# How CFX calculates forces

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 January 21, 2004, 08:39 How CFX calculates forces #1 Marco Aurelio Melo Guest   Posts: n/a Does anyone know, how does CFX calculate the forces that are reported in the post.I'm simulating a car and i want to know the drag and lift forces.

 January 21, 2004, 18:06 Re: How CFX calculates forces #2 Glenn Horrocks Guest   Posts: n/a Hi Marco, The force is a variable at each element face, which represents the force acting on that face of the individual element. This means the values of force at the element faces are mesh size dependant (ie, halve the element area and you halve the force). This means looking at the force on a per element basis is in general pretty useless. You can sum it up over the entire face, then that gives you the total force on that face. This is a far more useful parameter, and is mesh independant as long as you have done a good simulation. To calculate drag and lift on a car, do the simulation, and using the function calculator to sum the Force X, Y or Z components over the car surfaces to give you your lift, drag and side thrust components. Glenn

 January 22, 2004, 08:52 Re: How CFX calculates forces #3 Marco Aurelio Melo Guest   Posts: n/a But do you know if CFX calculates the forces in the point [0 0 0] or in other place?

 January 22, 2004, 17:50 Re: How CFX calculates forces #4 Glenn Horrocks Guest   Posts: n/a Hi Marco, I don't know what you mean. Are you trying to find the centre of pressure on your body or something like that? Glenn

 January 23, 2004, 15:59 Re: How CFX calculates forces #5 James Date Guest   Posts: n/a Hi Marco If i remeber correctly, CFX calculates the forces in the x,y & z directions about the point 0,0,0. I think you can change this, but there is no real need. All you need to do is resolve the forces in the coordinate system of your choice using a bit of simple trig. When calculating lift/drag on any body remember the lift acts perpendicular to the inflow direction and the drag parallel to it, the number of times I've seen students make this fundamental mistake and get incorrect results and wonder why! Good luck with your modelling. James

 January 26, 2004, 09:01 Re: How CFX calculates forces #6 Marco Aurelio Melo Guest   Posts: n/a James, I'm having a lot of problems with the volume mesh, even if I turn on the surface proximity, do you have some idea of wath can I do about this problem. Thanks. Marco

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