A short note on the discontinuous Galerkin discretization of the pressure projection operator in incompressible flow

This note reports on the issue of spurious compressibility artifacts that can arise when the popular pressure projection (PP) method is used for unsteady simulations of incompressible flow using the symmetric interior penalty discontinuous Galerkin (SIP-DG) method. Through a spectral analysis of the projection operator’s SIP-DG discretization, we demonstrate that the eigenfunctions of the operator do not form a basis that allows for the correct enforcement of the incompressibility constraint. This short-coming can cause numerical instabilities for inviscid, advection-dominated, and density stratified flow simulations, especially for long-time integrations and/or under-resolved situations. To remedy this problem, we propose a local post-processing projection that enforces incompressibility exactly to allow for stable and robust long-time integrations.