say you need to compute drag coefficient on a car body and let us assume that the flow direction is x,

Drag Coefficient = Traction_x ( on car body ) / ( 0.5  * rho * Uref * Uref * Aref ) Here Aref is the projected area.

To create the required UDF in AcuProbe,

1. Edit -> User Function

2. In the Name field, enter “cd”

3. In the Function area type as below. For getting x-traction on surface “Body”, right click on x_traction under Surface Output – Body and select copy name and paste it in the text area.

4. Click on “Apply” and AcuProbe will plot the value of Cd.

One of the things we do in VWT application is report both Drag Area and Drag Coefficient.  Sometimes, differences in drag coefficient are because of differences in reference area used.