# Car Lift Coefficient Problem.

 User Name Remember Me Password
 Register Blogs Members List Search Today's Posts Mark Forums Read

 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 k-e), and the solution method is (first order upwind). Could you give me any advice. Thank you in advance.

 March 18, 2013, 20:54 #2 Senior Member   Join Date: Aug 2011 Posts: 315 Rep Power: 13 Why is this result incorrect/inappropriate? What should be the reasonable value? It is my suggestion that never use the first-order scheme to produce the final result.

 March 18, 2013, 22:10 #3 New Member   Mi Baigang Join Date: Mar 2013 Posts: 11 Rep Power: 5 may be you can use the other turbulence model and try to use the second-order scheme!

March 19, 2013, 01:46
#4
Member

Join Date: Apr 2012
Posts: 54
Rep Power: 6
Quote:
 Originally Posted by blackmask Why is this result incorrect/inappropriate? What should be the reasonable value? It is my suggestion that never use the first-order scheme to produce the final result.
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
Member

Join Date: Apr 2012
Posts: 54
Rep Power: 6
Quote:
 Originally Posted by xiaomi may be you can use the other turbulence model and try to use the second-order scheme!
I've tried to use RNG K-e but it gave me the same results.

 March 19, 2013, 10:02 #6 Senior Member   OJ Join Date: Apr 2012 Location: United Kindom Posts: 475 Rep Power: 12 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
Member

Join Date: Apr 2012
Posts: 54
Rep Power: 6
Quote:
 Originally Posted by oj.bulmer 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
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 New Member   Mi Baigang Join Date: Mar 2013 Posts: 11 Rep Power: 5 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 Member   Thiagu Join Date: Oct 2012 Location: India Posts: 59 Rep Power: 6 Why dont you try with one-equation model spallart allmaras, recommended for external flows

April 5, 2013, 01:20
#10
Member

Join Date: Apr 2012
Posts: 54
Rep Power: 6
Quote:
 Originally Posted by xiaomi if possible, you can upload you .cas and .dat files then we can check them to find the problem
Unfortunately, i can't upload them because of the big size, but i've uploaded some pictures of the mesh and B.C
Attached Images
 1.png (66.8 KB, 25 views) 2.jpg (93.0 KB, 29 views) 3.jpg (64.5 KB, 28 views)

 April 5, 2013, 05:14 #11 New Member   Mi Baigang Join Date: Mar 2013 Posts: 11 Rep Power: 5 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 Rep Power: 5 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.

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post colopolo FLUENT 5 April 12, 2013 10:59 LittleBart CFX 4 June 29, 2011 02:33 Nick R CFX 5 March 8, 2011 03:21 Rif Main CFD Forum 4 March 9, 2010 11:52 pc Main CFD Forum 8 October 26, 2006 10:47

All times are GMT -4. The time now is 14:59.

 Contact Us - CFD Online - Top