Application of the Cubed-Sphere Grid to Tilted Black-Hole Accretion Disks

TL;DR

This paper introduces a cubed-sphere grid implementation in GRMHD simulations of tilted black-hole accretion disks, enabling better resolution of polar regions and analysis of jet formation, with validation tests showing expected performance.

Contribution

The authors developed and validated a cubed-sphere grid for GRMHD simulations, improving resolution of tilted disks and polar regions compared to traditional spherical-polar grids.

Findings

The cubed-sphere grid performs as expected in validation tests.

Disk evolution depends on orientation relative to grid alignment.

Differences in current sheet treatment affect early disk evolution.

Abstract

In recent work we presented the first results of global general relativistic magnetohydrodynamic (GRMHD) simulations of tilted (or misaligned) accretion disks around rotating black holes. The simulated tilted disks showed dramatic differences from comparable untilted disks, such as asymmetrical accretion onto the hole through opposing "plunging streams" and global precession of the disk powered by a torque provided by the black hole. However, those simulations used a traditional spherical-polar grid that was purposefully underresolved along the pole, which prevented us from assessing the behavior of any jets that may have been associated with the tilted disks. To address this shortcoming we have added a block-structured "cubed-sphere" grid option to the Cosmos++ GRMHD code, which will allow us to simultaneously resolve the disk and polar regions. Here we present our implementation of this grid and the results of a small suite of validation tests intended to demonstrate that the new grid performs as expected. The most important test in this work is a comparison of identical tilted disks, one evolved using our spherical-polar grid and the other with the cubed-sphere grid. We also demonstrate an interesting dependence of the early-time evolution of our disks on their orientation with respect to the grid alignment. This dependence arises from the differing treatment of current sheets within the disks, especially whether they are aligned with symmetry planes of the grid or not.