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sam May 20, 2005 04:26

Coefficient of drag
Dear all, I am doing aerodynamic analysis of car body. I would like to calculate Co-efficient of drags and lifts when wind in blowing at different angles. I tried with ACOF command but it's giving negative cd value along flow!! is there any other method to calculate cd values? please help me DK

Ant May 20, 2005 05:38

Re: Coefficient of drag
You can calculate force coefficients integrating forces around a necessary body part and projecting in n-direction! If you've got a negative value of drag coeff, most likely, you made some mistakes processing obtained data or if your solution is transient, you can observe a negative drag coeff. in some time steps. For understanding why so occurred you should give more information!

shyam May 22, 2005 22:38

Re: Coefficient of drag
Hi Ant, My run is Steady case only. Convergence criteria 10e-5.How can I project forces in some direction?

Ant May 24, 2005 15:00

Re: Coefficient of drag
For that you have to collect necessary wall cells generated Star-cd and load wall forces using post registers!

The steps:

1) Using command getw,... PROSTAR generate the different shells (wall, no wall, symmetry, etc).

2) Now, your task is to collect necessary cells from created shells above. This may be done several ways!

3) Use post registers to load forces (getw,... or using a menu)! You can load a component of either shear force or total force! Don't forget to select a necessary coordinate system (for that, use command psys,...)

4) Now, when you have got the loaded post registers you can integrate n-component of the force summing values in any post register (for that, use the command *get,...)

5) To get a coefficient, divide the result obtained above by dynamic pressure and base area!

Better, type a script for it!

Anton Lyaskin May 26, 2005 05:47

Re: Coefficient of drag
Or you can simply assign your surface of interest to a separate wall boundary region and turn on engineerng data output for them (i.e. forces). After the solution you'll get an .erd file, containing forces in global coordinate system, system 1. You need last free values - total forces. You can convert them to coefficients and project on any coordinate system

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