1. A computer-implemented method of generating a fluid-flow simulation over a computer-generated surface, the surface comprised of a surface mesh of polygons, the method comprising: obtaining a boundary-layer fluid property for a subset of the polygons of the surface mesh of polygons, the subset of polygons representing a portion of the computer-generated surface; determining, using a computer, a pressure-gradient vector for a selected polygon, the selected polygon belonging to the surface mesh of polygons but not one of the subset of polygons for which a boundary-layer fluid property has been obtained; defining a minimum diffusion rate along the direction of the pressure-gradient vector of the selected polygon; defining a maximum diffusion rate in a direction perpendicular to the direction of the pressure-gradient vector of the selected polygon; defining a diffusion-path vector from a point in the selected polygon to a neighboring point in a neighboring polygon, the neighboring polygon belonging to the subset of polygons for which a boundary-layer fluid property has been obtained; and determining, using the computer, an updated fluid property for the selected polygon using a variable diffusion rate, the variable diffusion rate based on the minimum diffusion rate, maximum diffusion rate, the diffusion-path vector, and the pressure-gradient vector.