MODEL TESTING OF ROAD TUNNEL VENTILATION
IN NORMAL TRAFFIC CONDITIONS
Miroslav Sambolek, Brodarski institut, Zagreb, Croatia
1 INTRODUCTION
Model tests of the functionality of complete tunnel ventilation systems are rare [1].
However, data relating to tests of the parts of a system, i.e., relating to the modeling of
individual physical phenomena, such as the testing diffusion of gas jets [2] and research in
the aerodynamics of jet fans [3] are available in the literature. For the Učka Tunnel the
influence of the distance between the fan batteries and the angle of inclination of fan
nozzles on ventilation efficiency was tested [4]. However, although in the available
literature we did not find examples of ventilation system design based only on the results
of model experiments, model testing can contribute much to the successfulness of the
project.
The facility tested is the Sveti Rok Road Tunnel with two-way single-track traffic, 5670 m in
length, with a transverse section area equal to 58.05 m2. In the first phase of the building
the tunnel is bored as a single tube and in the second phase of the building it will be a
twin-tube tunnel. More detailed data about the tunnel can be found in references [5], [6],
and [7].
2 CONDITIONS OF NORMAL TRAFFIC TESTING
Under normal conditions, there are three ways in which ventilation functions. The
conditions in turn depend on the current meteorological situation and on traffic density
through the tunnel1:
Natural ventilation only; in such a case, there is sufficient pressure difference at the
portals, capable of overcoming the longitudinal component of the weight of air in the tunnel, which is the consequence of the tunnel’s inclination, and the resistance of the airflow.
Forced ventilation only, i.e. atmospheric conditions do not support natural ventilation;
therefore fans alone produce the gas flow.
Combination of natural and forced ventilation; reversible fans enable the natural flow
through the tunnel, which can have one direction or the other, to be assisted.
The efficiency of natural ventilation depends on the pressure difference at the tunnel
portals, which depends on current weather conditions on one and on the other side of Mt
Velebit. Weather conditions include barometric pressure at both portals and, if existing,
dynamic air pressure as the consequence of wind. Also important is the air density (in fact
the density of the mixture of air and exhaust gases), which depends on the atmospheric air
1
These three conditions were experimentally investigated, but cannot be met in natural conditions.
1
temperature, on the thermal emissions of motor vehicles and on thermal transfer through
the tunnel walls.
3 THE PHYSICAL MODEL
Reliability of test results depends in great measure on the quality of the applied physical
model; therefore, we devoted particular attention to the model, especially because of the
lack of an appropriate example.
3.1 Choice of model scale
We chose the scale of the model taking into consideration a number of conditions. After
detailed consideration it was decided to make a model of the central part of the tunnel, 675
m long, on a scale of 1:25, which should ensure that the measured relevant physical
quantities and flow visualization are sufficiently reliable while avoiding unmanageably large
dimensions.
3.2 Conception of the model
At the designer’s suggestion, we chose for the testing a tunnel section near the middle.
This section includes one turnaround, two shelters for passengers, three niches for
stopping vehicles and five fan batteries.
In order to achieve a difference of pressure on both sides of the modeled section, one
section end was opened to the atmosphere, while the other ended in a sealed air
chamber. By a suitable fan (‘chamber fan’) the air can be pressed into the chamber.
During the experiment, the pressure difference between the chamber and the atmosphere
was continually measured. The chamber fan simulates the natural ventilation, and its
orifice meter I measures the total flow through the tunnel. Five fan batteries at a distance
of 5 m distributed along the modeled section and model vehicles moving through the tunnel
made the direct measuring of the airflow velocity impossible. Therefore, mean velocities
had to be calculated by means of the total flow and the area of model’s cross section.
The speed of model vehicle motion must satisfy the requirement of kinematical similarity
(see Chapter 5), that is, that the ratio between mean flow velocity and vehicle speed must
be the same in nature and in the model. From the condition of tunnel traffic load and
maintenance of the allowed concentration of harmful ingredients at the end of the tunnel at
which the gas exits, for the worst case, the designer calculated the necessary gas flow
velocity from 5 to 8 m/s. A speed of vehicles in the tunnel equal 80 km/h is assumed. In
order to obtain such high flow velocities (approximately 0.4 to 0.5 m/s) through the tunnel
model, enabling reliable measurement results, it has been decided that the top speed of
vehicle models equals 2 m/s.
3.3 Realization of the model
On the base of the tunnel model U-shape sections made from 3 mm thick Plexiglas, were
attached. Plexiglas was chosen for the tunnel model in order to enable visual observation
2
during tunnel fire experiments (these experiments will be described in separate article),
and to enable immediate location and removal of potential mechanical trouble.
3.4 Device for the simulation of two-way traffic
The device for vehicle model motion comprises an endless transport ribbon and two
pulleys. The electric motor gives 1kW at 930 min-1; its rpm can be reduced in a ratio of
1:12 to 77.5min-1. Vehicle model speed can be continuously varied at intervals from 0.15
m/s to the top speed of ca 2.2m/s. The device for the simulation of two-way traffic is
presented in diagram form in Fig. 1.
Fig.1. Sketch of the airtight chamber and the system for the simulation
of two-way traffic
3.5 Modeling of the impulse fans
Jet fans in the tunnel have the form of a tube of 1250 mm in diameter in which an electric
motor is installed, driving the fan impeller; inlet and outlet nozzles have been placed at the
ends of this tube. Two such jet fans are placed parallel to the central axis of the tunnel,
and form one fan battery.
A problem arose with jet fans because very small electric motors with a sufficiently large
power output were not available on the market. The problem was solved using fan
imitations composed of two pipe elbows, directed in opposite directions. Through one
elbow the air (gas) from the tunnel flows to a centrifugal fan located outside the tunnel
tube, further to the orifice meter and back to the tunnel, with high velocity, through another
elbow, creating an air jet.
The configuration and manner of the work of the system, which in the model simulates one
battery of jet-propelled fans, is presented in Fig. 2.
3
Fig. 2. Diagram of the simulation of a jet fan battery in the tunnel model
4. MEASUREMENT TECHNIQUE
During the testing on the model of the tunnel, several different quantities were measured:
flow rates, pressures, temperature and humidity of the air, and the speed of the motor
vehicle models.
The flow rate was measured in orifice meter made of Plexiglas. The pressure difference in
sections in front of and behind the flow meter was measured with inductive differential
manometers connected to the amplifier via the integrator.
Before the testing, the correlation between the regulated voltage on the terminals of the
fan’s electric motor and the airflow rate, q=f(U) was established.
This dependence (correlation) was established for each fan battery and (later, during the
experiments) for the setting of specified fan’s flow rate has been used.
The relative pressure in the chamber was also measured by an inductive differential
manometer, and measurement data were collected by means of the program AEDLAB
P20. At all seven sites provided for pressure measuring (5 fan batteries, chamber fan flow
and relative pressure in the chamber) the differential pressure was measured by
LANDIS&GYR pressure transducers, using the appropriate program for a PC.
4
5. SIMILARITY CONDITIONS
The first and essential condition is that there is geometrical similarity between the model
and the life-sized facility [8], [9]. It should be noted that complete geometrical similarity
also includes micro-geometric similarity, i.e., roughness similarity. Unfortunately,
roughness similarity is usually not achievable, so the scaling of the model results up to the
full-scale object needs special adjustments.
The second condition for complete similarity is kinematical similarity, that is, the
geometrical similarity of the pattern of the velocity vector field. In other words,
proportionality of absolute values of all corresponding (homologous) velocity vectors in the
model and in the full scale is needed, as well as the parallelism of corresponding vectors.
Regarding the model of impulse ventilation, kinematical similarity conditions demand the
same proportionality of jet velocity and mean flow velocity in the model and in the full-scale
object. Since the model simulated the influence of traffic by using models of road vehicles,
kinematical similarity demands the fulfilling of proportionality of streaming velocity in tunnel
and vehicle speed, that is:
The dynamic similarity condition (which is here the sufficient complete similarity condition,
since heat transfer processes are out of the scope of this model) demands the
proportionality of different natural forces. In the case of streaming through the tunnel,
pressure forces, viscous forces and inertial forces act on the mass of air. Therefore, the
Reynolds number ‘Re’ and Euler number ‘Eu’ are decisive dimensionless characteristics of
dynamic similarity.
In this case, the Euler number is equal to the flowing resistance coefficient. This resistance
is not only the result of shear stresses in the boundary layer, but also contains the
elements of “profile resistance”. Greater pressure decline is the result of resistance of the
hollows and niches in the tunnel wall, ventilation equipment, and of vehicles moving
through the tunnel.
It is impossible to achieve the Reynolds number in the tunnel model, as, indeed, in almost
every model experiment. Accordingly, the appropriate corrections are needed.
6. THE PROGRAM OF VENTILATION TESTING IN NORMAL TRAFFIC
CONDITIONS
During the testing of the ventilation system in normal traffic conditions (not in an
emergency case) the following quantities were measured:
o
o
o
speed of the vehicle models in motion
flow rate through the fan batteries
flow rate through the chamber fan.
Experiments were conducted for following speed of vehicle models:
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vv = 0; 1; 1.41; 1.73 and 2 m/s.
The flow rate through the fan batteries in the model depended practically only on the
centrifugal fan rotational speed. Therefore, the desired flow rate through the fan batteries
can be set by adapting the voltage on the rpm controllers, not taking into account the
streaming velocity through the tunnel model. The flow rates through the fan batteries were
chosen as follows:
qFB = 0; 0.010; 0.015; 0.020 m3/s.
Instances with a chamber fan working against the jet fans were not examined. Therefore
only the positive values of the flow rate were measured (the stream out of the chambers
into tunnel), reaching 0,095 m3/s.
7. EXPERIMENTAL RESULTS
The broad, well-covered testing domain enabled reliable smoothing of measured values,
whereby the influence of random errors diminishes, and at the same time, analysis of the
behavior of the ventilation system in different operating conditions.
7.1 Conditions of natural ventilation without moving vehicles
Testing when only natural ventilation is present, i.e. when the streaming is realized only
because of the pressure difference between the two tunnel portals, is very important. It
gives the clearest insight into the influence of traffic on streaming resistance.
In Fig. 3, experimental points and regression curves given by following function are
presented:
This formula describes the dependence of airflow through the tunnel model (qt in l/s) on
the pressure difference (
); k1 and e are the parameters obtained by the procedure
of nonlinear regression. Aside from this, the same diagram shows the calculated curve
representing flow through a tunnel with technically smooth walls.
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air flow rate in tunnel model l/s
200
calculated flow for smooth tunnel
150
empty tunnel
100
resting vehicles
50
2
4
6
8
pressure difference Pa
Fig. 3. Values measured on model and scaled to natural object, and
the regression curves representing air flow rate in tunnel for
conditions of natural ventilation. These velocities depend on
the pressure difference
.
On the basis of the measured rate of flow through the tunnel and pressure differences in
the case with and without the traffic and with idling fans, the resistance coefficients of
streaming f are determined and presented in the diagram on the Fig. 6, as a function of the
Reynolds number.
Symbols in the preceding expression have the following meaning:
vt
the average air flow velocity
l
the length of tunnel model section,

density of air
rh
hydraulic radius of the tunnel model.
In the same diagram the curve of the resistance coefficient for smooth wall pipes is drawn.
Diagram shows that the resistance coefficient of the tunnel without vehicles is larger than
the same coefficient for a smooth pipe. At Re = 25000 the resistance in the tunnel without
vehicles is ca 4 times larger, and in the tunnel with resting vehicles (2400 mm distant in
the model, which corresponds in nature to 60 m) the resistance is as much as about seven
times as great.
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f
0.2
resting vehicles
0.15
empty tunnel
0.1
0.05
pipe with technically smooth wall
3.4
3.6
3.8
log Re
4.2
4.4
Fig. 4. Dependence on Re of the empirical friction coefficient f for empty
tunnel and for resting vehicle models at natural ventilation is
presented. The curve of the friction coefficient for a pipe with a
technically smooth wall is also drawn.
The additional resistance falls with the enlargement of the Re number, with a tendency
towards a very slow inclination decrease. This fact is the result of experiments conducted
at the transitional region of streaming. In order to obtain the needed appropriate resistance
coefficient f for the tunnel in life size it is necessary to add the value of the additional
resistance coefficient to the value of the resistance coefficient for rough concrete surfaces
of tunnel.
The following group of experiments was performed with natural ventilation and with moving
vehicle models. In Fig. 5, the velocity of the air streaming through the tunnel model as a
function of the pressure difference is presented. This group of experiments was performed
a) without the vehicle models and b) for different vehicle model speeds vv .
The same figure also shows the curves approximating the experimental data. It is worth
noticing the already mentioned difference in the curves relating to the instance of moving
vehicles and the curves relating to the instance when there are no vehicles or the vehicles
are resting.
8
1
vt m/s
empty
vv=0 m/s
0.8
0.6
vv=2.0 m/s
vv=1.73 m/s
vv=1.41 m/s
vv=1.0 m/s
0.4
0.2
2
4
6
8
10
p Pa
F
Fig. 5.The measured values scaled to natural object and regression
curves presenting the dependence of air velocity in the tunnel
model on the pressure difference p. The experiments were
conducted simulating the natural ventilation, without traffic and for
different vehicle speed vv .
This difference in the character of the curves is the consequence of physical differences in
the streaming process, which influenced the form of the mathematical regression models.
Results of measurement for given testing conditions (very small differences of pressure, of
order 10-1 Pa) are very well grouped, which proves adequate precision of measurement.
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log f
vt /vv=0.05
0.08
0.5
3.6
3.8
4.2
0.11
0.17
4.4
log Re
-0.5
0.26
0.88
0.39
1.32
0.58
-1
2.00
3.00
-1.5
Fig. 6. Dependence of logarithm of the friction coefficient f from the Reynolds
number logarithm; the parameter is ratio of the flowing velocity in the tunnel
and vehicle speed vt /vv . Tunnel natural ventilation was simulated.
7.2 Forced ventilation and moving vehicles
Forced ventilation without any natural ventilation at all can appear only very seldom. There
always exists at least some small natural circulation that supports, or obstructs the
functioning of the ventilation system. Such combined ventilation, as compared to only
forced ventilation, has another changeable parameter; which is the difference of pressure
at tunnel portals and therefore the result of measurement is difficult to be well laid out.
Therefore from the experimental results, only conclusions about the functioning of the
ventilation system in conditions of forced ventilation only, with no difference of pressure at
the tunnel portals, can be drawn.
The dots in Fig. 7, mark the measured flow velocities through the tunnel model as function
of the pressure difference
at the ends of the tunnel model. Experiments were
conducted for the rate of flow through fan batteries
= 0.010; 0.015 and 0.020 m3/s,
and vehicle model speeds were vv = 0; 1.0; 1.41; 1.73 and 2.0 m/s. The measured values
were approximated by regression functions with arguments: the pressure difference
and the rate of flow through fan batteries
.
10
vt m/s
1
vv=0.0 m/s
vv=1.0 m/s
vv=1.41 m/s
vv=1.73 m/s
vv=2.0 m/s
0.8
0.6
0.4
0.2
qVB=15
l/s
qVB=20 l/s
qVB=10 l/s
-15
-10
-5
p Pa
5
Fig. 7. Dependence of streaming velocity in the tunnel vt on the pressure
difference p. Parameters are the flow rate through the fan batteries qVB
expressed in l/s and the vehicle models speed vv in m/s.
Fig. 7 shows that with fan batteries with a flow rate equal to 0.010 m3/s complete forced
ventilation can be realized, but for fan batteries with a flow rate equal to 0.020 m3/s the
capacity of the chamber fan was insufficient to realize the atmosphere pressure in the
chamber. Therefore, the measured value must be extrapolated, which is not desirable. For
a flow rate equal to 0.015 m3/s forced ventilation could be achieved only for relatively
small vehicle speeds, when the additional resistance was smaller.
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1
vt m/s
0.8
vv=2.00 m/s
vv=1.73 m/s
vv=1.41 m/s
vv=1.00 m/s
0.6
0.4
0.2
5
10
15
20
qVB l/s
Fig. 8. Dependence of streaming velocity through the tunnel model vt on
the flow rate of fan batteries. Vehicle speed is the parameter. The
diagram refers to conditions of forced ventilation only, and p=0.
The numerical calculation using the regression equation gave streaming velocities in the
tunnel vt as the function of fan batteries rate of flow qVB, for different vehicle speeds.
This speed has been presented diagrammatically in Fig. 8 for vehicle model speeds from 1
to 2 m/s. Here, as in the case of the natural ventilation, a clear, although not large,
influence of vehicle speed on the enlargement of resistance was established. This
enlargement is manifested as airflow velocity diminution, with the maintenance of
. The augmentation of the vehicle model speed from 1 m/s to 2 m/s leads to
a decrease of air flow velocity through the tunnel up to 20 %.
According to the experimental results it can be said that Re strongly, and vehicle speed
moderately influences the flow resistance coefficient.
8. CONCLUSIONS
The purpose of testing the road tunnel longitudinal ventilation system model, was to collect
data important for the design of this system. In the design process the testing results serve
as a supplement to the empirical values quoted in the literature. The following conclusions
can be drawn about the model experiment system itself, and the results obtained:
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o
It was demonstrated that the functioning of a tunnel ventilation system could be
successfully investigated with a physical model.
o
The measuring techniques, especially with respect to the pressure transducers, are
now so sophisticated that they enable successful measurement of the very small
pressure differences that occur in such experiments.
o
For practical use, it is interesting to note the additional flow resistance through the
tunnel that is the consequence of niches in tunnel wall, fan batteries and vehicles;
these resistances are embraced by the term “conditional roughness”.
o
It has been established that in the case of two-way traffic, the flow resistance
through the tunnel also grows to some extent.
o
For the testing of resistance in the event of resting vehicles, when there is no
limitation for the air flowing velocity established by the kinematical similarity
condition, it would be interesting to repeat the investigation at higher air velocity in
order to achieve bigger Reynolds number values and so to emerge from the
transient streaming regime.
o
It would be interesting to investigate the traffic through the tunnel in one direction for
both tracks and compare the results with the present ones.
9. LITERATURE
[1] *** Road Tunnels, PIARC (Permanent international association of road congresses), Montreal,
1995.
[2] Malmström, T.,G., Kirkpatrick, A., T., et al. : Centerline velocity decay measurements in lowvelocity axisymmetric jets, J. Fluid Mech., vol.246, pp363-377, 1997.
[3] Mutama, K. R.: The experimental investigation of jet fan aerodynamics using wind tunnel
modeling, Transactions of the ASME, vol. 118, pp 322.-28., 1996.
[4]*** “Societe du Tunnel Učka” - Ventilation longitudinale - verification experimentale,
SOFRAIR, Report n. RP/GH - CM 523 V 73 - 2/6/75, 1975.
[5] Drakulić, M., et al.: Sveti Rok Tunnel - Study of Ventilation System and Preliminary Analysis of
Fire Protection Meausre. (in Croatian), Brodarski institut, Zagreb, ožujak, 1997.
[6] Sambolek, M., Čizmek, M.: Model testing of road tunnel »Sveti Rok « ventilation, (in Croatian),
Izvještaj Brodarskog instituta WP50-02-000, Zagreb, 1998.
[7] Sambolek, M., Čizmek, M.: Model testing of road tunnel ventilation in normal traffic conditions,
(in Croatian), Građevinar 53(2001) 11, 711-716
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[8] Pope, A.: Wind-tunnel Testing, John Wiley & Sons, New York, 1954.
[9] Prandt, L.: Führer durch die Strömungslehre, Fridr. Vieweg & Sohn, Braunschweig, 1965.
Symbols
At
f
Fr
O
qt
qVB
Re
rh
vm
vt
vv


m2
m
m3/s
m3/s
m
m/s
m/s
m/s
m
kg/m3
Tunnel model section area
Flow resistance coefficient
Froude number
Perimeter of tunnel model
Air flow through the tunnel model
Supply of fan batteries in the model
Reynolds number
Hydraulic radius of tunnel model
Velocity of air jet from the fan outlet in the model
Average air flow velocity through the tunnel model
Speed of vehicle model
Absolute wall roughness
Air density
Key words
road tunnel, longitudinal ventilation, model testing, Sveti Rok tunnel,
normal traffic conditions
Miroslav Sambolek
Brodarski institut
Av. V. Holjevca 20
10020 Zagreb
Croatia
Tel:
+385 1 6504 110
Fax:
+385 1 6504 230
Email:
sambolek@hrbi.hr
The mistaken ‘Rn’ has been changed in the paper to read ‘Re’.
The answer to the question of the second referee is: "diffusion of gas jets" is a term different from
the "flow of gas jets".
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