
Under basal conditions, that is with the
subject at rest in an average environment, nearly one half of the heat
loss due to evaporation may be contributed by evaporation from the moist
membranes of the nose and throat.
This percentage is materially altered
under varying atmospheric conditions, but all evaporative loss has two
components - one from the mucous menbranes, the other from the sweat given
off at the skin surface.
RELATIONSHIPS UNDER VARYING CONDITIONS
We have seen that there are five factors influencing the heat exchange
between the body and its
environment.
These are the heat production,
or
metabolism, and the four avenues of heat loss or gain from the surroundings.
As is customarily done by physiologists, we shall combine conduction
with convection, and since conduction is the lesser of the two, the term
convection shall be understood to include both.
heat production by metabolism,
(2)
That gives us:
(1)
heat gain or loss by convection,
(3)
gain or loss by radiation, and (4) loss by evaporation.
Winslow and Herrington have expressed this relationship as a formula:
Metabolism minus Evaporation plus or minus Convection plus or minus
Radiation equals Zero; or
bI-E-C-tR:o
when a state of equilibrium exists.
The following chart gives all the relationships between the three methods
of heat transfer and the nine physical and physiological factor's involved
in the heat interchange between the human body and its environment.
Evapo
Physical
Air Temperature
X
Air Movement
X
Relative Humidity
X
Conv4
Rad.
X
Mean Radiant Temperature
X
Physiological
DuBois Surface Area
X
Effective Radiation Area
Area of Evaporative Surface
X
X
Mean Skin Temperature
Available Moisture for Evap.
Frca the foregoing chart it
X
X
X
can be seen that air movement across the body
can influence only evaporation and convection.
And these two methods,
in
turn, are in
some way altered by seven of the nine physical and physio-
logical factors.
It will be impossible to eliminate radiation from our
considerations, but emphasis will be placed on those elements which are
enclosed in boxes on the chart.
However, in the total picture of body heat loss, air movement can influence
radiation.
That is,
temperature of the radiant surfaces which add or take-
away heat from the body can be altered by the movement of air.
This will
be taken up later, and will not be considered at this time.
Apparently several attempts have been made to develop a single figure for
the combined influence of air temperature,
and mean radiant temperature.
according to
air movement,
relative humidity,
These attempts have been abandoned because,
inslow and Herrington,
"only an independent determination of
the four distinct factors mentioned above can give a real measure of the
thermal demands of the environment".
There have been, however,
several more-or-less successful efforts to
combine two or more of these factors.
Ventilating Engineers have developed,
temperature".
The American Society of Heating and
and use, a tea
odled "effective
Effective temperature shows the relationships of air
temperature, relative humidity, and air velocity which give an equal
sensation of comfort.
This is of course a subjective measurement, and
was determined from votes of a numberof subjects tested by the ASHVE under
varying conditions.
This quantity makes no reference to radiation,
since
the temperature of air and walls was always the seme in the experiments.
The Pierce Foundation has developed a term called "operative temperature".
Operative temperature is a figure representing the combined influence of
air temperature and mean radiant temperature.
A formula involving con-
stants for both radiation and convection has been devised, but for most
suual situations a mean between air and wall temperature gives approximately
the same figure as operative temperature.
Effective temperature and operative temperature should not be thought of
in terms of one being better than the other.
for different things,
and are useful in
They simply give figures
different ways.
They are cited
here to reduce any confusion that might result from reference to them
in this discussion or elsewhere.
The "ideal" climate
Since this thesis is limited to the cooling effects of air movement, it is
necessary to attempt to locate the comfort zone as a point of departure.
Some physiologist and climatologist seem to be especially wary in discussing the ideal thermal environment.
variables involved,
this is
Considering the number of
a natural attitude.
However, we can get
some indication of the ingredients of the ideal climate from a reference
made to it by Dr. L.P. Herrington in a speech to the Building Research
Advisory Board conference held in Washington, D.C.
In this talk Dr.
Herrington said, "The ideal indoor climate occurs in this general latitude
in the months of October,
early November,
or in May,
to use any artificial heating or cooling at all.
the air outside is
about 600.
when it
is
not necessary
Under these circumstances
We have sunny days and,
due to radiation
effect, the structure itself has a radiation temperature 40 to 50 above the
air temperature.
Due to the moderate temperature outside, the windows are
open, air movement is considerable, and the air changes through a structure
Mao=&%"
are perhaps 15 to 20 an hour, but variable from moment to moment.
"In
this internal environment with wall temperatures having infra-red
radiation values,
characteristic
of radiation frcm surfaces around 700
to 720,
and with air tenperatures about 40 lower, people feel very good.
This is
the ideal indoor climate for this latitude,
and for people with
the kind of seasonal weather experience typical of our geographical zone".
Wolfgang Langewiesche,
in
one of his articles for the Climate Control
21
Project of House Beautiful magazine, gave a less technical but more
generalized criterion of the ideal climate.
He said that the ideal
climate is simply where the individual is not conscious of climate at
all.
You don't want to take off your shirt or put on an overcoat;
you don't want to move in front of the fan or over the floor register;
you don't want to take a cool shower or a hot bath;
you just don't think
about the climate.
Assuming that both the above observations are correct, we need to know
whether other combinations of the factors involved in heat interchange
will give equivalent sensations of comfort,
and how far we can deviate
from the optimum before the climate becomes intolerable or unhealthy.
Cofort Chart,
ASHVE
Any amount of physiological data would be valueless unless the conditions
it prescribed also produced a feeling of comfort for the individual.
Therefore, the comfort zone should be determined primarily by subjective
measurement.
The Research Laboratory of the American Society of Heating
and Ventilating Engineers has used such a system of measurement in the
development of their comfort chart, shown on the following page.
The
special conditions to which this chart apply, given in the note accompanying the chart, should be carefully considered.
In addition to those
qualifications, recent information has indicated other possible variations.
There is some evidence to indicate that the comfort zone could be extended
beyond the 30% and 70% relative humidity lines.
There seems to be a rather wide spread of effective temperature called the
summer comfort zone, ranging from a preference of 30% on the cool side to
50% on the warm.
However, the graph shows all percentages, so the designer
will know how many people are likely to be satisfied in any given condition.
This chart makes no provision for the influence of radiant heat.
experiments condt.cted in
connection with this chart,
tures were approximately the same.
The A.S.H.V.E,
In the
air and wall temperaGuide states,
"Radiation
from occupants to room surfaces and between the occupants has an important
bearing on the feeling of warmth and may alter to some measurable degree
the optimum conditions for comfort previously indicated.
radiant temperature of a space is
Since the mean
affected by cold walls and windows,
as
well as by the warm surfaces of heating units placed within the room or
embedded in the walls, these factors must be compensated.
Likewise,
in
'?K.
D01
0
DRY BULB
TEMPERATURE*F
A.S.H.V.E. COMFORT CHART
FOR
STILL AIR
Note.-Both summer and winter comfort zones apply to inhabitants of the United States only. Application of winter comfort line is further limited to rooms heated by central station systems of the convection
type. The line does not apply to rooms heated by radiant methods. Application of summer comfort line
is limited to homes, offices and the like, where the occupants become fully adapted to the artificial air conditions. The line does not apply to theaters, department stores, and the like where the exposure is less than
3 hour.. The optimum summer comfort line shown pertains to Pittsburgh and to other cities in the northern
portion of the United States and Southern Canada, and at elevations not in excess of 1000 ft above sea level.
An increase of one deg ET should be made approximately per 5 deg reduction in north latitude.
densely occupied spaces, such as classrooms, theaters and auditoriums,
temperatures somewhat lower than those indicated by the comfort line may
be desirable because of counter-radiation between the bodies of occupants
in
close proximity to each other.
Such radiation will also elevate the
mean radiant temperature of the room".
The Guide does not indicate any method of determining the quantitative
compensation which should be made for significant variation in radiation.
A short discussion of how this might be done will be given in later
paragraphs.
Comfort Zone and Air Movement
This comfort chart is for minimal air movement of 15 to 25 feet per minute.
We are, however, given data vhich can be used to convert the comfort
chart for higher air movement.
The fish-shaped chart shows how effective
temperature varies with air velocities up to 700 ft. per min,
the line A-B on the chart.
Consider
We see that for a dry bulb temperature of
760 and a wet bulb temperature of 620
,
the effective temperature at an
air velocity of 20 to 30 ft. per min. is slightly over 700
When air
velocity is increased to 600 ft. per min., effective temperature drops
to 640.
It
should be remembered that effective temperatures are equivalent com-
fort conditions.
In the above example, we have an air temperature,
humidity, and air movement producing 640 ET.
feeling of comfort as still,
This would give the same
saturated air at 640.
Obviously, effective
temperature has no practical meaning unless used in conjunction with the
-120
I
2
Do-
0709
70-1
-6o50
6o-
kii
o*
~
EFFECTIVE TEMPERATURE CHART SHOWING NORMAL SCALE OF EFFECTIVE
TEMPERATURE, APPLICABLE TO INHABITANTS OF THE UNITED STATES
UNDER FOLLOWING CONDITIONS:
A. Clothing: Customary indoor clothing. B. Activity:
Methods:
Sedentary or light muscular work.
Convection type, i.e., warm air, direct steam or hot water radiators. plenum systems.
C. Heating
comfort chart.
Few people have any occasion to know how it feels to be
in still, saturated air at various temperatures.
Our objective is to determine the influence of air movement on the comfort
Assuming that we l-ave the conditions to which the ASRVE comfort
zone.
chart applies,
let's
take one example for illustration.
Suppose we want
80% of our tenants to be comfortable (we're mad at the other 20%).
relative humidity remains constant at 70%.
The
The chart indicates that an
effective temperature of 730 must be maintained.
For still
air, our wall thermometer must not go above 760 ;
ponding wet bulb temperature will be 690 ..
the corres-
Now if we can in some way,
natural or otherwise, increase air velocity to 700 feet per minute
(approximately 8 mph), the room thermometer may rise to 830 and our tenants
will be equally comfortable.
It's something of a nuisance to check it,
but the figures on the effective temperature chart which give this condition are:
83.50 dry bulb, 75*50 wet bulb, giving 730 ET at 700 ft. per
min.
Thus,
an increase of 8 mph in
air velocity allows us an increase of 70 dry
bulb temperature for an equivalent comfort condition.
The 30% relative
humidity at the other end of the 730 ET line does not give quite as good
results.
By increasing to the same velocity, dry bulb temperature goes
up only 60 , from 820 to 880.
These figures give us, very roughly, the upper limits of the effectiveness of air movement for an almost ideal (80o satisfied) comfort condition.
That is, 880 at low humidity, and 830 at high.
However, it is believed
that the importance of natural air movement will not be in satisfying an
ideal condition, but rather in bringing an intolerable heat down to an
acceptable one.
This would lead us to believe that even higher dry bulb
temperatures will enter the comfort picture, and they might under certain
conditions.
But there is another joker in the deal.
Allen and Walker
say, "the maximum air velocity which can be tolerated with comfort by
human beings at rest is
approximately two feet per second".
Converted,
two ft. per sec. equals 120 ft. per min. or about 1.4 mph.
This low air velocity may be a desirable goal, but it. is believed that if
the choice is between a higher air velocity and disagreeable heat, most
people would "tolerate" at least twice that velocity.
W7hile no data have
come to the authorts attention regarding the effects of air velocity on
various normal activities, such data undoubtedly exist;
and it
would seem
advisable for the designer to determine such things as: at what velocity
does a newspaper become unmanageable;
leave the desk;
when do letters and other papers
what air movement makes lighting a cigarette difficult;
etc.
All the preceding discussion has been on the basic assumption that the
temperature of air and walls is the same.
It would apply directly to
those rare situations when a building ig well insulated, well oriented,
well shaded, etc.
In other words, air and wall temperatures will not be
the same in most situations,
and especially at the elevated temperatures
with which we are primarily concerned.
Therefore, it seems reasonable to
look for data showing the influence of radiant heat on the comfort zone.
We are now in trouble.
A thorough search has revealed no comprehensive information on the
subjective measurement of radiation and comfort.
The physiological
reactions of the human body to radiant heat are well documented by
Winslow and Herrington, but references to which conditions produce a
feeling of comfort are rather sketchy.
We can get a hint by comparing
their charts relating skin temperature to operative temperature and skin
temperature to sensations of pleasantness.
Remembering that, for engineering purposes, operative temperature may be
taken as the mean between air and radiant temperature, the above comparisons would give some indication of how radiation affects comfort.
However,
generalizat*ins from this data will not be hazarded in this
study.
In reference to the ASHVE comfort chart, Winslow and Herrington have this
to say, "'%ork now in progress under the Society's direction will probably
result in twro scales.
One of these, the familiar effective temperature
scale in present use, will quite possibly be restricted to use insituations
-where traffic in and out of conditioned spaces places a premium on shortperiod contrast sensations.
It is expected that to this will be added a
similar scale for equilibrium conditions applicable to the comfort-conditioning
of spaces with relatively long periods of occupancy.
This added equilibrium
scale will probably base its equivalent combinations of dry-bulb temperature
and relative humidity on lines of equal skin temperature, experimentally
determined by methods similar to those which we have developed in connection
3'C
with partitional calorimetryt .
In the absence of more precise information, it appears that for the present
10
10C
901
s0
Ws7C
(r6 0
.5
5
50
RADIANT
60
70
80
90
100
TEMPERATURE (*F)
1. RADIANT AND AIR temperatures are inter-related in their
effect on winter comfort zone. Rise in either temperature
requires corresponding drop in temperature of opposite
factor to maintain equivalent comfort condition. The
resulting comfort temperature is simply the arithmetical
average of the two factors.
40
RELATIVE
60
80
100
HUMIDITY (%)
2. HUMIDITY as it affects comfort band. Using preceding
graph as basis for temperatures, lower humidities are
shown to raise upper comfort levels but to have almost
no effect on lower. Upper level is further raised by
increased air movement. Greatest cooling effect is obtained
when air is relatively dry and moving rapidly.
0
100
VELOCITY
OF AIR
200
INFEET
300
400
500
PER MINUTE
3. AIR MOVEMENT as it affects body cooling. As velocity
increases, upper comfort limit is raised. Air movement
becomes still more effective as relative humidity goes
down. These graphs are not intended to show precise
lines of upper and lower comfort range, but nature and
trend of band under influence of various factors.
we might make some adjustment for radiant heat by a rough combination of
That
the ASHVE comfort chart and the concept of operative temperature.
is,
take operative temperature
(mean of radiant and air) and use it
as
dry bulb temperature in the comfort chart.
One might assume that because we can simply average radiant and air temperatures and somehow relate this figure to comfort,
that it wculd be
equally desirable to have high radiant and low air temperature as it would
be to have low radiant and high air temperature.
human body, marvelous machine that it
is,
and bring about thermal equilibrium.
This is
not so.
can adjust for either condition
In doing so, the amount of heat
dissipated by each method of heat-loss varies over a wide range.
cue is
The
The
that the stress on the body is not the same when losing heat by
evaporation as it
is when losing it by radiation or convection.
As a starting point, let's
see what the porcentage of heat loss is
the various avenues of heat interchange under basal conditions;
for
that is,
for a resting subject, moderate temperatures, little air movement, low
relative humidity, and air and walls at about the same temperature.
In
this situation, radiation accounts for 2/5 of the total heat loss from the
body, convection 2/5, and evaporation 1/5.
of conditions.
these percentages.
It
But that's a whole mouthful
seems almost impossible to make any generalization about
Some results of experiments made by the Pierce
Laboratory of Hygiene will indicate why.
Air Tem.
Wall Temp.
Percent Heat Loss Due to
U
3
_Evag.
Rad.
Cony.
A
17.1
19.0
10
40
50
B
16.0
49.1
21
C
22.8
22.8
17
D
29.4
52.4
78
E
35.4
36.6
100
Series
79
13
70
22
In Series A - Body temperature was falling.
"
It
- There was more gain by radiation than was produced by metabolism.
"
"t
"
"
"
- High air movement of 264 cm per sec.
D - Gain by radiation was 66% above metabolic rate.
E - Heat gain from both air and walls.
These figures show how widely the percentages can vary under different
conditions.
But although generalizations are hard to make, we can tell
what will happen if we know the conditions beforehand.
To do this we
must know how each avenue of heat loss varies as temperature, humidity,
air movement,
etc.
are altered.
The chart on the following page shows how changes in operative temperature
influence the heat loss or gain by evaporation, radiation and convection.
This chart is for a single subject, so the values given should not be
applied to people in general, but the relationships would be similar.
Figures above the zero horizontal line indicate either heat production by
metabolism or heat gain from the environment.
loss.
Those below indicate heat
Radiation and convection are shovm as one sum because operative
temperature governs both processes.
Considering the curve for radiation and convection, we notice that as
operative temperature increases, less and less heat is lost by this method.
120
100
80
60
40
a!
20
0.
(n
0
I
o -20
U -40
-80
-100
-120
-140
-160
-180
-200
-220
T.
*
TOTAL
o HEAT
METABOLISM
CHANGE
o
*
RAD. AND CONVEC.
EVAPORATION
Factors in heat balance between the unclothed human body
and its environment at various Operative Temperatures.
In this case, at an operative temperature of about 840 F no heat is lost,
and the body is beginning to gain heat by radiation and convection.
Evaporation, on the other hand, remains fairly constant at lower temperaAt this
tures, until an operative temperature of about 800 F is reached.
point the rate of increase by this method jumps very rapidly.
Obviously,
evaporation is beginning to take over all the heat losses from the body not only that demanded by metabolism, but that gained from the environment
thru radiation and convection.
Thus we see why it is said that heat loss
by evaporation is accomplished only by greater stress on the body.
Note where the curve for evaporation crosses the curve for radiation and
convection.
This area has been termed the "zone of thermal equibibrium".
Operative temperatures below equilibrium has beeniermed the "zone of body
cooling" and above equilibrium the "zone of evaporative regulation".
Mean values for thermal equilibrium, using a group of subjects, have been
determined to be as follows:
Oper. Temp. for
Equilibrium, OC
Nude subjects in reclining position
Clothed subjects in reclining position
Nude subject performing active work
The question might well be asked:
-
29 to 33
25 to 29
19 to 21
what significance do the preceding facts
have in a discussion of air movement?
They tell us that at operative tem-
peratures above 210 C to 330 C (or 700 -90o
F) we are dealing only with
evaporation, and that evaporative regulation produces the greatest stress
on the body.
Therefore, we should give attention to means of reducing
operative temperature by air movement.
PART
II
BEHAVIOR OF MOVING AIR
Before attempting to control the wind, it
seemed wise to try to get a fairly
accurate picture of what the wind is like.
the properties of air must be known.
physics of air.
And before studying the wind,
So the first step was to review the
Following this, a search was made for information regarding
the characteristics of unconfined, natural breezes flowing without obstruction.
Finally, the effects of obstructions, such as buildings, will develop.
In this section, we will be concerned only with the air and natural air movement.
The following section will deal with the flow of air in and around
buildings.
Air and Tenperature
It might be well to make a brief summary of the dlementary physics relating to
air and temperature, in order to have this well-known informationfresh in our
minds.
Air is composed of about 80% nitrogen, 18% oxygen, small amounts of
carbon dioxide and other gases,
and water vapor.
When it comes to cooling,
the most important variable is that water vapor.
The psychrometric chart (page
) shows how an increase in the temperature of
the air increases its ability to hold moisture.
ture against grains of moisture in
the air.
This chart plots air tempera-
Among other things,
it shows the
amount of moisture required to saturate the air at various temperatures.
This
is shown by the top curve of 100% relative humidity, which is saturation.
Notice that only about 80 grains of moisture are required to saturate air at
600 f, whereas some 160 grains are required to bring 800 air to saturation.
a:
< 160
I
0
S120
Ix
~0
80
z
DRY BULB
TEMPERATURE
F
PSYCHROMETRIC CHART, PERSONS AT REST, NORMALLY CLOTHED, IN STILL AIR
As air is heated, it expands - that is, it becomes less dense, lighter, and
has a tendency to rise.
The low pressure area thus created must be filled
by the cooler, heavier air adjacent.
movement.
This is the basic cause of all air
The worldwide air currents have been shown by meteorologists to
be the result of the changing temperatures of the air.
Strong upward currents
occur over the hot land masses, particularly deserts, and cooler air from over
water or from polar regions rushes in to fill the low pressure area.
Of
course this is a super-simplification, but it illustrates the basic principle.
These two facts about the air-expansion and ability to hold moisture - account
for most weather phenomena, on large or small scale.
The proper utilization
of these facts can be extremely helpful in an effort to provide greater
thermal comfort.
This study will not attempt to discuss whether a certain locality will have
natural air movement.
There are existing works which give comprehensive
pictures of regional climates throughout the United States.
However,
these studies are-for sizeable regions, and may not give a true picture of the
microclimate, that is, the climate of the immediate neighborhood, block, or
Therefo;e,
lot on which a building is to be erected.
the air acts as it does will be given.
a few facts about why
This is not an attempt to survey
microclimatology, but rather to give same hints as to the value of this
field to design.
The most significant relationship is between air, land, and water.
The
land changes temperature at a much faster rate than water, gaining heat fran
the sun more rapidly during the day, and radiating it to the sky faster at
night.
This accounts for the sea breeze.
As the land heats during the day,
it
raises the temperature of the air next to the ground.
This air rises,
and the low pressure thus created must be filled by the cooler air from over
the water.
This breeze may extend inland from the ocean as far as twenty
miles, whereas in the case of a small lake, its effects may not be felt over
a few hundred yards.
water is
The whole procedure is reversed at night when the
warmer than the land.
This sea breeze may not be the"prevailing" breeze in some areas, and its
benefits may be cancelled by attempting to oppose a larger force.
has been cited as a good example of this opposition of forces.
the prevailing breeze is toward Lake Michigan.
Chicago
In summer
Due to the conditions
mentioned above, the purely local "sea" breeze is of course fraam Lake
Michigan.
Therefore, the two have a tendency to cancel each other, leaving
the city with little natural air movement.
And when the rush of cool air
fram the lake is felt at all, it carries for only a few blocks.
Obviously,
on the opposite side of Lake Michigan, where the two forces are cooperating,
the weather is
much more ideal.
Another phonomenon worth mentioning is
the downward flow of cool air.
While this effect is either negligible or non-existmnt during the summer,
in areas where the land is relatively flat, it contributes to significant
velocities in mountainous areas.
The so-called "mountain breeze" is a
result of air being cooled' by the lower temperature surfaces at high
altitude, and descending into the valley.
It has been said that residents
of such areas seek to place their houses at the mouth of a valley in order to
benefit from this breeze.
It
heat from direct
should be noted that the air gains relatively little
solar radiation.
Finch et. al. state "...
the atmosphere absorbs directly
only about 10 to 15 per cent of the solar energy that comes to it.
absorption takes place mainly in the upper layers of the air.
Such
This process,
therefore, is not very effective in heating the layers of air close to the
earth."
However, the same reference goes on to say that the re-radiation
of solar energy from the earth is readily absorbed by the atmosphere - "some
It would seem that this method of heat transfer
90 per cent" is estimated.
from the earth to air by radiation would heat the air more or less uniformly.
That is,
the radiant energy would have to travel same distance before it
would become completely absorbed.
On the other hand,
comes in
air is
a very poor conductor.
Therefore,
the air which
contact with the heated surface of the earth trqnsmits this heat
very slowly to adjacent air, end a film of warm air forms.
assume that this film would always rise immediately,
in contact with the heated surface.
One might
allowing more air to come
This is not true in all cases.
In one
of his articles for the House Beautiful Magazine Climate Control Project,
Wolfgang Langewiesche refers to the "stagnation" of hot,
low,air.
Apparently, this hot air forms in a sort of bubble, and in open areas it
balloons away periodically.
However, when this air is confined in a snall
space such as a patio or enclosed garden, this bubble sometimes doesn't
rise.
The comparison is made to a soap bubble, which will remain stationary
until activated by some small breeze.
mentioned above is
The renedy for such stagnation as
to allow access to even a small air movement - by a gate,
breezeway, or similar opening.
Many more interesting examples of the influence of the surroundings on
air movement and temperature could be made - the cooling effect of trees
and lawns,
radiation from paved areas and buildings,
not relate directly to this subject;
etc.
- but they do
so let it be sufficient to say that
the importance of the surroundings to the otal design picture cannot be
overestimated.
Velocity and Altitude
In the search for information about the behaviour of natural breezes, an
interesting study came to light.
The practical application to the cooling
field of the results of this study are obscure, but since so little information of any kind is readily available, it is cited here for the possible
bearing it might have on future work.
In their paper, "Air Conditions Close to the Ground, and the Effects on
35
Airpla.Ine Landings," the authors report finding that average air velocities
will be greater at higher altitudes than they are at ground level at the
same time.
formula
.
They have even expressed this relationship in terms of a
= (h)"
A graph of this formula and their experimental
results are shown on the following page.
These experiments were conducted on an airfield where the breeze could
travel for about
mile over unobstructed, flat terrain before hitting the
test instruments.
Measurements were made at intervals fram six to fifty
one feet above the ground.
Fluctuations in vertical and horizontal
direction were recorded, as well as fluctuations in velocity.
The test
runs were of some 36 seconds duration, and results were computed from
.36
Q-2
-
o,
0
- )
3 20
/0
0 .,
4
'?4
<20
0
0
/0
......
-
4
/0,4
-
-
50
0
/2
8
4
/6
20 24
Time, seconds
28
32
36
-Wind speed and inclination fluctuations for an average ground wind
of 8
niles per hour.
Run
1,July 8,
1932.
112
ExperimnentaI
1z
')1V
b
0~
0
0
/0
20
30
40
Height above gr-ound, feet
50
60
Average wind speeds expressed in terms of the average wind specd
at altitude of 51 feet.
photographs.
Results of one test run of average velocity of 8 mph at ground
level are reproduced here as an example.
No question is made of the validity of these results when used under the
conditions outlined above.
The procedure of these experiments is mentioned
to explain caution in attempting to apply the results to any other conditions
than those under which the experiments were made.
Certainly an unobstructed
approach is demanded, and there is some doubt in the mind of the writer
whether similar figures would come of longer test periods and measurements
at higher altitudes.
As was said, the relation of this pheonomenon to ventilation is not quite
clear.
It probably has no bearing on the problem of the one-story building.
But it seems that it could be important in taller buildings, particularly
when the building is not divided by floors, as in some industrial structures.
'What would be the indoor effect of air entering the five foot level at 10
mph and at the seme time entering the fifty foot level at nearly 15 mph?
Changes in Velocity and Direction
Fluctuations in wind speed and inclination are of interest also.
The
preceding charts of these variations show changes in inclination sometimes
as often as every second, and rarely farther apart than three or four
seconds.
This would indicate that the wind has a sort of wave motion of
rather high frequency.
Changes in wind speed are not as frequent, but do
indicate a definite pulsation.
It is doubtful whether these changes would
be conspicuous to the observer, and records for such short periods would
not indicate large gusts, but this is evidence that the behavior of wind is
far from the smooth flow found in the wind tunnel.
It was expected that the fields of meteorology and aerodynamics would offer
many references on the behavior of the wind, and they do.
But unfortunately,
the vast majority of this information cannot be used in connection with
Meteorology concentrates on the large-scale causes of
natural ventilation.
winds, and the wind patterns over vast areas.
Present-day aerodynamics has
little use for any air movement of less than about a hundred miles per hour.
Not a single comprehensive study of wind behavior at low velocities (below
20 mph) was discovered.
In the search of aerodynamics literature, after being confronted with subsonic, super-sonic, much numbers, and many, many airfoils, it gradually
In
dawned that the very early literature might be of a different nature.
1893 The Smithsonian Institution published,
as one of its
"Contributions to
Knowledge", a pamphlet by S.P. Langley called "The Internal Work of the Wind."
Aside from its bearing on this study, the paper is thoroughly fascinating.
It is a story of observations made by a man who was later to make some of the
most important contributions to the development of heavier-than-air craft.
Of course the date precedes the first flight of man by about ten years.
Langley was trying to find an explanation of the soaring flight of birds.
Since he had assumed that this phenomenon was due to the vagaries of the
wind, he set about to determine the precise nature of air currents.
His
paper offers data on the changes in wind velocity at very short intervals
of time.
The technique was to build an extremely light cup-anemometer so
as to reduce the effects of inertia.
Recordings were made as often as every
Graphs
half-revolution, which would sometime mean as often as every second.
of these recordings, one example of which is shown on the following page, are
very similar to those of Thompson, et al, mentioned above.
Langley's results
were substantiated by other findings, though not extended materially, and
apparently little
more is known about the wind today than was known in 1893.
Irminger and NAkentved
it is
state, "The wind having such an irregular structure,
obviously necessary to study it most thoroughly, and to correlate the
results with those obtained by experiment, in order that data applicable to
They go on to report essentially
practical conditions may be evolved."
the same characteristics mentioned above;
and in reference to pressures on
structures, say "The available knowledge, however, is insufficient to enable
any definite statement to be made."
This same inadequacy can be applied
to ventilation.
Summry
In the investigation of natural air movement, we first looked for the
basic properties of air and their relatonship to temperature.
found that when air is
heated,
it
It
was
expands and rises, causing a low pressure
area which must be filled by other air.
This is the basic cause of all air
movement, from worldwide currents to the draft in your fireplace.
The
natural means of heating air were discussed, and reasons given for the hot
film of airthat forms on surfaces and sometimes does not rise.
This same change in temperature governs the ability of the air to retain
moisture.
Hot air can hold more moisture than cold, therefore having a
greater power to evaporate moisture from a surface.
L111
PLATE II.
5
m
5
53-
541"
55m"
56rn
57"'
bw"
Wind velocities recorded January 14, 1893, at the Smithsonian Institution with a light Robinson anemometer
(paper cups) registering every revolution.
Abscisso = Time.
Ordinates = Wind velocities in miles per hour.
The present state of knowledge about the behavior of unconfined, natural
air movement is generally inadequate.
far from a auooth steady current.
We know that a natural breeze is
Its absolute velocity changes rapidly,
wide fluctuations having been recorded as often as every second.
Mean
values over longer periods of time also give a changing picture.
At the
same time the velocity is pulsating, the wind is changing direction, both
horizontally and vertically.
It was shown also that the mean velocity of the wind will increase with
altitude, within certain limits at least.
With regard to the rapid fluctuation of velocity and direction, it
that no generalizations can be made from existing data.
observation will be made here:
to demand that it
ventilation.
Only this
the evidence of fluctuation is
be accounted for in
seems
sufficient
any theoretical solution to natural
This does not mean that the phenomena will necessarily
influence engineering applications, but proof that it either does or does
not must be shown sooner or later.
The already complicated procedure
of making comparisons between wind tunnel tests and full-scale results
is not made easier when this fluctuation must be considered.
PART
III
AIRFLOW AROUND OBSTRUCTIONS
The preceding section discusses some of the properties of air, and its
behavior when unrestricted and unconfined.
The following discussion
will deal with the happenings when an obstruction is placed in the path of
air currents.
The possible number of shapes and combination of shapes is
infinite, so an attempt was made to select examples which would have the most
bearing on building considerations.
Certain simple geometric shapes are discussed first, with the thought that
these will serve for generalizations about airflow around more complex
shapes.
Another reason for using these geometric forms is that most
experimental investigations on building shapes use only the standard
rectangle and gable roof.
It should be noted that, with rare exceptions, all of the results used in
the following discussion were derived from steady-state wind conditions,
either produced in a wind tunnel or assumed for calculation.
Such a
procedure ignores the actual fluctuation of the natural wind which was
shown in the preceding section.
HoweVer, we must be content to use
existing information, keeping in mind the limitati ons as well as the
possibilities.
We are, in general, looking for two things:
(2)
variations in pressure.
(1) the path of air flow, and
Without changing the temperature of the air,
one way to maintain air movement is simply not to obstruct it.
"When an
Y
obstruction is
placed in
an air stream,
it
has been found that the new
currents form high and low pressure areas;
and as has been shovn, air will
rush toward an area of lower pressure.
Bernoulli and Venturi
Another general statement that can be made is:
there is low static pressure;
pressure.
where there is high velocity,
and conversely,low velocity means high static
This fact gives rise to the Bernoulli Principle and the Venturi
tube, which take cognizance of the compresibility of air.
Since many of
the effects of obstructions on air flow will be illustrated by smoke-stream
pictures, it should be noted that "In all air-flow pictures, wide, diverging
smoke streams indicate decreasing velocity andhigh static pressure;
whereas
narrow,
converging smoke streams are a sign of high velocity and reduced
I7
static pressure."
The photographs on the following page illustrate Bernoulli's Principle,
or the Venturi Effect.
As air passes thru a construction of decreasing
area (such as a funnel or between two inclined surfaces) it moves faster
than the rest of the stream because it is compressed into a smaller space.
Streamlines
Everyone is familiar with "streamlining" although it
t
is
often confused with
rounding the corners" - witness streamlined coffee pots and baby buggies.
True streamlining is a recognition of the fact that air resists an interruption
of its movement, and if interrupted, seeks to regain its smooth homogenous
flow as quickly as possible.
On the second page following will be found two
forms which offer the least resistance to air flow, and allow the air to
regain its "equilibrium" in the easiest way.
The lenticular section is also
A. Flat Plate at right angles to air stream.
Drag 100%
B. Entering wedge. Drag still 75%
A
I
C. Entering wedge turned around. Drag still 75%
Plate XX. Illustrations of Drag-Flat Plate and Entering Wedge
A. Stationary cylinder. Drag only 50%
B. Half-cylinder and wedge. Drag reduced to 25%
Plate XXI. Illustrations of Drag-Cylinder and Half-Cylinder with Wedge
A. Air velocity= 15 mph. Airfoils stay far apart
B. Air velocity = 30 mph. Airfoils move closer
C. Air velocity= 45 mph. Airfoils move still closer
Plate V. Demonstration of Bernoulli's Principle
A "streamlined"
section, particularly efficient at high velocities.
Ludington
27
states,
It
is
"There is
no such thing as the perfect section for all speeds."
interesting to note that when a non-streamlined body is
flow of air, the air has a tendency to streamline itself.
placed in the
Smoke stream
pictures show the lines diverging befare reaching the obstruction,
and a cloudy
area behind the obstruction (on the leeward side) around which the snooth flow
lines gradually converge.
Thus,
in
a steady stream of air, turbulent areas
are formed around an obstruction (called vortex regions) allowing the rest of
the air to follow a path of least resistance.
In connection with streamlines, it
diould be mentioned that the patterns
around a body freely suspended in a wind tunnel are not the sane as those
where the body is
resting on the "ground".
And it
is
not accurate to take
the symmetrical picture of a freely suspended body and assume that half of
it
will show the ground conditions.
"For a body freely exposed to the air current, the irrotatt onal stream flow
on the windward side will almostreach the body itself,
and only form a xnall
vortex region;
further, there will always be a point at the surface of this
region which is
normal to the flow,
giving rise to velocity pressure on a
small area.
"This does not apply to a body placed on the ground with which the windward
vortex layer issues from the ground,
and consequently velocity pressure is
not fully developed at any point.
"Another and very important difference between a body placed on the ground
and a body freely exposed to the air current is the formation of the shape
of the leeward region."
Simple Geametric Forms
On the next pages will be seen the air patterns around a flat
and cylinder.
These photographs of smoke streams by Ludington
objects freely suspended in the air.
reduced velocity and high pressure.
27
are of
Notice that the streams are wide and
diverging in front of all the forms but the "entering wedge".
This indicates
In each instance there is a foggy, tur-
bulent area on the side away from the wind.
or suction,
plate, prism,
This indicates a low pressure,
area.
The illustrations following are from Irminger and N)kkentved i
and show
the measured pressures of a plate, prism, and cylinder under two conditions.
As stated on the graphs, one curve is for the object freely exposed, the
other is for the same object placed on the ground.
While patterns for the
two conditions have a certain similarity in appearance, the pressure (suction)
for the model on the ground is
pended model.
generally sailler than the one for the sus-
With more complicated shapes, even the shape of the curve
may be markedly different.
Although deviations from these forms should be investigated separately in
order to get an accurate picture, a study of these patterns will give an
indication of what might happen around various architectural shapes.
example,
the flat plane on the ground is a fence;
cabination of the flat
surface is
the shed roof may be a
plane and either of the wedges,
toward the wind.
For
depending upon which
25
.50
75
/x? % n
71
Pr-e-asre
curves for an ,>finife/y /ong poAne.
The upper curve-s; o./aed w/h a p/ane placed on /he ground
The low-er curves; ob/aned wi/h a /win model of /he p/ane freely
expoied
0
25
50
75
/00%
4/
P-esure curveJ for 6n infn/e/y lon9 cy/mnde,The upper curves, ob/aired w/h a semicy//rder p/aced on 4he ground
The /ower curves, ob/a/ned w/h a
whole cy/inder freely expofsed.
5
0
25
50
2
75
f00%/?/
Pres-ure
curve.s
for an 'nfini/ely /on9 pr/sr
w/lh an pe
away fron the wmnd
he upper cur ves; ob/aine d w1h the prism p/aced on the grouind
The lower curves5 ob/ained w//h a /w/n
mode/ of the prism
freely exposed
0
Pressure
curves
an apexr towards
for an
25
75
50
in//in//ely
long
/00","l/
triangular prism
wi/h
the w/nd
The upper ctirve;, obtained with the prism p/aced on the ground
The lower c urve-s; ob/aned w/h o wh nodel o/. heprism freely
exposed
Wind around Fences
It is doubtful that fences can be used to increase the cooling effect of
the breeze, but they are often detrimental to cooling;
often necessary for privacy or security reasons,
around then should be understood.
and since they are
the behavior of the air
We will first look at the patterns
around a fence standing isolated, and then consider the influence of a
fence on the air movement around a nearby structure.
The subject of fences
seems a rather prosaic one until it is realized that it may be an indication
of the sheltering effect of any structure on another.
In an article for the House Beautiful Magazine's Climate Control Project,1
Dr. Joseph E. Howland gave illustrations of several fences, showing how
each affected the air temperature on the leeward side.
on the following page.
These are reproduced
The test procedure is not given, so no generaliza-
tions will be hazarded from this information;
however the Thermal Radiation
Laboratory of the University of California was the source,
so it
is
assumed
that a scientific procedure was used, and that the figures are comparable.
Unfortunately, this article was discovered at a late date,
so the original
source material could not be obtained in time to be included here.
Around Buildings
This discussion will be limited to the flow of air around buildings with
all windows and doors closed.
This is obviously as far from ventilati.on
as you can get, and no defense is offered for such an approach.
The only
reason for handling the subject this way is that much of the existing
literature is concerned with wind stresses on the structure of a building,
and deals with air flaw in this manner.
It is believed that the patterns
and pressures around a "solid" building will be of some value in ventilation
Herimstal
1. The cdhking
ee o uemd is proportionate
to its speed.So youM be warmer in the lee of any fence.
Scientists call this "comfort temperature." The solid
fence was a surprisingly ineicient windbreak. Maximum warmth was at distance about equal to height.
4.
&p added to top of solid fence
increases protested area in lee quite a bit. You also
get higher cemfort temperatures than with fence above.
This is due to way baMe deflets wind up in gentle
are. avoids downward. wave-like whirlpooling.
5. Trfl t
2. A aophasg
ione, tilted up away from wind,
proved better. all-around windbreak than solid fence
at left. Temperatures were not quite as high but protected area was wider. This fence would be good where
wind velocities had to he reduced but ventilation kept.
sheams deen, however, and you've
get auotner kettle of lien.
fais maxes the least effeetive windbreak of all fences tested because the louvers
bounce the wind right down into the area to be protested. This is not a "warm" fence.
.. 2..0...1
S. Shut the befe toward the wind and you get
a somewhat sumalter protected area. But note that,
close in lee of fence, you get highest temperature
readings in any fence tested: 67.5* F. Even at three
times its height behind fence, protection was noticeable.
6. Veried latha, spaced a half inch apart, made
good iwindbreak. Although temperatures were fairl
low in immediate lee, they rose steadily as you moved
away. At a distance of three times its height, this
fence gave best protection of all.
but absolutely no assurance has been given that the patterns will be the
same,
or even similar, when openings are made in the building.
Mention has been made of the tendency of moving air to create its
streamlines.
Graphic illustration of this phenomena is
photographs of the following page.
solid model,
shown in the
The pictures are made by drawing a
attached to the tground" thru a tank of water on which
aluminum powder was sprinkled.
as the model,
own
The camera was attached to the same frame
so that when a long exposure was made, the grains of powder
appear as streaks,
showing the streamlines.
The top photograph is made at a lower velocity than the bottom.
Notice
how the vortex regions are beginning to form - a small triangular area
on the windward (left) side, and a whirling eddy next to the leeward
slope of the roof.
In the lower photograph the triangular region at
the base of the building toward the wind has increased in size, and the
leeward area is greatly enlarged.
These vortex regions will not continue
to grow indefinitely, but will reach a fixed shape characteristic of the
building shape and wind velocity.
It should be noticed that the grains of powder in the forward triangle
do not show as streaks, indicating
that they remain fairly still.
This
shows the combination of low velocity and high pressure mentioned previously.
The condition on the side away from the wind is just the opposite, high
velocity and low pressure.
lip
Two and three Dimensions
Thus far, we have seen how the stream patterns vary with the shape of the
obstruction.
wind velocity.
We know that the total pressure (suction) varies with the
What we want to know is the value of the pressure at each
point of the surface of the building.
All the foregoing illustrations have shown a two dimensional stream flow
-- that is, the models would imply a building of infinite length.
The
most we could assume fram such studies would be that the pressure distribution around the center cross-section of the building would be somewhat
similar.
The next step, then, is
to examine the three-dimensional flow
around a building.
In determining the pressure on all surfaces of a building, the following
variables are involved: (1) the shape of the building;
the slightest overhang, should be considered;
(3)
all surfaces, even
(2) the wind velocity;
the angle at which the wind strikes the building;
obstructions, such as fences, trees, other buildings.
(4) nearby
We will consider
each of these variables, and cite a few generalizations that seen fairly
well-proven.
Such generalizations are very difficult to make, and are
done with same hesitancy,
because of the myriad of possible variables.
For those who wish to examine examples other than the few given here,
references 17 and 18 give the pressure distribution on rectangular
buildings with roof slopes from O0 to 600, with angles of incidence of the
wind frm 00 to 900.
Also given are graphs for open sheds, buildings
with curved roofs, as well as prisms, cylinders, pyramids, etc.
Pressure Distribution
In dealing with the four variables above, we will consider first a building
with a sloping roof and with the breeze striking it
long dimension.
perpendicular to the
Mention will be made of the effect of changing velocity.
As an illustration of difference in shape, a similar building with a flat
roof will be shown.
This same flat roof building will be used to illustrate
a change in angle of incidence of the wind.
Diagrams on the following page are for a building (model) 50x50xlO0 mm,
with a 200 roof slope.
shown.
as it
The air current is
normal to the long side, as
At the top of the page the building is
would look in
a paper pattern.
shown "flattened-out"
Each hole in the surface where
measurements were taken is shown with the maximum value recorded at that
point.
The average of maximum values for each face is shown in the box.
Each wall and roof surface is
line of holes is
lines, numbered I,
given a letter, A thru F.
given a roman numeral.
II,
horizontal sections,
III.
Notice that each
There are three cross-section
Numbers IV thru VII refer to the four
beginning at the ground.
It should be noted that the left section in the middle of the page is for
external pressures, whereas the same section on the right of the page shows
the difference between external and internal pressures.
Both diagrams at
the bottan of the page are for pressure difference.
Consider the section showing external pressure.
There is pressure on the
windward wall, and all other surfaces show a suction.
On the windward
wall, pressure is greatest at the middle of the building (curve II),
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dropping off toward the two sides.
The greatest suction is developed at
the leading edge of the roof.
The cross section showing difference in pressure has much the same shape
curves, but the windward pressure is greater, roof suction less, and at
the leeward wall suction has changed to slight pressure.
Horizontal sections show a rather uniform pressure difference on the
windward wall, but on the side wvlls the greatest suction is near the
leading edge.
Change in Shape
It has been found that roof slopes up to 450 will show curves similar
to the ones just illustrated.
Above 450, however, suction will change
to pressure, meaning that the roof is
now acting more as a sloping wall.
The next illustration shows a building of the same dimensions, wind still
at 900, but this building has a flat roof.
Curves for wall pressures are
very similar to those of the preceding example, with same shift in the
position of the maximum suction on the side walls.
The roof is
all in
a
saction area, and test results indicate that this will always be the case,
regardless of the height of the building.
Proportionally wide flat-
roofed buildings may show that part of the roof away from the wind to be
in a pressure area.
Change in Windle
Now let us consider the same flat roof building, but with the wind striking
it at 300 instead of 900.
torted.
As might be expected, all curves are now dis-
The lower right hand diagram (sections at right angles to the
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roof) is particularly interesting.
eave;
Curve VIII is for a line near the
curve IX is for a line parallel to it, nearer the center. From
these curves we can see that near the center of the roof on the right
side there will be a point of slight pressure.
Nearby will be a point of
rather great suction.
When we lot the wind strike the building from the end (angle of incidence
of 00), we are in for some surprises.
tribution.
The following page shows the dis-
As would be expected, the long walls now have an external
suction instead of a pressure.
But look at the horizontal section of
differences in the internal and external pressures.
We find that the
leeward wall, formerly indicating suction, now is under pressure.
The
long "side" walls show a suction toward the wind, uniformly decreasing
until a pressure is developed toward the leeward end.
The Sheltering Effect
In the first part of this section are references to the pressure distribution on a flat plane, and to the influence of various fences on air
temperature.
Now we want to consider how an obstruction on the windward
side of a building effects the pressure on that building.
An attempt to
generalize on this subject was made by Irminger and Nokkentved by experiments with solid and perforated screens in the wind tunnel.
Several
screens of varying heights were placed in front of models and pressures
on the model were recorded.
The screens were moved farther and farther
from the building, so that the effect of distance could be shown.
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These experiments were made with the screens extending completely across
the tunnel, and plates were affixed to each side of the model, so the results are for a two-dimensional stream flow only.
The authors are aware
of the limitations of these results, and emphasis is given to the need
for further studies.
However, the figures will be accurate for a center
cross section through a building, and will positively show how the sheltering effect changes with the height of the screen and its distance from
the building.
The following illustration shows both a solid and perforated screen of
height h equal to the height of the building, at a distance a from the
windward face of the building.
The section is through the center of the
building, and 14 numbered points are shown at which pressure was recorded.
Charts beneath the sections show the magnitude of pressure or suction for
these points.
Five curves are shown:
curve 0 is without any screen, and
curves 1 through 4 are for increasing distances from the building (indicated as ratios in the lower right of the page).
Pressures on the windward face of the building sheltered by a solid screen
are pretty much as would be expected.
Where there was great pressure with-
out the screen, there is a large suction when the screen is close to the
building.
This suction decreases more or less uniformly as the screen
is moved farther and farther from the building--apparently approaching the
condition without the wall.
On the roof the situation is not quite the same.
With the screen near,
suction on the roof increases over that with no screen (curve 0); as the
Building h- b
Solid screen
slope of roof 00
Height-h
-too
Perforated
screen
,
Height =h
44
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.
I
14
Ve.t
0 without screen
I
-
6
2
3
4
0
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-
loo
.8.
*1t
screen is moved away, the suction drops below curve 0 and continues to
drop.
It might be assumed that when the screen is somewhere beyond 11
times the height of the building (curve 4) a minimum value would be
reached, and then the suction curves would climb back to the original
value.
There is no positive indication that this would be so, however.
On the leeward wall, the influence of the screen is relatively small,
except in the case where the screen is very close to the building.
There are two particularly significant observations to be made about the
influence of a perforated screen.
4 and 5.
First, notice the curves between points
Where most of the windward wall is under pressure, here at the
eave line there is a sudden change to suction.
Experiments with other
building shapes and other screen heights show that this phenomenon occurs
near any edge where one surface has a positive pressure and the adjacent
surface has a negative pressure.
In the case of a steeply sloping roof
where the windward roof surface is under pressure, this curious happening
takes place immediately next to the ridge.
The second observation about the perforated screen is the fact that curves
1 through 4 are grouped together.
When the screen is first placed in front
of the building, there is a sharp change in pressures, but as the screen
is moved farther away, the curves remain parallel and close together.
Several general observations can be made about these test results for solid
and perforated screens.
In the case of the solid screen, what is taking
place is that the building falls completely within the rear vortex region
of the screen, creating suction on all sides.
So long as the building
under consideration continues to be completely enveloped by this vortex
region, the screen could be assumed to be the windward face of another
relatively thin building.
In the case of the perforated screen, the leeward side is not a rear vortex region, but a region of reduced velocity.
"Naturally, the sheltering
influence just behind the screen is much greater for a solid than for a
perforated screen, but the flee' behind a solid screen soon disappears and
is followed by a very disturbed vortex region.
Behind the perforated screen
the sheltering influence is less, but it is very constant for a long distance--at least 8 times the height of the screen."
As the solid screen
can be taken for a building (with limitations), so can the perforated
screen be likened to planting.
Trees and shrubs will have much the same
influence.
It will be noticed that the curve for a screen distance of 11 times the
building height (curve 4) still does not equal the results where no screen
was used.
The authors explain that their wind tunnel did not allow them
to make experiments at greater screen-to-building distances.
Therefore,
while it is obvious that the influence of either screen is felt at a
greater distance than llh, and it is expected that the influence of the
perforated screen will be felt for the greatest distance, we do not know
at this time what the maximum values are.
Other investigators have indi-
cated that the sheltering effect of trees may extend to 20 times their
height.
Summary
As the necessary preface to the study of airflow around buildings, some
of the general principles of aerodynamics should be known.
One of the
most significant of these principles is that of Bernoulli.
It will be
shown later how the increase of air velocity by directing it through an
opening of decreasing size can be of vital importance to cooling by natural ventilation.
The general procedure for interpreting smokestream
patterns was shown also--how vortex regions are formed, and the importance of them.
As a body "streamlines itself", the forward vortex region
becomes an area of high velocity and low pressure.
The reason for such a lengthy discussion of these pressure areas, and
subsequently the more precise pressure distribution, is that this
pressure distribution is one of the few opportunities at our disposal
for coaxing air through a building.
As was shown in a preceding section
of this study, air will move from a high pressure area toward a low
pressure area.
If we can determine with sufficient accuracy where the
high and low pressure areas exist around a building, we can locate openings to take advantage of the pressure difference.
It should be remembered
that the examples shown were for buildings practically closed.
hone of the
literature examined gave any evidence for buildings with normal openings.
Irminger and Nokkentved show
that when one complete side, or one com-
plete gable, of a curved-roof building is removed, the external pressures
on the other surfaces do not change.
They also show that when the only
opening is a ridge ventilator, a powerful suction develops at the ventilator.
But these cases of open side and ventilator are so far removed from
the usual conditions of building occupancy, no particular importance can
be placed upon them now.
This does not mean that a knowledge of pressure areas gained in this way
is valueless.
On the contrary, a great many buildings exist which show
proof that when openings are placed in the high and low pressure areas
shown on the preceding charts, air will travel between these openings.
However, there seems to be no practical method at the moment for predicting the velocity and pattern of air movement through a building for any
given set of conditions.
We can give assurance that by using pressure
patterns we can get more air movement than by opposing them.
The sheltering effect on a building, by placing a solid or perforated
screen on the windward wide, was illustrated.
viously mentioned is the change with velocity.
Another variable not preSince the sheltering effect
is governed largely by the leeward vortex region on the screen, and since
this vortex region varies with velocity, it is obvious that velocity must
enter any design consideration of the effect.
We found that the solid screen has the stronger influence on the building,
but that the influence of the perforated screen is more constant as the
distance from the building is increased.
Also, it is probable that the
perforated screen will effect the building from a greater distance.
PART IV
AIRFLOW THROUGH BUILDINGS
In this section an attempt will be made to correlate the bits of information in the preceding sections.
As has been done throughout this
report, general statements will be made as much as possible, rather
than specific ones for certain buildings.
It should be remembered,
however, that it was necessary to limit the study to small buildings
(one or two stories), and examples will be taken from this category.
We must have a clear picture of just what we are trying to do with
this natural ventilation, so certain objectives will be outlined first.
It will then be shown that there are two forces at our disposal for
controlling air currents--temperature difference and the wind.
Next
will be a discussion of how the shape of the building effects these
forces.
Finally, a few of the big unknowns will be mentioned, with
some recommendations for further study.
Objectives.
(1) Currents at body level
As was stated in the first part of this study, we are not interested
in cooling buildings--for the sake of cooling buildings.
We are only
interested in cooling buildings if this results in also cooling people.
It is possible to reduce the temperature of building surfaces by natural ventilation, and this is a very important aspect of the problem;
but it is not the most important aspect.
Our primary concern is to
get air movement around the human body.
The physiological data in the first section assumes that the velocities
mentioned are striking the body, and here "a miss is as good as a
mile."
Perhaps others have had the experience of sleeping on a low
bed in a bedroom where window sills are two or three feet above the
In such a case, the air velocity may be sufficient
level of the bed.
to hold the drapes away from the wall, but there is little benefit
from this breeze at the level of the bed.
This is an indication that one of the currently popular techniques of
design may be questioned.
This is the practice of placing low windows
on the windward wall for the entry of the air, and using a high clearstory on the other side to exhaust the air.
In cross section a smooth
arrow is drawn through the window, diagonally across the room, and out
the clearstory.
It is also questionable whether the air will take this
route, but for the moment let's assume that it does.
room divided into two triangles by the air current.
We now have the
If the bed is
placed near the window wall, maximum cooling should be derived from
the air, the sleeper receiving a direct blast.
however, if the bed is
placed against the wall opposite the window, this air current will be
moving high overhead, and the cooling benefits seriously reduced.
Obviously, the turbulence created by the air stream will be of some
benefit, so cross ventilation is better than no cross ventilation. The
point is that the air stream should be at body level wherever possible.
(2)
Reduction of radiant temperatures
By directing air across the body, more heat will be lost by convection
and conduction.
The Pierce Laboratory experiments mentioned previously
have shown that radiation must also be considered, so we must look for
some means of reducing radiant temperatures by natural air circulation.
The technique is to lower wall (ceiling, floor) temperature by moving
Actually the wall will be losing its
cooler air pest the surface.
heat by convection, but the fact that the temperature is lowered means
that the radiant temperature is lowered also.
(3)
Maintenance of air temperature
This third reason for moving air through buildings has been called
"maintenance" of air temperature because the goal is the lowest possible temperature of the air, and no method is now known to reduce air
temperature by air movement alone.
In other words, the objective is
to prohibit increase over the natural air temperature.
The entering air may be heated by the walls, which have gained their
heat from solar radiation.
It may be heated by people, who must always
be giving off heat in some manner.
And of course it may be heated, he
many mechanical devices found in buildings, such as lights, motors,
kitchen ranges, water heaters, etc.
Therefore, if the entering air
is allowed to remain inside the building for extended periods, it will
probably become hotter than the outside air.
This hot, stagnant air
will, of course, be found at the ceiling, where it will transmit its
heat to the surfaces in contact with it, and will gradually heat the
lower layers of air.
emerature-difference forces
To quote the ASHVE Guide, "The stack effect produced within a building
when the outdoor temperature is lower than the indoor temperature is
F
due to the difference in weight of the warm column of air within the
building and cooler air outside.
The flow due to stack effect is
proportional to the square root of the draft head, or approximately:
Q = 9.4 A
h (t-t.)
where
Q equals air flow, cubic feet per minute.
A
h
t
t
9.40
"
"
"
"
free area of inlets or outlets (assumed equal),
square feet.
Height from inlets to outlets, feet.
average temperature of indoor air in height h, OF.
temperature of outdoor air, OF.
constant of proportionality, including a value of
65% for effectiveness of openings. This should be
reduced to 50% (constant equals 7.2) if conditions
are not favorable."
We have, then, these variables: (1) area of openings, (2)
vertical
distance between openings, (3) effectiveness of openings, and (4) the
difference in outdoor and indoor air temperatures.
The best use can
be made of the temperature-difference force in the design of industrial buildings where the processes involved produce large amounts of
heat.
The temperature difference will, of necessity, be rather high,
and the vertical distance between openings is often great.
With regard to the combined effect of temperature-difference forces and
wind forces, the ASHVE Guide states further, "When the two heads are
about equal in value and the ventilating openings are operated so as
to coordinate them, the total air flow through the building is about
10% greater than that produced by either head acting independently under conditions ideal to it.
This percentage decreases rapidly as one
head increases over the other and the larger will predominate."
relationship is shown as a chart on the following page.
This
40
1 oll
30
---
z
az 20
CL)
- -
10
5
4
3
2
RATIO OF OUTLET TO INLET OR VICE-VERSA
1
6
INCREASE IN FLOW CAUSED BY EXCESS OF ONE OPENING OVER ANOTHER
7
6
CL
0
2
0
20
40
6o
so
FLOWDUETOTEMPERATURE
DIFFERENCE
AS PERCENTOFTOTAL
0
DETERMINATION OF FLOW CAUSED BY COMBINED FORCES OF
WIND
AND TEMPERATURE DIFFERENCE
In most other buildings, where the heat being produced in the building
is small, and the ceiling heights are not great, the advantages of this
force is relatively insignificant.
As was shown previously, it is de-
sirable to have no temperature difference, and it is obvious that as
this value in the formula approaches zero, the temperature-difference
force will become no force.
Wind forces
In considering the natural wind force available for moving air through
a building, four variables must be taken into account:
(1) average
wind velocity, (2) prevailing wind direction, (3) variations in velocity and direction, and (4) local interference by trees, buildings,
hills, and other obstructions.
It is a rare case when an architect
can have recordings of wind velocity and direction for a specific
building site.
The number of buildings that can be placed on the sites
of existing post offices and airport terminals is relatively small (and
even in these cases, it is doubtful that the architect will have any
usable information for anything but the roof).
However, the recent
"Regional Climate Analysis" being published in the A.T.A. Bulletin and
in House Beautiful magazine will give much valuable information regarding the seasonal and daily averages for the first three variables.
Data, such as that given in this report, on the sheltering effect of
obstructions will be of assistance in allowing for the fourth variable.
As an indication of the range of velocities under consideration, a
quick check of a table of average wind velocities showed that a maximum of 12.0 mph is found at Ft. Smith, Arkansas, and minimum averages
are found at Birmingham, Alabama and Washington, D. C. of 5.2 mph.
The ASHV
offers the following relationships as a means of arriving at
the quantitative value of wind forces.
Q
where:
Q, equals
"
A
"
V
"
E
- EAT
air flow, cubic feet per minute.
free area of inlet openings, square feet.
wind velocity, feet per minute, (mph x 88).
effectiveness of openings. (0.50 to 0.60 for
perpendicular winds, 0.25 to 0.35 for diagonal
winds)
This formula assumes that inlets are facing the prevailing wind and
outlets are in the low pressure areas of the building.
It is cited
here as a means of getting a first approximation of the quantitative
value, and since no other evidence along this line could be found, it
must also be cited as a last approximation.
From the information given
in this report, one could determine whether a given condition would
produce a greater or lesser air flow, but how much is not known.
Shape of the building
On the following page will be seen a wind tunnel smoke test of a building model.
It is necessary first to cast some doubt on the validity
of this test, since there is apparently no "ground."
However, it is
believed that the addition of a ground plate would influence the magnitude more than the pattern on the air flow.
There are two interest-
ing features about this experiment--the influence of the leading wall,
and the ratio of inlet to outlet.
In the tests of solid buildings discussed in the preceding section, it
WIND
DIRECTION
A%
-...
wMMMMJ d
was shown that the windward wall is all under pressure, and that relatively flat roofs are all in a suction area.
In this case, apparently
the streamlines is tangent to the "sill" of the opening rather than the
eave, contributing to the high velocities (converging streamlines)
through the opening and across the roof.
The other, and perhaps larger
contribution to the high velocity through the opening is due to the Venturi action.
This is because the inlet area is smaller than the outlet
area.
The Venturi effect has been mentioned a number of times in this report,
but no quantitative value for it has been given.
On the preceding page
of illustrations is shown a graph that can be used for calculation.
Notice that the curve levels off at something less than 40% when the
ratio of inlet to outlet (or vice versa) reaches about 5.
From this
it appears that the extreme example of a small slit for an opening on
the windward side, and no wall on the leeward would not give the terrific velocities that might have been expected.
On the following page are shovm two more illustrations of how this
effect might be used.
It is interesting to note that in common prac-
tice the adjustment of openings on one wall will be all that is necessary.
The air wash
Mention has been made of the importance of getting air movement at
body level.
Considering body height when sitting, this will mean that
openings should generally be low in the wall.
But we have also mentioned
BREZI
the detrimental effects of stagnant air near the ceiling.
To move this
air we mast have openings near the ceiling.. When there is an air cur-
rent, the hot air will be swept away; when there is little or no air
movement, these openings will allow the hot air to drift out as it
rises.
The double-hung window suits the above requirements surprising-
ly well.
Another important use of the air wash idea is to circulate air inside
a hollow wall or roof.
The attic fan is based on this principle.
Of
course the reason for trying to circulate air through the air space in
a wall is to reduce the radiant temperature.
The true values of radiant
temperatures of a wall are not well understood.
Mackey and Wright con-
ducted experiments with various solid walls to determine the influence
of "time lag."
There is not time to go into a discussion of these
findings at any length, but generally it was shown that this time lag
varied with the material used and the orientation of the wall.
The
total amount of heat transmitted over a 24 hour period was about the
same for a 16" brick wall and a 1" cellular glass wall.
Hourly chan-
ges in temperature for the brick wall were relatively slight as compared
to the glass wall.
For example, compare the two walls located on the
west side of a building.
In one experiment, the highest temperature
for the brick wall was 87.50 at 3 a.m., lowest 860 at 3 p.m.
Highest
temperature for the cellular glass wall was 93.50 at 3 p.m., lowest
830 at 4 a.m.
The times of maximum and minimum are almost exactly re-
versed, and the values widely divergent.
No similar studies for hollow walls came to light, but it is assumed
that the pattern will be similar.
Mention is made of it here in the
hope that the time of maximum heat for such walls might be coordinated
with the times of maximum air movement, and produce a significant lowering of the heat radiated into a room.
The unknowns
Throughout this report there have been references to the inadequacies
of present knowledge.
categories;
The more important ones seem to fall into three
(1) the influence of radiation on the feeling of comfort,
(2) the natural behavior of moving air, and (3) the influence of architectural shapes on air flow at low velocities.
Qu.ite
possibly there
are a number of individuals and agencies working on these problems. In
the first category, it appears that the Pierce laboratories and the
ASHVE laboratories may be expected to find a method of taking radiation
into account.
As for the natural behavior of moving air, great strides have been made
by those connected with the House Beautiful Climate Control Project,
and it is expected that the consulting metiorologist will enter the long
list of consultants now working together to produce buildings.
The Texas Engineering Experiment Station is known to be conducting ex-
periments, under the direction of Mr. W. W. Caudill, of the influence
of architectural shapes on air movement at low velocities.
Any contributions to any of these three fields will be welcomed by
those who must design for the wind.
Perhaps it will not be too strange
that the strongest recommendation is for a continuation of the study
at hand.
Correlation of the information from the various fields in-
volved in cooling by natural ventilation, of which this report is a
beginning, should be extended in an attempt to find quantitative values
for the many variables cited.
Searchers can be assured of a most in-
teresting and instructive experience.
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