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-   -   how to calculate cd -- drag coefficient?????? (https://www.cfd-online.com/Forums/cfx/22405-how-calculate-cd-drag-coefficient.html)

 vijesh joshi April 6, 2006 03:12

how to calculate cd -- drag coefficient??????

Hi, I have simulated the flow over an aerospace vehicle. Now I can get the drag force on the surface of the vehicle(with the help of readymade options that are available in CFX-post). I know that, cd = drag force / (dynamic pre * reference area). Whatever the cd that I have obtained is being half of the exact value. I am not getting the mistake. Plz help. Thank you.

 Stuman April 6, 2006 05:55

Re: how to calculate cd -- drag coefficient??????

OK, quick question, have you used a symmetry plane? If so, then you have to double the force. Stu

 Michael Bo Hansen April 6, 2006 08:01

Re: how to calculate cd -- drag coefficient??????

Remember this computing task list: <Li> Lift is easy[*] Drag is hard[*] Moment is imposibble

- Michael

 PetrK April 6, 2006 08:17

Re: how to calculate cd -- drag coefficient??????

Hi I have obtained for a 3D wing a good results even for Cm Coeff. However its difficult to compare it to other methods since every numeric method has serious problems with moments.

 PetrK April 6, 2006 08:18

Re: how to calculate cd -- drag coefficient??????

 Glenn Horrocks April 6, 2006 19:07

Re: how to calculate cd -- drag coefficient??????

Hi,

Yes, check your reference area; but also most people calculate drag from Cd = D / (A * .5 * rho * V^2) (http://www.grc.nasa.gov/WWW/K-12/airplane/dragco.html) and that has a factor of 2 in it. If you left it out you would be out by exact 2! Maybe that's it.

Regards, Glenn Horrocks

 vijesh April 6, 2006 23:35

Re: how to calculate cd -- drag coefficient??????

Thanks to all. I obtained the results.

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