# External vehicle flow - help with set up

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 April 10, 2017, 13:01 External vehicle flow - help with set up #1 New Member   Jake Join Date: Apr 2017 Posts: 2 Rep Power: 0 Hi, I'm relativley new to using CFD software and am having issues trying to get good values for Cd and Cl. I'm using Star CCM 11.02. I used Beta ANSA to clean up and mesh my geometry with inlfation layers growing from the vehicle and road. There's a large area for the wake and in front of the vehicle with a refined mesh area around, infront of and behind the vehicle too. I have a velocity inlet of 70mph, a pressure outlet, walls for the road and vehicle and symmetry planes for all else. As for physics settings I have used; gas, constant density, coupled flow, k-epsilon turbulence as well as the standard settings. I have left the defualt initial condiditons the same except for initial velocity at 70mph and pressure of 1.03 bar. For results so far I am getting a spike at around 350-400 iterations of Cd to a value of 320 and for Cl to 6. Residuals are between 1 and 0.01 which I know should be more 0.001 - 0.0001. Any help would be appreciated! Thanks Jake

 April 10, 2017, 18:17 #2 Senior Member   Join Date: Nov 2010 Location: USA Posts: 1,232 Rep Power: 24 Problems like this are almost always mesh related, please post pictures of your mesh.

 April 11, 2017, 04:17 #3 Senior Member   Vangelis Skaperdas Join Date: Mar 2009 Location: Thessaloniki, Greece Posts: 287 Rep Power: 21 When you write "a spike" you mean that the solution explodes completely? 400 iterations are not enough to get a converged solution. Also have you set correct reference values for Velocity, Density and reference Area for the calculation of Cd and Cl? Do you have correct units? Have you created the mesh in mm? Are you solving in metres? Have you checked the quality of the mesh in ANSA before outputting it?

 Tags automotive aerodynamics, cfd, external flow, star ccm, vehicle aerodynamics