We present a higher order discretization scheme for the compressible Euler and Navier-Stokes equations with immersed boundaries. Our approach makes use of a discontinuous Galerkin discretization in a domain that is implicitly defined by means of a level set function. The zero iso-contour of this level set function is considered as an additional domain boundary where we weakly enforce boundary conditions in the same manner as in boundary-fitted cells. In order to retain the full order of convergence of the scheme, it is crucial to perform volume and surface integrals in boundary cells with high accuracy. This is achieved using a linear moment-fitting strategy. Moreover, we apply a non-intrusive cell-agglomeration technique that averts problems with very small and ill-shaped cuts. The robustness, accuracy, and convergence properties of the scheme are assessed in several two-dimensional test cases for the steady compressible Euler and Navier-Stokes equations. Approximation orders range from 0 to 4, even though the approach directly generalizes to even higher orders. In all test cases with a sufficiently smooth solution, the experimental order of convergence matches the expected rate for discontinuous Galerkin schemes. 4 B. MÜLLER ET AL.intersected by the immersed boundary, which allows us to avoid integration sub-cells and, as a result, to extend the scheme to higher approximation orders. Second, we do not require any reformulation of the boundary conditions, which allows us to evade the limitation to adiabatic slip walls. Finally, we have extended the approach to the compressible Navier-Stokes equations.
State of the artMethods in the spirit of the original IBM by Peskin [1] have drawn much interest ever since their introduction in 1972 [2]. Within the present work, the term IBM is used in the general sense of methods making use of computational domains that do not conform with the problem domain, hence separating the aspects of discretization and geometry representation to a large extent. This idea is the core of many modern developments in the context of the finite element method such as the extended finite element method (XFEM) [5], the finite cell method [6], Nitsche-type methods [7], and even the finite difference method [8]. In the following, we will focus on numerical methods of the finite element type that are able to deliver higher order convergence rates in the presence of curved immersed problem geometries.First efforts in this direction using XFEM/Nitsche-type approaches have been presented in [9,10] in the context of linear elasticity and in [11] in the context of Stokes flow. Both approaches rely on planar surface triangulations for the numerical integration of the weak forms, which renders the introduction of quadrature sub-cells inevitable if higher order convergence of the scheme is desired. An interesting alternative has been proposed in [12] where special enrichment functions for common features such as circular sections or corners are introduced. Unfortunately, the presented scheme appears to be...