How to determine flow direction through plane surface
Client has defined plane. When mass flux through plane surface is reported, client cannot tell which is positive (or negative) direction for the face. The bulk flow direction is difficult to determine by inspection of the velocity vectors on the face.
mdot = - SUM_i (rho V_i . n_i A_i) for all i on the surface.
Thus, mdot = (+) for V_inlet, (-) for P_outlet.
For internal surface, n is arbitrary. So we do the following:
1. Initialize V = V_guess so that it is approximately perpendicular to the face in question
2. Compute mdot_surface using Reports->Surface Integral... Mass flow rate
3. SOLVE Problem
4. Compute mdot_actual using Reports->Surface Integral... Mass flow rate
5. If sign(mdot_actual) = sign(mdot_surface), then flow is predominantly in the direction V_guess.