# Drag prediction

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 March 22, 2009, 18:37 Drag prediction #1 New Member   Martin Gariepy Join Date: Mar 2009 Location: Laval Posts: 13 Rep Power: 8 Dear all, I've got two questions for you. First off all, does any one know how Fluent calculate drag on a profil? 1)Based on a converged solutionn from Fluent, I integrated myself the pressure on the profile with the concept of finite volume (ie Pressure is constant on the control volume, so I did the sum of (P-P_inf)*nx*ds where P is the constant pressure in the control volume) but my answer is far from the one from Fluent. I though i made a mistake while integrating the pressure so I imported the solution in Tecplot but I found exactly the same drag as I found.... So I wondering if Fluent doest something special while intragrating the pressure. 2) In a second hand, based on theory of the momentum, the integration of pressure (x direction) on the profile (nearfield drag) must equal the integration of pressure + momentum around a control volume including the profile (Farfield method of drag). This equaltity must be precise up to machine precision in case of a conservative scheme. (In fact, what I mean is simply that the force the profile applied on the fluid is equal and opposite to the force the fluid exerced on the profile). The problem is that this statement doest seem to be true inside the converged solution of fluent. I have difference up to 100N beetween the farfield and nearfield integration. Anybody can help ? Thanks

 March 23, 2009, 05:38 #2 Senior Member   Join Date: Mar 2009 Posts: 225 Rep Power: 9 To answer your question, try Fluent User Manual 22.2.1 Equations of Motion for Particles. Hope it helps.

 March 25, 2009, 20:02 #3 New Member   Martin Gariepy Join Date: Mar 2009 Location: Laval Posts: 13 Rep Power: 8 Thanks but that not exactly what I'm searching. I talking about how the drag is compute (what kind of interpolation) on a profil of a wing in a 2d simulation.... Anyway, thanks for your support

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