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Cola1988 January 10, 2012 17:02

Downforce Underbody Diffusor
Hello Everybody,

I'm from a Formula Student Team and i'm simulation the aerodynamic concepts. At the time i'm simulating the underbody diffusor.
My problem is I can't display the downforce in Y-direction... It normally brings totaly unnormal values of about 50000N....

My Expression says: force_y()@underbody

underbody is the bottom surface ( I selected it as a separate part in ICEM) of the underbody...
Do i have to select gravity or somthing other?

Am i doing it wrong the hole way?
Thanks and regards


singer1812 January 10, 2012 18:25

Your picture is showing 0.5 N not 50000 N?

ghorrocks January 10, 2012 20:39

The force variable is not meaningful as you have shown it. What you are showing is the force on each element face, and the areaAve() function is giving the average force on a element - neither of which is meaningful as it changes with element size.

What you want to do is do an area integral of force over the surface. This will give you the total force on the body which is the number of physical importance.

singer1812 January 11, 2012 10:00

I think Glenn mean areaInt of Pressure, unless I am mistaken.

Cola1988 January 11, 2012 12:06


Thanks very much for your help! i don't know why i missed the equation for the pressure^^ :(:eek:

The Problem is i'm getting really unnormal values..

for: areaint(pressure)@underbody = around 90000N ... That can't be true..

I calculatet it in the CFD post function calculator..
Do i have to set on gravity in CFX-pre?

The inlet is set to 22 m/s.

Thanks, Phil

singer1812 January 11, 2012 12:43

Why would you need gravity?

Is the Underbody the entire structure shown in your picture or is it just one side of the structure.

If it is just one side, you need to include the other side because you are not including the backside pressure in your force calc.

Also another way to get your force out is force_y@Underbody or use force with axis y in the function calculator

ghorrocks January 11, 2012 17:52

I meant the force_x/y/z functions. The areaInt function will return only the pressure component of the force and not the wall shear. Force gives you the total force. But it won't make much of a difference in this case, the pressure force should be just about all the force.

But if you are getting massive forces, what is your reference pressure? If you have a zero reference pressure the force on a surface like this would be massive. To fix this set the reference pressure to the local ambient pressure, or better still the pressure which acts on the other side of the diffuser (if different to ambient).

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