A Hybrid Advection Scheme for Conserving Angular Momentum on a Refined Cartesian Mesh

TL;DR

This paper introduces a hybrid advection scheme that conserves angular momentum with high precision on a Cartesian mesh, improving simulation accuracy for astrophysical fluid flows like accretion disks.

Contribution

The paper presents a novel hybrid advection scheme that combines the advantages of Cartesian meshes with precise angular momentum conservation, validated through eigenfrequency measurements.

Findings

Achieves conservation of angular momentum to machine precision.

Shows good agreement with linear stability analysis and nonlinear simulations.

Requires lower grid resolutions for accurate results.

Abstract

We test a new "hybrid" scheme for simulating dynamical fluid flows in which cylindrical components of the momentum are advected across a rotating Cartesian coordinate mesh. This hybrid scheme allows us to conserve angular momentum to machine precision while capitalizing on the advantages offered by a Cartesian mesh, such as a straightforward implementation of mesh refinement. Our test focuses on measuring the real and imaginary parts of the eigenfrequency of unstable axisymmetric modes that naturally arise in massless polytropic tori having a range of different aspect ratios, and quantifying the uncertainty in these measurements. Our measured eigenfrequencies show good agreement with the results obtained from the linear stability analysis of Kojima (1986) and from nonlinear hydrodynamic simulations performed on a cylindrical coordinate mesh by Woodward et al. (1994). When compared against results conducted with a traditional Cartesian advection scheme, the hybrid scheme achieves qualitative convergence at the same or, in some cases, much lower grid resolutions and conserves angular momentum to a much higher degree of precision. As a result, this hybrid scheme is much better suited for simulating astrophysical fluid flows, such as accretion disks and mass-transferring binary systems.