K. R. Arun

K. R. Arun, S. Samantaray

We consider two distinguished asymptotic limits of the Euler equations in a gravitational field, namely the incompressible and Boussinesq limits. Both these limits can be obtained as singular limits of the Euler equations under appropriate scaling of the Mach and Froude numbers. We propose and analyse an asymptotic preserving (AP) time discretisation for the numerical approximation of the Euler system in these asymptotic regimes. A key step in the construction of the AP scheme is a semi-implicit discretisation of the fluxes and the source term. The non-stiff convective terms are treated explicitly whereas the stiff pressure-gradient and source term are implicit. The implicit terms are combined to get a nonlinear elliptic equation. We show that the overall scheme is consistent with the respective limit system when the Mach number goes to zero. A linearised stability analysis confirms the $L^2$-stability of the proposed scheme. The results of numerical experiments validate the theoretical findings.

Megala Anandan, K. R. Arun, Amogh Krishnamurthy et al.

We design an energy-stable and asymptotic-preserving finite volume scheme for the compressible Euler system. Using the relative energy framework, we establish rigorous error estimates that yield convergence of the numerical solutions in two distinct regimes. For a fixed Mach number $\varepsilon>0$, we derive error estimates between the numerical solutions and a strong solution of the compressible Euler system that are uniform with respect to the discretisation parameters, ensuring convergence as the underlying mesh is refined. In the low Mach number regime, we analyse the error between the numerical solutions and a strong solution of the incompressible Euler system and obtain asymptotic error estimates that are uniform in $\varepsilon$ and the discretisation parameters. These results imply convergence of the numerical solutions toward a strong solution of the incompressible Euler system as $\varepsilon$, and the discretisation parameters simultaneously tend to zero. Numerical experiments are presented to validate the theoretical analysis.

Sebastian Noelle, Georgij Binev, K. R. Arun et al.

We propose a low Mach number, Godunov-type finite volume scheme for the numerical solution of the compressible Euler equations of gas dynamics. The scheme combines Klein's non-stiff/stiff decomposition of the fluxes (J. Comput. Phys. 121:213-237, 1995) with an explicit/implicit time discretization (Cordier et al., J. Comput. Phys. 231:5685- 5704, 2012) for the split fluxes. This results in a scalar second order partial differential equation (PDE) for the pressure, which we solve by an iterative approximation. Due to our choice of a crucial reference pressure, the stiff subsystem is hyperbolic, and the second order PDE for the pressure is elliptic. The scheme is also uniformly asymptotically consistent. Numerical experiments show that the scheme needs to be stabilized for low Mach numbers. Unfortunately, this affects the asymptotic consistency, which becomes non-uniform in the Mach number, and requires an unduly fine grid in the small Mach number limit. On the other hand, the CFL number is only related to the non-stiff characteristic speeds, independently of the Mach number. Our analytical and numerical results stress the importance of further studies of asymptotic stability in the development of AP (asymptotic preserving) schemes.