Fully and semi-implicit robust space–time DG methods for the incompressible Navier–Stokes equations

We carry out a stability and convergence analysis of a fully discrete scheme for the time-dependent Navier–Stokes equations resulting from combining an [Formula: see text]-conforming discontinuous Galerkin spatial discretization, and a discontinuous Galerkin time-stepping scheme. Such a scheme is proven to be pressure robust and Reynolds semi-robust. Standard techniques can be used to analyze only the case of lowest-order approximations in time. Therefore, we use some nonstandard test functions to prove existence of discrete solutions, unconditional stability, and quasi-optimal convergence rates for any degree of approximation in time. In particular, a continuous dependence of the discrete solution on the data of the problem, and quasi-optimal convergence rates for low and high Reynolds numbers are proven in an energy norm including the term [Formula: see text] for the velocity. In addition to the standard discontinuous Galerkin time-stepping scheme, which is fully implicit, we propose and analyze a novel high-order semi-implicit version that avoids the need of solving nonlinear systems of equations after the first time slab, thus improving the efficiency of the method. Some numerical experiments validating our theoretical results are presented for both schemes.