All computations used for this report have been made with Fluent 5.4, with a car velocity of 108 km/h, a k-epsilon turbulence model, with a second order scheme for pressure and momentum. The Reynolds number associated is 3,000,000.

We also sucessfully made a test run using Star-CD.
 

The following tables report all the computed aerodynamic values, for the three different meshes:

The geometry has been defined with two separate zone: car and glasses. Car roughly regroups all the steel part of the car body, while glasses contains all the windows. The board has its own zone (named.... board). Net is the sum of all the forces.
 

Car without board
 

zone name 
pressure force Fpx (N)
viscous force Fvx (N)
total force Fx (N) 
pressure coefficient Cpx
viscous coefficient Cvx
total coefficient Cx
car
153.96747
 12.554846
 166.52232
 0.23275506
 0.018979359
 0.25173442
glasses
 25.999348
4.068213
 30.067561
 0.03930362
 0.0061499819
 0.045453607
net
 179.96682
 16.62305
 196.58988
 0.27205868
 0.025129341
 0.29718802
Tab 1.a: Efforts in X-direction (without board)

The car has got a coefficient Cx (or Cd) about 0.30, which is very low if compared to real vehicles (about 0.35 to 0.40; VW Polo: 0.37). This value is acceptable if we consider that this car has been designed without wheels or accessories such as rear view mirrors, calender, handles, windscreen wipers...
For exemple, rear view mirrors can account for as much as 5% of the drag.

The main contribution in the total force Fx is due to pressure effects, much greater than viscous effects.
 
zone name
pressure force Fpz (N)
viscous force Fvz(N)
total force Fz (N)
pressure coefficient Cpz
viscous coefficient Cvz
total coefficient Cz
car
-90.95401
0.55450565
-90.399504
-0.13749661
0.00083825495
-0.13665836
glasses
211.78152
1.0603678
212.84189
0.32015347
0.0016029748
0.32175644
net
120.82751
1.6148735
122.44238
0.18265686
0.0024412297
0.18509809

Tab 1.b: Efforts in Z-direction (without board)


Car with board in first configuration
 

zone name 
pressure force Fpx (N)
viscous force Fvx (N)
total force Fx (N)
pressure coefficient Cpx
viscous coefficient Cvx
total coefficient Cx
board
0.50581014
2.7991452
3.3049553
0.00079100812
0.0043774262
0.0051684343
car
169.82745
12.231901
182.05935
0.26558363
0.019128784
0.28471241
glasses
32.161106
4.0656862
36.22679
0.05029495
0.0063580987
0.056653049
net
202.49437
19.096732
221.591
0.31666959
0.029864309
0.34653389
Tab 2.a: Efforts in X-direction (with first configuration)

The computed drag coefficient is now about 0.346. The efforts on the board are not significant compared to car forces, but the presence of this new object on the roof deeply modifies the flow around the car. That's why forces on the car have increased.
 
zone name
pressure force Fpz (N)
viscous force Fvz (N)
total force Fz (N)
pressure coefficient Cpz
viscous coefficient Cvz
total coefficient Cz
board
-47.95068
0.072627157
-47.878053
-0.0724878
0.00010979162
-0.072378009
car
-109.6974
0.47383898
-109.22356
-0.16583129
0.00071630987
-0.16511498
glasses
277.78122
0.84630829
278.62753
0.41992626
0.0012793776
0.42120564
net
120.13314
1.3927744
121.52591
0.18160717
0.0021054791
0.18371264

Tab 2.b: Efforts in Z-direction (with first configuration)


Car with board in second configuration
 

zone name
pressure force Fpx (N)
viscous force Fvx (N)
total force Fx (N)
pressure coefficient Cpx
viscous coefficient Cvx
total coefficient Cx
board
6.6780057
3.1740925
9.8520982
0.010095247
0.0047983258
0.014893572
car
161.84656
12.315351
174.16191
0.244666
0.018617311
0.26328331
glasses
66.516296
3.8547502
70.371046
0.10055374
0.0058272868
0.10638102
net
235.04086
19.344194
254.38506
0.35531498
0.029242923
0.38455791
Tab 3.a: Efforts in X-direction (with second configuration)

The drag coefficient has now increased until 0.385. The major effect of this configuration is that it creates a great overpressure on the windscreen (twice more important than with the first configuration). This overpressure can be seen on the contours of pressure displayed.
zone name
pressure force Fpz (N)
pressure force Fvz (N)
total force Fz (N)
pressure coefficient Cpz
viscous coefficient Cvz
total coefficient Cz
board
-59.732327
0.10096034
-59.631367
-0.090298302
0.00015262334
-0.090145679
car
18.550703
0.39320984
18.943913
0.028043391
0.00059442153
0.028637812
glasses
165.97876
0.65207964
166.63084
0.25091271
0.00098575909
0.25189847
net
124.79714
1.1462498
125.94339
0.1886578
0.001732804
0.19039061

Tab 3.b: Efforts in Z-direction (with second configuration)

The underpressure zone under the board is then larger than for the previous configuration, and the force on the board is then greater. See contours of pressure.


The main results are reported in the following table:
 
 
Cx
Overcost in Cx (Cx_ref= 0.297)
Car without board
0.297
0 %
Car with board in first configuration
0.347
16.8 %
Car with board in second configuration
0.385
29.6 %

Conclusion:

The first configuration is consequently much more economic than the second one. In conclusion, it seems better to put the board nose forward.
Nevertheless, it makes the drag coefficient increase, compared to the same car without anything on its roof.
 
 

Link to pictures: