# Downforce Underbody Diffusor

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 January 10, 2012, 17:02 Downforce Underbody Diffusor #1 New Member   Philipp Bolay Join Date: Jan 2012 Posts: 6 Rep Power: 5 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? http://www.bilder-hochladen.net/file...xs-2c-9778.jpg http://www.bilder-hochladen.net/file...hxs-2d-fe9f.jp http://www.bilder-hochladen.net/file...xs-2e-68d3.jpg Thanks and regards Phil

 January 10, 2012, 18:25 #2 Senior Member   Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 494 Rep Power: 10 Your picture is showing 0.5 N not 50000 N?

 January 10, 2012, 20:39 #3 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 10,824 Rep Power: 85 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.

 January 11, 2012, 10:00 #4 Senior Member   Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 494 Rep Power: 10 I think Glenn mean areaInt of Pressure, unless I am mistaken.

 January 11, 2012, 12:06 #5 New Member   Philipp Bolay Join Date: Jan 2012 Posts: 6 Rep Power: 5 Hey, Thanks very much for your help! i don't know why i missed the equation for the pressure^^ 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

 January 11, 2012, 12:43 #6 Senior Member   Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 494 Rep Power: 10 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

 January 11, 2012, 17:52 #7 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 10,824 Rep Power: 85 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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