How to calculate drag/lift forces, using SIMPLE method.
Could anyone explain to me how to calculate drag and lift forces on an object in my system, I am using the SIMPLE method to solve the incompressible NS equations. I know they depend on both the pressure and viscous force but my problem is in calculating these forces. I know the pressure force is the integration of the pressure over the surface area of the object, which I attempted to get by using the summation of P(i,j)*dx (or dy for the y component of the force), but that didn't seem to give me good results. Thanks for any help, even if you could explain one of the forces.

Use Gauss quadrature instead of merely summing up p*dx ( summing up of p*dx will be accurate only when the number of points is large).
Dominic 
I'm not sure I understand why this would make a difference? Since the only "accurate" points are the points on the grid anyway, any other generated points used for the Gauss quadrature method would only be interpolations of those grid points. Am I right to believe this?

Maybe I am just integrating over the incorrect area? I am doing the numerical integration over the surface of the object in my system. But inside the object the pressure is undefined, so any pressure gradient around there would also be undefined I think. I think this is the real problem... do I do it at the surface, just outside the surface, or something else I am missing? Thanks for any help.

See "Fundamentals of Aerodyanmics" by J.D. Anderson for a good explanation of the aerodynamic forces.

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