[ganesh chakravarthi]

how to calculate 'drag coefficient' in CFX? I am working on different car models to calculate 'cd',but am unable to get.for each car model i have created Domain, and used this expression:
dragcoeff: force_x()@body/(0.5*airrho*speed^2*area)
airrho: 1.2250[kg/m^3]
speed: 16.67[m/s]
area: which area i have to choose for my model?whether carbody area or domain area?
bodydrag: force_x()@body
these are the expressions i have given for my model..whether these expressions are correct OR not?? please suggest the expression for finding drag coeff.

[singer1812]

I suggest first year fluids book.

The area is typically the projected frontal area of the body, to the flow direction.

[ganesh chakravarthi]

how to calculate frontal area for a car body??

[singer1812]

Dude! Its your model....

[Lance]

You should know the frontal area of your own model (from drawings, cad or whatever), but you can also get it from the solver. If your front area is a separate surface called front, and the car is travelling in the x-direction, the the frontal area is given by the CEL expression:

area_x()@front

[ghorrocks]

I am not sure of the behaviour of area_x - if you have something like a spherical surface and you do area_x on it, what does it return?

* Does it return zero (as the positive and negative sides of the face cancel out)
* Does it return the projected area of the sphere, ie pi*r^2
* Does it return 2x the projected area (ie 2*pi*R^2) as it gives you the absolute value of the projected area of the front and back faces?

I guess I could just do a simple test and find out but if somebody already knows the approach CFX takes that would be better.