I need help regarding a practical problem I'm facing. It's not a homework/coursework, It's a real-life problem. I have the values of a vector field (wind) given in six coordinates in 3D space. Those coordinates are the vertices of a triangular right prism (looks similar to my problem: http://i.imgur.com/2TU7D.png). I would like to find the flux of this vector field through each lateral face of the prism. The components of the vector field may be approximated by linear functions. My goal is to find the flux through the top face, given that the flux through the bottom face is null. I'm trying to solve this numerically, i.e., using tools such as Matlab/Octave. How could I do this? Any suggestion (a reference for a method, a similar example, etc.) is welcome. Sorry if this isn't the best place to post this kind of question, but since it's about flux I thought it would be included in General Physics section.