Resolving fluid boundary layers with particle strength exchange and weak adaptivity

Most fluid scenarios in graphics have a high Reynolds number, where viscosity is dominated by inertial effects, thus most solvers drop viscosity altogether: numerical damping from coarse grids is generally stronger than physical viscosity while resembling it in character. However, viscosity remains crucial near solid boundaries, in the boundary layer, to a large extent determining the look of the flow as a function of Reynolds number. Typical graphics simulations do not resolve boundary layer dynamics, so their look is determined mostly by numerical errors with the given grid size and time step, rather than physical parameters. We introduce two complementary techniques to capture boundary layer dynamics, bringing more physical control and predictability. We extend the FLIP particle-grid method with viscous particle strength exchange[Rivoalen and Huberson 2001] to better transfer momentum at solid boundaries, dubbed VFLIP. We also introduce Weakly Higher Resolution Regional Projection (WHIRP), a cheap and simple way to increase grid resolution where important by overlaying high resolution grids on the global coarse grid.