Efficient Semi Implicit Time Stepping Schemes For Incompressible Flows PDF Download

This thesis presents a semi-implicit method for simulating inviscid compressible flow and its extensions for strong implicit coupling of compressible flow with Lagrangian solids, and artificial transition of fluid from compressible flow to incompressible flow regime for graphics applications. First we present a novel semi-implicit method for alleviating the stringent CFL condition imposed by the sound speed in simulating inviscid compressible flow with shocks, contacts and rarefactions. The method splits the compressible flow flux into two parts -- an advection part and an acoustic part. The advection part is solved using an explicit scheme, while the acoustic part is solved using an implicit method allowing us to avoid the sound speed imposed CFL restriction. Our method leads to a standard Poisson equation similar to what one would solve for incompressible flow, but has an identity term more similar to a diffusion equation. In the limit as the sound speed goes to infinity, one obtains the Poisson equation for incompressible flow. This implicit pressure solve also lends itself nicely to solve for the pressure and coupling forces at a solid fluid interface. With this pressure solve as the foundation, we then develop a novel method to implicitly two-way couple Eulerian compressible flow to volumetric Lagrangian solids. The method works for both deformable and rigid solids and for arbitrary equations of state. Similar to previous fluid-structure interaction methods, we apply pressure forces to the solid and enforce a velocity boundary condition on the fluid in order to satisfy a no-slip constraint. Unlike previous methods, however, we apply these coupled interactions implicitly by adding the constraint to the pressure system and combining it with any implicit solid forces in order to obtain a strongly coupled system. Because our method handles the fluid-structure interactions implicitly, we avoid introducing any new time step restrictions and obtain stable results even for high density-to-mass ratios, where explicit methods struggle or fail. We exactly conserve momentum and kinetic energy (thermal fluid-structure interactions are not considered) at the fluid-structure interface, and hence naturally handle highly non-linear phenomenon such as shocks, contacts and rarefactions. The implicit pressure solve allows our method to be used for any sound speed efficiently. In particular as the sound speed goes to infinity, we obtain the standard Poisson equation for incompressible flow. This allows our method to work seamlessly and efficiently as the sound speed in the underlying flow field changes. Building on this feature of our method, we next develop a practical approach to integrating shock wave dynamics into traditional smoke simulations. Previous methods for doing this either simplified away the compressible component of the flow and were unable to capture shock fronts or used a prohibitively expensive explicit method that limits the time step of the simulation long after the relevant shock waves and rarefactions have left the domain. Instead, using our semi-implicit formulation allows us to take time steps on the order of fluid velocity. As we handle the acoustic fluid effects implicitly, we can artificially drive the sound speed c of the fluid to infinity without going unstable or driving the time step to zero. This permits the fluid to transition from compressible flow to the far more tractable incompressible flow regime once the interesting compressible flow phenomena (such as shocks) have left the domain of interest, and allows the use of state-of-the-art smoke simulation techniques.