1. Method for forming a fixed geometric grid for a fluid dynamics system multi-cell computer model, the system having one or more surfaces, the method including: representing the system surfaces by an array of contiguous polygons; creating a single model cell that encompasses the entire system; assigning all the surface polygons to the single model cell; subdividing at least the portion of the single model cell encompassing the surface polygon array, including subdividing all intersected assigned polygons, using one or more subdividing planes; reassigning all of the surface polygons to respective subdivided cells resulting from the subdividing step; identifying those subdivided cells having two or more surface polygons from a common system surface; and calculating effective boundary areas and normal vectors for the surface polygons for those identified cells.