INDUSTRIAL
AERODYNAMICS
Mr.B.Navin Kumar
Senior Lecturer
Rajalakshmi Engineering College
Thandalam-602 105
UNIT - I
ATMOSPHERIC
BOUNDARY LAYER
ATMOSPHERIC CIRCULATION
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Atmospheric circulation is the large-scale movement of air, and the means (together with the smaller
ocean circulation) by which thermal energy is distributed on the surface of the Earth.
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The large-scale structure of the atmospheric circulation varies from year to year, but the basic
climatological structure remains fairly constant. However, individual weather systems - mid-latitude
depressions, or tropical convective cells - occur "randomly and it is accepted that weather cannot be
predicted beyond a fairly short limit: perhaps a month in theory, or (currently) about ten days in
practice (see Chaos theory and Butterfly effect). Nonetheless, as the climate is the average of these
systems and patterns - where and when they tend to occur again and again -, it is stable over longer
periods of time.
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As a rule, the "cells" of Earth's atmosphere shift polewards in warmer climates (e.g. interglacials
compared to glacial), but remain largely constant even due to continental drift. Tectonic uplift can
significantly alter major elements of it, however - for example the jet stream -, and plate tectonics shift
ocean currents. In the extremely hot climates of the Mesozoic, indications of a third desert belt at the
Equator has been found; it was perhaps caused by convection. But even then, the overall latitudinal
pattern of Earth's climate was not much different from the one today.
LOCAL WINDS
Local winds blow over a much smaller area than global
winds and have a much shorter time span. Hot windsoriginate
in vast anticyclones over hot deserts and include the Santa
Ana (California), the Brickfielder (south-east Australia), the
Sirocco (Mediterranean), the Haboob (Sudan), the Khamsin
(Egypt), and the Harmattan (West Africa).
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The causes of local winds are because of tempreture and air
pressure. The greater one of these are, the stronger the winds
are.
A local wind is a little zephyr that one can get occasionally. A
global wind would be the jet streams.
TERRAIN TYPESTerrain Type
Modifier
Forest
3
Jungle
4
SandDesert
2
RockDesert
1.5
Plains
1.5
Tundra
2
Icesheet
2
Hills
3
Mountains
5
HighMountains
6
Swamp
5
Bog
4
City
1.3
Ocean
2
Lake
1
River
1.5
PavedRoad
1
DirtRoad
1.3
MEAN VELOCITY DISTRIBUTION
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The mean velocity distribution in a low-speed three-dimensional
turbulent boundary-layer flow was investigated experimentally. The
experiments were performed on a large-scale model which consisted of
a flat plate on which secondary flow was generated by the pressure
field introduced by a circular cylinder standing on the plate. The
Reynolds number based on distance from the leading edge of the plate
was about 6 x 106.
It was found that the wall-wake model of Coles does not apply for flow
of this kind and the model breaks down in the case of conically
divergent flow with rising pressure, for example, in the results of
Kehl (1943). The triangular model for the yawed turbulent boundary
layer proposed by Johnston (1960) was confirmed with good
correlation. However, the value of yuτ/v which occurs at the vertex of
the triangle was found to range up to 150 whereas Johnston gives the
highest value as about 16 and hence assumes that the peak lies
within the viscous sublayer. Much of his analysis is based on this
assumption.
The dimensionless velocity-defect profile was found to lie in a fairly
narrow band when plotted against y/δ for a wide variation of other
parameters including the pressure gradient. The law of the wall was
found to apply in the same form as for two-dimensional flow but for a
more limited range of y.
POWER LAW AND LOGARITHM LAW
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Employing the Shannon entropy, this note derives the
well-known power law and the Prandt–von Kármán
universal (or logarithmic) velocity distribution equations
for open channel flow. The Shannon entropy yields
probability distributions underlying these velocity
equations. With the use of this entropy, one obtains an
expression for the power law exponent in physically
measurable quantities (surface flow velocity and average
logarithmic velocity) and an expression for shear flow
depth in flow depth, surface flow velocity and shear
velocity, thus obviating the need for fitting.
WIND SPEEDS-TURBULENCE
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Atmospheric turbulence causes vertical mixing of the
atmosphere. it can be caused by convection (heating at the
surface causing warm air parcels to lift) or by friction with
objects on the earth's surface and shear stress between layers
of the atmosphere moving at different wind speeds.
Since the atmosphere is mixing, the temperature will be more
or less constant with height (or at least closer to constant than
it would be without turbulence).
not sure what to tell you for wind speed, but in general the
wind speed increases logarithmically with height due to
friction with the earth's surface. Shear between layers with
different wind speeds in the atmosphere causes turbulence
ROUGHNESS PARAMETERS
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Ra: Ra is the arithmetic average of the absolute values of the roughness profile
ordinates. Also known as Arithmetic Average (AA), Center Line Average (CLA).
The average roughness is the area between the roughness profile and its mean
line, or the integral of the absolute value of the roughness profile height over the
evaluation length
Rz: Rz is the arithmetic mean value of the single roughness depths of
consecutive sampling lengths. Z is the sum of the height of the highest peaks and
the lowest valley depth within a sampling length.
Cutoff λc: of a profile filter determines which wavelengths belong to roughness
and which ones to waviness.
Sampling Length: is the reference for roughness evaluation. Its length is equal to
the cutoff wavelength.
Traversing Length: is the overall length traveled by the stylus when acquiring
the traced profile. It is the total of Pre-travel, evaluation length and post travel
Evaluation Length: is the part of the traversing length from where the values of
the surface parameters are determined.
Pre-Travel: the first part of the traversing length.
Post-Travel: The last part of the traversing length
Wind tunnel Techniques
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Aerodynamicists use wind tunnels to test models of proposed aircraft and engine components. During a test, the
model is placed in the test section of the tunnel and air is made to flow past the model. Various types of
instrumentation are used to determine the forces on the model. There are four main types of wind tunnel tests.
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In some wind tunnel tests, the aerodynamic forces and moments on the model are measured directly. The model
is mounted in the tunnel on a special machine called a force balance. The output from the balance is a signal that
is related to the forces and moments on the model. Balances can be used to measure both the lift and drag forces.
The balance must be calibrated against a known value of the force before, and sometimes during, the test. Force
measurements usually require some data reduction or post-test processing to account for Reynolds number or
Mach number effects on the model during testing. It is very important in data reports to always specify the
reference value of variables used in data reduction.
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In some wind tunnel tests, the model is instrumented with pressure taps and the component performance is
calculated from the pressure data. Total pressure measurement is the normal procedure for determining aircraft
inlet performance. Theoretically, the aerodynamic force on an aircraft model could be obtained using pressure
instrumentation by integrating the pressure times an incremental area around the entire surface of the model. But,
in practice, pressure integration is not used because of the large number of taps necessary to accurately resolve
pressure variations. Airfoil drag can be determined by integrating the total pressure deficit in the wake created by
a wing model.
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In some wind tunnel tests, the model is instrumented to provide diagnostic information about the flow of air
around the model. Diagnostic instrumentation includes static pressure taps, total pressure rakes, laser Doppler
velocimetry, and hot-wire velocity probes. A diagnostic test does not provide overall aircraft performance, but
helps the engineer to better understand how the fluid moves around and through the model. There are a variety of
flow control devices that are employed to improve performance of the aircraft, if the local flow conditions are
known. Depending on the type of instrumentation used in the experiment, steady state flow or unsteady, timevarying, flow information can be obtained. The engineer must use some experience when employing flow
diagnostic instrumentation to properly place the instruments in regions of flow gradients or separations.
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In some wind tunnel tests, flow visualization techniques are used to provide diagnostic information.
Visualizaation techniques include free stream smoke, laser sheet, or surface oil flow. The assumption is made
that the flow visualization medium moves exactly with the flow. Shadowgraphs or schlierin systems are used to
visualize the shape and location of shock waves in compressible flows. For low speed flows, tufts or surface oil
indicate the flow direction along the surface of a model.
UNIT - II
BLUFF BODY
AERODYNAMICS
BOUNDARY LAYERS AND SEPARATION
In most situations it is inevitable that the boundary layer becomes
detached from a solid body. This boundary layer separation results in a
large increase in the drag on the body. We can understand this by
returning to the flow of a nonviscous fluid around a cylinder. The
pressure distribution is the same on the downstream side of the cylinder
as on the upstream side; thus, there were no unbalanced forces on the
cylinder and therefore no drag (d'Alembert's paradox again). If the flow
of a viscous fluid about a body is such that the boundary layer remains
attached, then we have almost the same result--we'll just have a small
drag due to the skin friction. However, if the boundary layer separates
from the cylinder, then the pressure on the downstream side of the
cylinder is essentially constant, and equal to the low pressure on the top
and bottom points of the cylinder. This pressure is much lower than the
large pressure which occurs at the stagnation point on the upstream side
of the cylinder, leading to a pressure imbalance and a large pressure
drag on the cylinder. For instance, for a cylinder in a flow with a
Reynolds number in the range , the boundary layer separates and the
coefficient of drag is , much larger that the coefficient of drag due to
skin friction, which we would estimate to be about .
TWO DIMENSIONAL WAKE AND VORTEX
FORMATION
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A cylinder having mild variations in diameter along its span is subjected to controlled
excitation at frequencies above and below the inherent shedding frequency from the
corresponding two-dimensional cylinder. The response of the near wake is
characterized in terms of timeline visualization and velocity traces, spectra, and phase
plane representations. It is possible to generate several types of vortex formation,
depending upon the excitation frequency. Globally locked-in, three-dimensional
vortex formation can occur along the entire span of the flow. Regions of locally
locked-in and period-doubled vortex formation can exist along different portions of
the span provided the excitation frequency is properly tuned. Unlike the classical
subharmonic instability in free shear flows, the occurrence of period-doubled vortex
formation does not involve vortex coalescence; instead, the flow structure alternates
between two different states.
An experimental study of the flow around a cylinder with a single straight perturbation
was conducted in a wind tunnel. With this bluff body, positioned in a uniform
crossflow, the vortex shedding frequency and other flow characteristics could be
manipulated. The Strouhal number has been shown to be a function of the perturbation
angular position, theta _{rm p}, as well as the perturbation size and Reynolds number.
As much as a 50% change in Strouhal number could be achieved, simply by changing
theta _{rm p} by 1^ circ. The perturbation size compared to the boundary-layer
thickness, delta, was varied from approximately 1delta to about 20delta. The Reynolds
number was varied from 10,000 to 40,000. A detailed investigation of the
characteristic Strouhal number variation has shown that varying theta_{rm p} had a
significant influence on the boundary -layer separation and transition to turbulence.
The Strouhal number St is a function of the Reynolds
number Re (although a sufficiently varying one that it may be
said that it is typically equal to 0.2, e.g. see figure below) and is
proportional to the reciprocal of vortex spacing expressed as a
number of obstacle diameter. It is used in the momentum
transfer in general, and in both Von Karmann vortex streets and
unsteady flow calculations in particular. It is normally defined in
the following form :
where : - n is the frequency of the observed
phenomenon,
- d is the characteristic length (which is the diameter of the
cylinder in the case of vortex streets),
- U is the velocity of the fluid.
POWER REQUIREMENTS AND DRAG
COEFFICIENTS OF AUTOMOBILES-
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The number of cars available on our planet is continuously increasing. But also
other factors are important for the emissions and the energy consumption: How
efficient is the motor of the car? How much fuel does it consume on a certain
distance? How long are the distances the car owner goes per year in average? What
driving style does the driver exhibit? What is the average speed?
The energy needed for optimal consumption of a car is not easy to calculate. The
most simple physical equation for the description of accelerating a body of the mass
m from the velocity v = 0 to the velocity v is the kinetic energy equation:
The energy required to accelerate a body increases with the mass of the body and
with the square of the velocity gained. We can deduce this formula from four simple
physical expressions:
a) work / energy = force × distance (E = F × s)
b) force = mass × acceleration (F = m × a)
c) acceleration = change of velocity with time (a = dv/dt)
d) velocity = change of distance with time (v = ds/dt)
If we change the velocity from 0 to v the energy invested over the distance s is given
according to:
The SI unit of an energy is 1 Joule [J] according to:
If the velocity of a mass is increasing continuously in time, which means 1 m/s after
1 second, 2 m/s after 2 seconds, we have a constant acceleration of 1 meter per
second squared. The force needed for this acceleration depends on the mass itself. If
the mass is 1 kg the required force is 1 Newton (N) = 1 kg × 1 m/s2. The total energy
required or the work we have to carry out depends on over which distance we
exacerbate the accelerating force. If it is one meter, the energy is 1 kg m 2/s2 = 1
Joule.
TRAIN AERODYNAMICS
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Railway train aerodynamic problems are closely associated with the
flows occurring around train. Much effort to speed up the train
system has to date been paid on the improvement of electric motor
power rather than understanding the flow around the train. This has
led to larger energy losses and performance deterioration of the train
system, since the flows around train are more disturbed due to
turbulence of the increased speed of the train, and consequently the
flow energies are converted to aerodynamic drag, noise and
vibrations. With the speed-up of train, many engineering problems
which have been neglected at low train speeds, are being raised with
regard to aerodynamic noise and vibrations, impulse forces
occurring as two trains intersect each other, impulse wave at the exit
of tunnel, ear discomfort of passengers inside train, etc. These are of
major limitation factors to the speed-up of train system. The present
review addresses the state of the art on the aerodynamic and aero
acoustic problems of high-speed railway train and highlights proper
control strategies to alleviate undesirable aerodynamic problems of
high-speed railway train system.
UNIT - III
WIND ENERGY
COLLECTORS
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Horizontal Axis Wind Turbines
Most of the technology described on these pages is related to horizontal axis wind
turbines (HAWTs, as some people like to call them).
The reason is simple: All grid-connected commercial wind turbines today are built
with a propeller-type rotor on a horizontal axis (i.e. a horizontal main shaft).
The purpose of the rotor, of course, is to convert the linear motion of the wind into
rotational energy that can be used to drive a generator. The same basic principle is
used in a modern water turbine, where the flow of water is parallel to the rotational
axis of the turbine blades.
Vertical Axis Wind Turbines Eole C, a 4200 kW Vertical axis Darrieus wind
turbine with 100 m rotor diameter at Cap Chat, Québec, Canada. The machine (which
is the world's largest wind turbine) is no longer operational.
Vertical axis wind turbines (VAWTs as some people call them) are a bit like water
wheels in that sense. (Some vertical axis turbine types could actually work with a
horizontal axis as well, but they would hardly be able to beat the efficiency of a
propeller-type turbine).
The only vertical axis turbine which has ever been manufactured commercially at any
volume is the Darrieus machine, named after the French engineer Georges Darrieus
who patented the design in 1931. (It was manufactured by the U.S. company FloWind
which went bankrupt in 1997). The Darrieus machine is characterised by its C-shaped
rotor blades which make it look a bit like an eggbeater. It is normally built with two
or three blades
The Power Coefficient
Very simply, we just divide the electrical power output
by the wind energy input to measure how technically
efficient a wind turbine is. In other words, we take the
power curve , and divide it by the area of the rotor to get
the power output per square metre of rotor area. For each
wind speed, we then divide the result by the amount of
power in the wind per square metre.
The graph shows a power coefficient curve for a typical
Danish wind turbine. Although the average efficiency for
these turbines is somewhat above 20 per cent, the
efficiency varies very much with the wind speed. (If there
are small kinks in the curve, they are usually due to
measurement errors).
 As you can see, the mechanical efficiency of the turbine
is largest (in this case 44 per cent) at a wind speed around
some 9 m/s. This is a deliberate choice by the engineers
who designed the turbine. At low wind speeds efficiency
is not so important, because there is not much energy to
harvest. At high wind speeds the turbine must waste any
excess energy above what the generator was designed for.
Efficiency therefore matters most in the region of wind
speeds where most of the energy is to be found.
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BETZ COEFFICIENT
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Betz was able to develop an expression for Cp in terms of the induction factors. This
is done by the velocity relations being substituted into power and power is substituted
into the coefficient of power definition. The relationship Betz developed is given
below:
Cp = 4a(1 − a)2
The Betz limit is defined by the maximum value that can be given by the above
formula. This is found by taking the derivative with respect to the axial induction
factor, setting it to zero and solving for the axial induction factor. Betz was able to
show that the optimum axial induction factor is one third. The optimum axial
induction factor was then used to find the maximum coefficient of power. This
maximum coefficient is the Betz limit. Betz was able to show that the maximum
coefficient of power of a wind turbine is 16/27. Airflow operating at higher thrust will
cause the axial induction factor to rise above the optimum value. Higher thrust cause
more air to be deflected away from the turbine. When the axial induction factor falls
below the optimum value the wind turbine is not extracting all the energy it can. This
reduces pressure around the turbine and allows more air to pass through the turbine,
but not enough to account for lack of energy being extracted.
The derivation of the Betz limit shows a simple analysis of wind turbine
aerodynamics. In reality there is a lot more. A more rigorous analysis would include
wake rotation, the effect of variable geometry. The effect of air foils on the flow is a
major component of wind turbine aerodynamics. Within airfoils alone, the wind
turbine aerodynamicist has to consider the effect of surface roughness, dynamic stall
tip losses, solidity, among other problems.
UNIT IV
BUILDING AERODYNAMICS
PRESSURE DISTRIBUTION ON LOW RISE
BUILDINGS
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the conditionally sampled actual wind pressure
distributions causing maximum quasi-static wind load
effects at the base of low-rise building models with
square and rectangular plans. The maximum normal
stresses in column members were also examined to
discuss the wind load combination of the along-wind,
across-wind, uplift, and three moments. Then, it
examines the actual wind pressure distributions causing
the maximum quasi-static stresses in structural frames.
These are compared with Kasperski’s load-responsecorrelation formula and the quasi-steady pressure
distributions.
Wind loads on buildings
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The design of buildings must account for wind loads, and these are
affected by wind shear. For engineering purposes, a power law wind
speed profile may be defined as follows:
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Vz = speed of the wind at height ;Vg = gradient wind at gradient
height = exponential coefficient
Typically, buildings are designed to resist a strong wind with a very
long return period, such as 50 years or more. The design wind speed is
determined from historical records using Extreme value theory to
predict future extreme wind speeds.
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Building code, or building control, is a set of rules that specify the
minimum acceptable level of safety for constructed objects such as
buildings and nonbuilding structures. The main purpose of building
codes are to protect public health, safety and general welfare as they
relate to the construction and occupancy of buildings and structures.
The building code becomes law of a particular jurisdiction when
formally enacted by the appropriate authority.
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Building codes are generally intended to be applied by architects and
engineers although this is not the case in the UK where Building
Control Surveyors act as verifiers both in the public and private
sector (Approved Inspectors), but are also used for various purposes
by safety inspectors, environmental scientists, real estate developers,
contractors and subcontractors, manufacturers of building products
and materials, insurance companies, facility managers, tenants, and
others.
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There are often additional codes or sections of the same building
code that have more specific requirements that apply to dwellings
and special construction objects such as canopies, signs, pedestrian
walkways, parking lots, and radio and television antennas.
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Ventilating (the V in HVAC) is the process of "changing" or replacing air in any space to provide high
indoor air quality (i.e. to control temperature, replenish oxygen, or remove moisture, odors, smoke,
heat, dust, airborne bacteria, and carbon dioxide). Ventilation is used to remove unpleasant smells and
excessive moisture, introduce outside air, to keep interior building air circulating, and to prevent
stagnation of the interior air.
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Ventilation includes both the exchange of air to the outside as well as circulation of air within the
building. It is one of the most important factors for maintaining acceptable indoor air quality in
buildings. Methods for ventilating a building may be divided into mechanical/forced and natural
types.[
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"Mechanical" or "forced" ventilation is used to control indoor air quality. Excess humidity, odors,
and contaminants can often be controlled via dilution or replacement with outside air. However, in
humid climates much energy is required to remove excess moisture from ventilation air.
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Natural ventilation is the ventilation of a building with outside air without the use of a fan or other
mechanical system. It can be achieved with openable windows or trickle vents when the spaces to
ventilate are small and the architecture permits. In more complex systems warm air in the building can
be allowed to rise and flow out upper openings to the outside (stack effect) thus forcing cool outside
air to be drawn into the building naturally through openings in the lower areas. These systems use very
little energy but care must be taken to ensure the occupants' comfort. In warm or humid months, in
many climates, maintaining thermal comfort solely via natural ventilation may not be possible so
conventional air conditioning systems are used as backups. Air-side economizers perform the same
function as natural ventilation, but use mechanical systems' fans, ducts, dampers, and control systems
to introduce and distribute cool outdoor air when appropriate.
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