CFD Time-Integration Strategies for Large Chemical Systems

To date, we have developed and implemented (into our GASP CFD Slow solver) a loosely-coupled strategy for the fluid-dynamic chemical systems. The approach uses the lower-upper, symmetric, Gauss-Siedel (LU-SGS) implicit scheme to advance the fluid-dynamic system. In most cases, the chemical reactions have much smaller time scales than those associated with the flow, result stiffness due to the source terms. Solution of the chemical system uses a time-operator splitting approach that allows isolation of the stiff source term from the transport terms. As a result, the solution process can be divided into two fractional steps - a transport fractional step and chemical production (or reaction) fractional step. The chemical transport fractional step is currently solved using simple, low-cost Euler explicit scheme. The chemical production fractional step involves solving a system of ordinary differential equations representing the chemical source term and is carried out using a variable coefficient ordinary differential equation solver (DVODE).