Vehicle Aerodynamics
To investigate the aerodynamic performance of a road vehicle, three basic methods can be taken into consideration: road testing, low speed wind tunnel testing and computational method.
All of these methods have their advantages and disadvantages, and which method to use is heavily dictated by the budget and availability of ones facilities.
In our case we have decided to go with a small-scale low speed wind tunnel testing method. The Aerospace Department at San Diego State University already owns this type of tunnel testing facility.
The advantage of low speed wind tunnel testing is that the environment can be controlled. However, inaccuracy in fabrication of the scaled model will not yield exact full-scale conditions. The scale must be chosen carefully based on the test section size of the tunnel. This would greatly reduce the magnitude of aerodynamic problems introduced by the wind tunnel walls (solid blockage) and stationary floor. Thus resulting in data that is different from full-scale road testing.
All that said; why wind tunnel testing? Practice shows that wind tunnel testing, when correctly proceeded has an accuracy of about 90%.
Blockage
The blockage (ratio of model frontal-area to test-section-area) shouldn’t exceed 7.5%. Knowing the frontal area of the vehicle, test section area and scale (chosen 1/5th), one now can calculate the blockage in percentage for the scaled vehicle.
Test Model Scale:
| Wind Tunnel cross section area: 9.764 ft2 |
| HEV cross section area: 12.5 ft2 |
| Chosen Scale: 1/5th scale |
| Calculated Blockage in Percentage with 1/5th scale: |
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Reflection effect/ Correction Method
In addition to the blockage effect there is a reflection effect that will cause a change in lift of lifting surfaces near solid boundaries (for example ground effect), therefore wind tunnel corrections have to be taken in account.
One of the correction methods is shown below:
where:
| A = Model frontal area | r = density of air |
| C = test-section, cross-section area | V = velocity of wind tunnel |
| c = corrected | |
| m = measured |
These corrections can be applied to any aerodynamic coefficient (cd,cl,q..).
Moving Ground
One of the problems in a wind tunnel testing facility might be the stationary floor. The velocity profile between on the road and the wind tunnel conditions looks different (see Fig. a, b).
| a) Velocity profile, Vehicle on road | b) Velocity profile, Vehicle in wind tunnel |
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One of the approaches to reduce the phenomena is to equip the wind tunnel with an elevated ground plane that creates a much thinner boundary layer than the wind tunnel floor. (see Fig. below )
Velocity profile with integrated ground plane:
Suction
Another effect that might occur while testing is when the gap between wheels and floor isn’t sealed. It will cause a reduction of lift up to dcl = -0.45, but doesn’t have a large effect on drag.< /FONT >
Adding a foam block in between wheels and floor can easily reduce the effect. (see Fig. below)
The primary reason for testing our vehicle in a wind tunnel facility is to get the drag coefficient of the body, which will be used for calculations in load reduction (part of total power loss).
The accuracy of the reduced testing data certainly depends on the design of the wind tunnel model. But being too detailed isn’t always possible on a scaled model. Therefore we have to find a compromise between them.
Testing configurations play an important role in the design of the model. In our case we would like to get a clearer picture on how and if a bottom pan as also targa-top will affect the total drag. To compare the data of those configurations, the design has to reflect the full-scale car in those areas very closely.
The sketch below is a first approximation of the test model. It shall give a basic idea on how to and what materials to use for manufacturing.
HEV Model Sketch (click on thumbnail for full-size drawing)
Test Configurations
As mentioned before the model was designed to allow different configurations (add on) while testing in the wind tunnel. The following list is a short description of configurations to be used.
Model Test-Configurations:
HEV Team
Department of Mechanical Engineering
San Diego State University
5500 Campanile Dr.
San Diego, CA 92182-1323
Fax: (619) 594-3599
E-mail: hev@kahuna.sdsu.edu