- Email: [email protected]
Design of a low-cost stratified boundary-layer wind tunnel
JOORI~.L OF
Journal of Wind Engineering and Industrial Aerodynamics 54/55 (1995) 483-491
ELSEVIER
~
~
Design of a low-cost stratified boundary-layer wind tunnel M. Schatzmann*, J. Donat, S. Hendel, G. Krishan Meteorological Institute, University of Hamburg, Bundesstrasse 55, 20146 Hamburg, Germany
Abstract
A new stratified boundary-layer wind tunnel has been developed and built. The tunnel is of closed circuit type with several layers, each of which is insulated against the neighbouring layers, heated individually and driven by a separate fan. The tunnel will be applied to solve environmental flow problems which are governed by stable atmospheric stability including elevated inversions.
1. Introduction Atmospheric boundary-layer wind tunnels play an important role in many meteorological and engineering applications. There are two main reasons for simulating the boundary layer in a wind tunnel. The first reason is to study the basic phenomena of micro-meteorological processes in the atmosphere. The second is to solve engineering problems of practical interest such as the dispersion of stack gases in complex terrain or in urban areas where buildings produce complex flow patterns. In contrast to the many neutrally stratified wind tunnels which presently exist at several research institutions (for a survey see Ref. [1]), wind tunnels which are able to generate thermally stratified layers are rather rare (worldwide about 15). This is certainly not due to a lack of interest in stratified flows since there exist many important applications for which stratification of the atmosphere is a key consideration. For example, air pollution problems are often critical in the presence of stable stratification or an inversion layer. Stratified boundary layer wind tunnels would be more widely used were it not for their high construction and operating costs. 2. Design of conventional stratified tunnels As an example of the conventional type of stratified boundary layer wind tunnel, Fig. 1 shows the tunnel of the Military University of Munich, the only stratified * Corresponding author. 0167-6105/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 1 6 7 - 6 1 0 5 ( 9 4 ) 0 0 0 6 1 - H
484
M. Schatzmann et al./J. Wind Eng. Ind. Aerodyn. 54/55 (1995) 483-491
&_ H
4 Air Intake 5,20 Sound ebserber
12 14
6
15
Heater
9,16 Flow straightener 10 Nozzle 11 Turn table
17 19 21
Flow establishment end test section Boftom hestlng or cooling Flexlb/e ceiling Fen
Diffusor Air outlet
Fig. 1. Stratified boundary layer wind tunnel at the Military University, Munich, Germany [2].
boundary layer tunnel which was formerly available in Germany [2]. This tunnel is equipped with a fan which sucks the air through a heating device and over a heated or cooled wind tunnel floor. To achieve a boundary layer of modest thickness with temperature, velocity and turbulence characteristics similar to nature, long tunnels are required [3]. The cost of the tunnel in Munich was about 8 million DM, the power consumption of the fan, the heating section and the cooling section totals up to 1 MW.
3. Design of the multi-layer stratified tunnel At the Hamburg University, a new type of tunnel has been designed and built, which is significantly cheaper than a comparable conventional tunnel and requires less laboratory space. The tunnel is based on an idea first introduced by Hertig [l]. The tunnel is of closed circuit type. Fig. 2 shows a principal sketch of the tunnel. The return section is made up of nine horizontal ducts of rectangular shape. The height and width of their cross sections is 12 cm and 2.3 m, respectively. The ducts are composed of 0.8 m long sheet metal boxes connected to each other and properly insulated against the wind tunnel hall and the neighbouring ducts by 2 cm thick sheets of insulation material. Small vertical pillars assure the accuracy to gauge and the stiffness of the boxes (Fig. 3).
M. Schatzmann et al./,1. Wind Eng. Ind. Aerodyn. 54/55 (1995) 483-491
~ .
16.5m
. Cooling SectiOns.7mTeStSection
485
" 2.3 m1~
.
FanSHeatersand
Fig. 2. Multi-layer stratified boundary layer wind tunnel of the Hamburg University. The flow direction in the test section is from left to right. Test and cooling section: length of 8.7 m, width of 2.3 m, height of 1.1 m.
Fig. 3. View from the test section into the ducts. In front one of the rakes equipped with IC-temperature transducers can be seen.
In each duct a separate fan and a set of six electrical heaters has been installed. The fans are driven by electromotors which are speed-controlled via microverters from a personal computer. Another PC controls the heaters and keeps the temperatures of the nine layers at pre-set values. Additional control mechanisms prevent the tunnel from being overheated. The (tangential) fans, the motors, microverters and (tubular) heaters were all customary available and therefore inexpensive. The installed electric power is in total about 100 kW for the heating system and 20 kW for the electric motors.
486
M. Schatzmann et al./.1. Wind Eng. Ind. Aerodyn. 54/55 (1995) 483-491
The working section is made up of wood panels. It contains large double-glass windows in the side walls through which flow visualization experiments can be observed. The side walls are completely removable when the tunnel is being set up for a new experiment. In the first 3.5 m of the working section floor a heat exchanger has been integrated which cools the floor down to about - 20°C. The coolant is circulated by a refrigerating equipment commonly used in the food processing industry. The power of the cooling system is about 15 kW. The subsequent test section floor is again wooden. It contains a turntable which is readily accessible from below the tunnel. The usable length of the test section is about 4.5 m. A three-dimensional traversing system has been constructed which allows the remote-controlled transport of probes within the entire working section.
4. Instrumentation of the tunnel
For the determination of the boundary layer characteristics, temperature and velocity measurements have to be carried out. Continuous temperature registration is done by means of IC-temperature transducers (analog devices 592 CN) which work in the r a n g e - 25°C ~< T ~< 105°C with about 0.5°C accuracy. The signals from the numerous sensors are processed by a personal computer equipped with an A/D card. The PC displays the actual vertical temperature profiles at the entrance and exit of the test section. Wind field and turbulence measurements are done with a DANTEC laser Doppler anemometer. The LDA used in Hamburg is equipped with a 300 mW argon-ion laser and a two-dimensional fiber optic probe. The probe is as small as a cigar and can be moved with the traversing system. According to the manufacturer, the probe operates safely in environments with temperatures up to about 70°C.
5. Modes of tunnel operation
The tunnel operates in three distinct modes: (a) Neutral stratification: The mean velocity profile is generated by appropriate choice of the revolutions per minute of the nine tangential fans. In addition, roughness elements and, if necessary, vortex generators are employed to achieve turbulence characteristics which correspond to the real atmosphere. (b) Stable stratification: In addition to (a), the individual layers are heated and the bottom is cooled. The system installed provides temperature differences between the top layer and the ground up to 100°C which is sufficient to meet the Richardson number requirement for most dispersion problems. (c) Elevated inversion: Only a few layers at the top of the flow will be heated. The strength of the inversion can be modified by changing the temperature in these layers accordingly.
M. Schatzmann et al./J. Wind Eng. Ind. Aerodyn. 54/55 (1995) 483-491
487
The convective boundary layer (which, however, prevails only a small percentage of the time of the year in central Europe) cannot be simulated in our new tunnel. This is the domain of another multi-layer tunnel which is presently in the test phase at the Karlsruhe University [4].
6. Boundary-layer characteristics In conventional stratified boundary-layer wind tunnels (as well as in nature) the free stream velocity, the bottom roughness and the vertical heat flux determine the mean and fluctuating velocity and temperature fields, provided the flow establishment section is sufficiently long that an equilibrium boundary layer can be achieved. This is different in a tunnel of multi-layer type. In such a tunnel arbitrary combinations of vertical velocity and temperature profiles are obtainable (at least within certain ranges), since the speed and temperature of the air in each layer are individually controlled. Only a small number of possible combinations correspond to natural boundary layers and are of interest here. At the beginning of the test phase we started with neutral stratification and the same velocity in all layers. Neither vortex generators nor bottom roughness elements were in the tunnel. The vertical profiles of time averaged velocities and turbulence intensities at various test-section locations and at several velocities were measured. The situation was subsequently made more complex. A mean velocity profile was generated, vortex generators and roughness elements were added and, finally, the layers were heated and the bottom was cooled. The results are too manifold to present here in detail. Subsequently only a few examples will be given. 6.1. Neutral stratification, smooth floor
For three free stream velocities Uo~ = 1.1, 1.4 and 2.9 m/s, measured at h = 52 cm above the ground) it was intended to generate a power-law profile with profile exponent n = 0.16. This value corresponds to boundary layers over smooth terrain. Fig. 4 shows the result at a position 3.6 m downwind from the entrance of the working section. Similar profiles were measured also further downwind. The desired mean velocity profile was easily achievable. The corresponding turbulence intensity profiles, however, were generally too low and not as smooth as expected. That means that much more time is needed until the turbulence characteristics adjust to the mean profiles of the flow. Similar observations have been made in conventional stratified tunnels I-5]. 6.2. Neutral stratification, rough floor, vortex generators
In order to obtain a boundary layer of sufficient height (h ~ 50 cm) and turbulence characteristics similar to nature, vortex generators in combination with sharp edged roughness elements (as they are used in our conventional unstratified boundary layer
488
M. Schatzmann et al./J( Wind Eng. Ind. Aerodyn. 54/55 (1995) 483-491 60
6O 5O
N
© U(h):l.lm/s []
U(h)=l.4m/$
~1 J~3
50
U(h)=2.~Vnls
e3
40
30
30
20
20
.J
10
%
o~2 o.4~o.6
z~
Q
U(h)=l.lm/s
,~
[] A
u(n)=1.a,m/8 U(h)=2Jm/s
~)
10
o.a
i
1.2
00
5
10
15
20
u'/O(z) [%]
U(z)/U(h) [/]
Fig. 4. Mean velocity and turbulence intensity profiles for various free stream velocities at the end of the cooling section. Smooth floor, neutral stratification.
6O
so
60
Q
conventional
[]
multi-layer
D
5O
0 0 O (~
4O 30
(~ conventional [ ] multhlayer
[] ©
40 30
0
N
20 10
lO
[] °o
0.2
0.4
, o.6
0.8
U(z)/U(h) [/1
1
1.2
0
1'o
~o 30 u'/U(z) [%1
Fig. 5. Comparison of mean velocity and turbulence intensity profiles measured in a conventional boundary-layer wind tunnel and in the new multi-layer tunnel. Vortex generators, rough flow, neutral stratification.
tunnel) were installed also in the multi-layer tunnel. Fig. 5 shows the results. Due to the larger roughness length Zo, the power-law exponent is now n = 0.28. The measurements from the conventional tunnel and from the multi-layer tunnel fall nearly on top of each other. Since the boundary layer characteristics from our conventional tunnel compare well with those known from field measurements [6], this comparison can be interpreted as an indirect proof for the boundary-layer quality of the multi-layer tunnel.
6.3. Stable stratification For a free-stream velocity U(h) = 1 m/s and mean and turbulence characteristics corresponding under neutral stratification to Fig. 5, tests have been carried out in order to determine the largest obtainable, steady state, linear, mean temperature
M. Schatzmann et al./J. Wind Eng. Ind. Aerodyn. 54/55 (1995) 483 491
60 50
Z N
489
~~
I-]x=3.6m
40 30 20 10
%
00
,." .
30
.
.
40
.
50
.
.
60
70
T [°C] Fig. 6. Typical mean vertical temperature profiles obtained in different tests at various locations of the test section.
gradient. Fig. 6 shows the results from different trials. In each case both the heaters and the bottom cooling system were switched on. As can be seen, temperature gradients of about 50 K/m were generated. The gradients are not yet everywhere linear but this will improve once the automatic temperature and velocity control system is finished. Since in undistorted models equal temperature differences over corresponding heights are required, this allows, e.g., small scale simulations up to 20 K/100 m in the scale 1:250, which is certainly more than needed. It is remarkable that the temperature at the lowest probe position at z = 4.3 cm above the ground was already in the range of 30°C although the coolant temperature was kept constant at - 15°C. Basing the stability parameters on the temperature difference between the free-stream temperature and the coolant temperature, as it is sometimes done in the literature, would, therefore, be extremely misleading.
6. 4. Elevated inversion Finally, the potential of the new tunnel to generate an elevated inversion was investigated. The same basic setup of the tunnel with vortex generators and bottom roughness elements was utilized but the freee stream velocity was reduced to U(h) = 0.7 m/s. Only the uppermost three layers were heated. The heaters of all other layers as well as the bottom cooling system were switched off in order to keep the temperature profile in the mixed layer below the inversion as neutral as possible. Fig. 7 shows the development of the inversion layer as a function of time. The strongest inversion layer was obtained 105 min after the start of the experiment with a temperature gradient at the base of the layer of the order of
100 K/m.
490
M. Schatzmann et al./J. Wind Eng. Ind. Aerodyn. 54/55 (1995) 483-491
~
IO0 8O 6O N
40 20
eaters
on
= 60 mln = 75 mln = 90mln = 105 min
o1'o
20
3b
4o
5o
8o
ro
80
r rCl Fig. 7. Development of an elevated inversion as a function of time.
7. Conclusions The new multi-layer stratified boundary layer wind tunnel was built for about 0.5 million DM which is much less than a conventional tunnel would have cost. With a maximal electric power consumption of 135 kW the operating costs appear to be reasonable. The tunnel is able to produce stable stratification sufficient for most environmental applications. There are, however, also disadvantages which should be noted. Due to the fact that the vertical distributions of temperature and velocity can be controlled independently from each other, a boundary-layer wind tunnel of multi-layer type has the potential of generating combinations of boundary layer parameters, which are purely artificial and would never occur in reality. Thus, in comparison with a conventional tunnel, the simulation of a stratified boundary layer in the new tunnel requires more work until all major boundary layer characteristics are equal to pre-set values. Therefore, a very efficient control system for the temperature and velocity field is an essential necessity for such tunnels. Since the length of the flow establishment section is usually not sufficiently long so that equilibrium between mean and turbulent properties can be achieved, additional measures (increased bottom roughness, vortex generators) will mostly be necessary to adjust the turbulence field to the mean field. Other disadvantages are the fact that it takes substantial time until steady state conditions are obtained. As is the case with all recirculating tunnels, background concentrations increase rapidly when tracer experiments are carried out.
Acknowledgement We are grateful to the German Ministry of Research and Technology (BMFT) for financial support.
M. Schatzmann et al. / J. Wind Eng. lnd. Aerodyn. 54/55 (1995) 483-491
491
References [1] J.A. Hertig, A review of physical modeling of atmospheric air flows, Report No. N507.117, Ecole Polytechnique F6d6rale de Lausanne, IENER (1982). [2] L. R6mer, Simulation von Ausbreitungsvorg~ngen im Windkanal, in: Ausbreitungsrechnung im Rahmen des Vollzugs der St6rfall-Verordnung, Texte des Umweltbundesamtes 1/89 (Umweltbundesamt, Berlin, 1989). [3] J.E. Cermak, Physical modeling of flow and dispersion over urban areas, NATO Advanced Study Institute on Wind climate in cities, Waldbronn, Germany, 5, 6 July 1993. [4] M. Rau, W. B~ichlin and E.J. Plate, Detailed design features of a new wind tunnel for studying the effects of thermal stratification, Atmos. Environ. A 26 (1991) 1257 1263. [5] R.N. Meroney and D.E. Neff, Plume behavior from tall stacks at Savannah River Laboratory, Aiken, South Carolina, Colorado State University, Internal Report, July 1987. I-6] M. Schatzmann and G. K6nig-Langlo, Application of scale modeling in environmental meteorology, in: Proc. Int. Symp. on Scale modeling, Tokyo, 18-22 July 1988.
Journal of Wind Engineering and Industrial Aerodynamics 54/55 (1995) 483-491
ELSEVIER
~
~
Design of a low-cost stratified boundary-layer wind tunnel M. Schatzmann*, J. Donat, S. Hendel, G. Krishan Meteorological Institute, University of Hamburg, Bundesstrasse 55, 20146 Hamburg, Germany
Abstract
A new stratified boundary-layer wind tunnel has been developed and built. The tunnel is of closed circuit type with several layers, each of which is insulated against the neighbouring layers, heated individually and driven by a separate fan. The tunnel will be applied to solve environmental flow problems which are governed by stable atmospheric stability including elevated inversions.
1. Introduction Atmospheric boundary-layer wind tunnels play an important role in many meteorological and engineering applications. There are two main reasons for simulating the boundary layer in a wind tunnel. The first reason is to study the basic phenomena of micro-meteorological processes in the atmosphere. The second is to solve engineering problems of practical interest such as the dispersion of stack gases in complex terrain or in urban areas where buildings produce complex flow patterns. In contrast to the many neutrally stratified wind tunnels which presently exist at several research institutions (for a survey see Ref. [1]), wind tunnels which are able to generate thermally stratified layers are rather rare (worldwide about 15). This is certainly not due to a lack of interest in stratified flows since there exist many important applications for which stratification of the atmosphere is a key consideration. For example, air pollution problems are often critical in the presence of stable stratification or an inversion layer. Stratified boundary layer wind tunnels would be more widely used were it not for their high construction and operating costs. 2. Design of conventional stratified tunnels As an example of the conventional type of stratified boundary layer wind tunnel, Fig. 1 shows the tunnel of the Military University of Munich, the only stratified * Corresponding author. 0167-6105/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 1 6 7 - 6 1 0 5 ( 9 4 ) 0 0 0 6 1 - H
484
M. Schatzmann et al./J. Wind Eng. Ind. Aerodyn. 54/55 (1995) 483-491
&_ H
4 Air Intake 5,20 Sound ebserber
12 14
6
15
Heater
9,16 Flow straightener 10 Nozzle 11 Turn table
17 19 21
Flow establishment end test section Boftom hestlng or cooling Flexlb/e ceiling Fen
Diffusor Air outlet
Fig. 1. Stratified boundary layer wind tunnel at the Military University, Munich, Germany [2].
boundary layer tunnel which was formerly available in Germany [2]. This tunnel is equipped with a fan which sucks the air through a heating device and over a heated or cooled wind tunnel floor. To achieve a boundary layer of modest thickness with temperature, velocity and turbulence characteristics similar to nature, long tunnels are required [3]. The cost of the tunnel in Munich was about 8 million DM, the power consumption of the fan, the heating section and the cooling section totals up to 1 MW.
3. Design of the multi-layer stratified tunnel At the Hamburg University, a new type of tunnel has been designed and built, which is significantly cheaper than a comparable conventional tunnel and requires less laboratory space. The tunnel is based on an idea first introduced by Hertig [l]. The tunnel is of closed circuit type. Fig. 2 shows a principal sketch of the tunnel. The return section is made up of nine horizontal ducts of rectangular shape. The height and width of their cross sections is 12 cm and 2.3 m, respectively. The ducts are composed of 0.8 m long sheet metal boxes connected to each other and properly insulated against the wind tunnel hall and the neighbouring ducts by 2 cm thick sheets of insulation material. Small vertical pillars assure the accuracy to gauge and the stiffness of the boxes (Fig. 3).
M. Schatzmann et al./,1. Wind Eng. Ind. Aerodyn. 54/55 (1995) 483-491
~ .
16.5m
. Cooling SectiOns.7mTeStSection
485
" 2.3 m1~
.
FanSHeatersand
Fig. 2. Multi-layer stratified boundary layer wind tunnel of the Hamburg University. The flow direction in the test section is from left to right. Test and cooling section: length of 8.7 m, width of 2.3 m, height of 1.1 m.
Fig. 3. View from the test section into the ducts. In front one of the rakes equipped with IC-temperature transducers can be seen.
In each duct a separate fan and a set of six electrical heaters has been installed. The fans are driven by electromotors which are speed-controlled via microverters from a personal computer. Another PC controls the heaters and keeps the temperatures of the nine layers at pre-set values. Additional control mechanisms prevent the tunnel from being overheated. The (tangential) fans, the motors, microverters and (tubular) heaters were all customary available and therefore inexpensive. The installed electric power is in total about 100 kW for the heating system and 20 kW for the electric motors.
486
M. Schatzmann et al./.1. Wind Eng. Ind. Aerodyn. 54/55 (1995) 483-491
The working section is made up of wood panels. It contains large double-glass windows in the side walls through which flow visualization experiments can be observed. The side walls are completely removable when the tunnel is being set up for a new experiment. In the first 3.5 m of the working section floor a heat exchanger has been integrated which cools the floor down to about - 20°C. The coolant is circulated by a refrigerating equipment commonly used in the food processing industry. The power of the cooling system is about 15 kW. The subsequent test section floor is again wooden. It contains a turntable which is readily accessible from below the tunnel. The usable length of the test section is about 4.5 m. A three-dimensional traversing system has been constructed which allows the remote-controlled transport of probes within the entire working section.
4. Instrumentation of the tunnel
For the determination of the boundary layer characteristics, temperature and velocity measurements have to be carried out. Continuous temperature registration is done by means of IC-temperature transducers (analog devices 592 CN) which work in the r a n g e - 25°C ~< T ~< 105°C with about 0.5°C accuracy. The signals from the numerous sensors are processed by a personal computer equipped with an A/D card. The PC displays the actual vertical temperature profiles at the entrance and exit of the test section. Wind field and turbulence measurements are done with a DANTEC laser Doppler anemometer. The LDA used in Hamburg is equipped with a 300 mW argon-ion laser and a two-dimensional fiber optic probe. The probe is as small as a cigar and can be moved with the traversing system. According to the manufacturer, the probe operates safely in environments with temperatures up to about 70°C.
5. Modes of tunnel operation
The tunnel operates in three distinct modes: (a) Neutral stratification: The mean velocity profile is generated by appropriate choice of the revolutions per minute of the nine tangential fans. In addition, roughness elements and, if necessary, vortex generators are employed to achieve turbulence characteristics which correspond to the real atmosphere. (b) Stable stratification: In addition to (a), the individual layers are heated and the bottom is cooled. The system installed provides temperature differences between the top layer and the ground up to 100°C which is sufficient to meet the Richardson number requirement for most dispersion problems. (c) Elevated inversion: Only a few layers at the top of the flow will be heated. The strength of the inversion can be modified by changing the temperature in these layers accordingly.
M. Schatzmann et al./J. Wind Eng. Ind. Aerodyn. 54/55 (1995) 483-491
487
The convective boundary layer (which, however, prevails only a small percentage of the time of the year in central Europe) cannot be simulated in our new tunnel. This is the domain of another multi-layer tunnel which is presently in the test phase at the Karlsruhe University [4].
6. Boundary-layer characteristics In conventional stratified boundary-layer wind tunnels (as well as in nature) the free stream velocity, the bottom roughness and the vertical heat flux determine the mean and fluctuating velocity and temperature fields, provided the flow establishment section is sufficiently long that an equilibrium boundary layer can be achieved. This is different in a tunnel of multi-layer type. In such a tunnel arbitrary combinations of vertical velocity and temperature profiles are obtainable (at least within certain ranges), since the speed and temperature of the air in each layer are individually controlled. Only a small number of possible combinations correspond to natural boundary layers and are of interest here. At the beginning of the test phase we started with neutral stratification and the same velocity in all layers. Neither vortex generators nor bottom roughness elements were in the tunnel. The vertical profiles of time averaged velocities and turbulence intensities at various test-section locations and at several velocities were measured. The situation was subsequently made more complex. A mean velocity profile was generated, vortex generators and roughness elements were added and, finally, the layers were heated and the bottom was cooled. The results are too manifold to present here in detail. Subsequently only a few examples will be given. 6.1. Neutral stratification, smooth floor
For three free stream velocities Uo~ = 1.1, 1.4 and 2.9 m/s, measured at h = 52 cm above the ground) it was intended to generate a power-law profile with profile exponent n = 0.16. This value corresponds to boundary layers over smooth terrain. Fig. 4 shows the result at a position 3.6 m downwind from the entrance of the working section. Similar profiles were measured also further downwind. The desired mean velocity profile was easily achievable. The corresponding turbulence intensity profiles, however, were generally too low and not as smooth as expected. That means that much more time is needed until the turbulence characteristics adjust to the mean profiles of the flow. Similar observations have been made in conventional stratified tunnels I-5]. 6.2. Neutral stratification, rough floor, vortex generators
In order to obtain a boundary layer of sufficient height (h ~ 50 cm) and turbulence characteristics similar to nature, vortex generators in combination with sharp edged roughness elements (as they are used in our conventional unstratified boundary layer
488
M. Schatzmann et al./J( Wind Eng. Ind. Aerodyn. 54/55 (1995) 483-491 60
6O 5O
N
© U(h):l.lm/s []
U(h)=l.4m/$
~1 J~3
50
U(h)=2.~Vnls
e3
40
30
30
20
20
.J
10
%
o~2 o.4~o.6
z~
Q
U(h)=l.lm/s
,~
[] A
u(n)=1.a,m/8 U(h)=2Jm/s
~)
10
o.a
i
1.2
00
5
10
15
20
u'/O(z) [%]
U(z)/U(h) [/]
Fig. 4. Mean velocity and turbulence intensity profiles for various free stream velocities at the end of the cooling section. Smooth floor, neutral stratification.
6O
so
60
Q
conventional
[]
multi-layer
D
5O
0 0 O (~
4O 30
(~ conventional [ ] multhlayer
[] ©
40 30
0
N
20 10
lO
[] °o
0.2
0.4
, o.6
0.8
U(z)/U(h) [/1
1
1.2
0
1'o
~o 30 u'/U(z) [%1
Fig. 5. Comparison of mean velocity and turbulence intensity profiles measured in a conventional boundary-layer wind tunnel and in the new multi-layer tunnel. Vortex generators, rough flow, neutral stratification.
tunnel) were installed also in the multi-layer tunnel. Fig. 5 shows the results. Due to the larger roughness length Zo, the power-law exponent is now n = 0.28. The measurements from the conventional tunnel and from the multi-layer tunnel fall nearly on top of each other. Since the boundary layer characteristics from our conventional tunnel compare well with those known from field measurements [6], this comparison can be interpreted as an indirect proof for the boundary-layer quality of the multi-layer tunnel.
6.3. Stable stratification For a free-stream velocity U(h) = 1 m/s and mean and turbulence characteristics corresponding under neutral stratification to Fig. 5, tests have been carried out in order to determine the largest obtainable, steady state, linear, mean temperature
M. Schatzmann et al./J. Wind Eng. Ind. Aerodyn. 54/55 (1995) 483 491
60 50
Z N
489
~~
I-]x=3.6m
40 30 20 10
%
00
,." .
30
.
.
40
.
50
.
.
60
70
T [°C] Fig. 6. Typical mean vertical temperature profiles obtained in different tests at various locations of the test section.
gradient. Fig. 6 shows the results from different trials. In each case both the heaters and the bottom cooling system were switched on. As can be seen, temperature gradients of about 50 K/m were generated. The gradients are not yet everywhere linear but this will improve once the automatic temperature and velocity control system is finished. Since in undistorted models equal temperature differences over corresponding heights are required, this allows, e.g., small scale simulations up to 20 K/100 m in the scale 1:250, which is certainly more than needed. It is remarkable that the temperature at the lowest probe position at z = 4.3 cm above the ground was already in the range of 30°C although the coolant temperature was kept constant at - 15°C. Basing the stability parameters on the temperature difference between the free-stream temperature and the coolant temperature, as it is sometimes done in the literature, would, therefore, be extremely misleading.
6. 4. Elevated inversion Finally, the potential of the new tunnel to generate an elevated inversion was investigated. The same basic setup of the tunnel with vortex generators and bottom roughness elements was utilized but the freee stream velocity was reduced to U(h) = 0.7 m/s. Only the uppermost three layers were heated. The heaters of all other layers as well as the bottom cooling system were switched off in order to keep the temperature profile in the mixed layer below the inversion as neutral as possible. Fig. 7 shows the development of the inversion layer as a function of time. The strongest inversion layer was obtained 105 min after the start of the experiment with a temperature gradient at the base of the layer of the order of
100 K/m.
490
M. Schatzmann et al./J. Wind Eng. Ind. Aerodyn. 54/55 (1995) 483-491
~
IO0 8O 6O N
40 20
eaters
on
= 60 mln = 75 mln = 90mln = 105 min
o1'o
20
3b
4o
5o
8o
ro
80
r rCl Fig. 7. Development of an elevated inversion as a function of time.
7. Conclusions The new multi-layer stratified boundary layer wind tunnel was built for about 0.5 million DM which is much less than a conventional tunnel would have cost. With a maximal electric power consumption of 135 kW the operating costs appear to be reasonable. The tunnel is able to produce stable stratification sufficient for most environmental applications. There are, however, also disadvantages which should be noted. Due to the fact that the vertical distributions of temperature and velocity can be controlled independently from each other, a boundary-layer wind tunnel of multi-layer type has the potential of generating combinations of boundary layer parameters, which are purely artificial and would never occur in reality. Thus, in comparison with a conventional tunnel, the simulation of a stratified boundary layer in the new tunnel requires more work until all major boundary layer characteristics are equal to pre-set values. Therefore, a very efficient control system for the temperature and velocity field is an essential necessity for such tunnels. Since the length of the flow establishment section is usually not sufficiently long so that equilibrium between mean and turbulent properties can be achieved, additional measures (increased bottom roughness, vortex generators) will mostly be necessary to adjust the turbulence field to the mean field. Other disadvantages are the fact that it takes substantial time until steady state conditions are obtained. As is the case with all recirculating tunnels, background concentrations increase rapidly when tracer experiments are carried out.
Acknowledgement We are grateful to the German Ministry of Research and Technology (BMFT) for financial support.
M. Schatzmann et al. / J. Wind Eng. lnd. Aerodyn. 54/55 (1995) 483-491
491
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