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March 18, 2013, 14:12 
Car Lift Coefficient Problem.

#1 
Member
AHMAD
Join Date: Apr 2012
Posts: 54
Rep Power: 6 
Hi,
I'm trying to simulate air flow around peugeot 206CC to calculate the values of drag and lift coefficients, but after finishing the simulation and doing the calculations i get good results of drag coefficeint(approximatly Cd=0.28), unlike the results of the lift coefficient(Cl=0.4) The turbulence model being used is (Standard ke), and the solution method is (first order upwind). Could you give me any advice. Thank you in advance. 

March 18, 2013, 20:54 

#2 
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Why is this result incorrect/inappropriate? What should be the reasonable value? It is my suggestion that never use the firstorder scheme to produce the final result.


March 18, 2013, 22:10 

#3 
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Mi Baigang
Join Date: Mar 2013
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may be you can use the other turbulence model and try to use the secondorder scheme!


March 19, 2013, 01:46 

#4  
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AHMAD
Join Date: Apr 2012
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Quote:
After searching on the internet i found that the car lift coefficient for this type of cars should be positive (around 0.1). I've tried to use second order scheme, but when using it i get convergence problems. 

March 19, 2013, 01:47 

#5 
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AHMAD
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March 19, 2013, 10:02 

#6 
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OJ
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Have you done mesh independence? Are you sure the solution is converged? Have you tried second order scheme along with RNG? Although, my choice of model would be Realizable model.
OJ 

March 23, 2013, 05:53 

#7  
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AHMAD
Join Date: Apr 2012
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Quote:
I've tried the Relizable model+Second order, but i still get the same results (negative lift coefficient), even though the drag coefficient results are good (which means that the pressure coefficient variation along the car should be true) 

March 25, 2013, 09:51 

#8 
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Mi Baigang
Join Date: Mar 2013
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if possible, you can upload you .cas and .dat files then we can check them to find the problem


March 25, 2013, 10:19 

#9 
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Thiagu
Join Date: Oct 2012
Location: India
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Why dont you try with oneequation model spallart allmaras, recommended for external flows


April 5, 2013, 01:20 

#10 
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AHMAD
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Unfortunately, i can't upload them because of the big size, but i've uploaded some pictures of the mesh and B.C


April 5, 2013, 05:14 

#11 
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Mi Baigang
Join Date: Mar 2013
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hi i think the result of lift may be reasonable due to the ground effect, as the ground is moving, the pressure between the car and ground may be less than the surface of the car, then the lift may be negative, you can check the pressure contour to see what happened!


April 7, 2013, 08:39 

#12 
New Member
Umut Can
Join Date: Oct 2012
Posts: 20
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i think negative lift coefficient is normal. For a car you dont want to fly. You want to feel the ground and attached to it. Race cars are designed some diffuser mechanisims to get more downforce. They have even higher (but negative) lift coefficient then you found. I think you should search more about car aerodynamics. I belive your results are okey. However, you should see mesh independence. Although i am not sure about spalart almaras it seems kind of enough for your case. And you should use second order methods. Furthermore the negative lift is partially is due to ground effect, therefore you must be very carefull when you are determining ground to car distance. There must be a some treshold value at which lift of the car becomes negative. Think like that; if ground clearance is very low flow velocity at the bottom of the car will be higher then the flow at the upper surface of the car which results in downforce(negative lift). But if your ground clearance is high(er then some specific value) the flow at the bottom of the car is slow and this results in positive lift coefficient.


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