Hello,

I would like to calculate the resultant forces that a viscous fluid exerts on a flat surface, suppose that the object is in 3D, is the total force calculated as follows?:

varFf([u1,u2,u3],[v1,v2,v3]) = int2d(Th, Allfaces)= (pressure*N.x*v1 + pressure*N.y*v2 + pressure*N.z.v3) + (tau_xy*(v1*N.y + v2*N.x) + tau_xz*(v1*N.z + v3*N.x) + tau_yz*(v2*N.z + v3*N.y))

I took into account the pressure and shear stress terms.