1. Method for forming a fixed geometric grid for a fluid dynamics system multi-cell computer model configured for use with finite volume transport equations for calculating the flow of one or more of mass, momentum, and energy into and out of each model cell, the system having at least one surface which is entirely closed, the method being executed in a digital computer and comprising the steps of electronically: representing the system surface by an array of contiguous polygons, wherein each polygon in the surface shares a side with only one other polygon; creating a single model volumetric cell that encompasses the entire system and is spatially fixed; assigning all the surface polygons to the single model volumetric cell; subdividing at least a portion of the single model volumetric cell encompassing the surface polygon array using one or more subdividing planes to provide a fixed grid having a plurality of subdivided volumetric cells, wherein said one or more subdividing planes also subdivide at least one surface polygon to provide subdivided surface polygon parts; reassigning all of the surface polygons and said subdivided surface polygon parts to respective ones of the subdivided volumetric cells resulting from the subdividing step; identifying those subdivided volumetric cells having two or more reassigned surface polygons and polygon parts from the system surface; calculating effective boundary areas and normal vectors for the reassigned surface polygons and polygon parts of those identified subdivided volumetric cells; wherein the calculated effective boundary areas and normal vectors are adapted to be used in the simulation of mass, momentum or energy transport for respective subdivided volumetric cells of the multi-cell computer model across one or more of said effective boundary areas.