cuHARM : a new GPU accelerated GR-MHD code and its application to ADAF disks

TL;DR

This paper introduces cuHARM, a GPU-accelerated GR-MHD simulation code enabling high-resolution accretion disk and jet studies on single servers, revealing insights into magnetic field effects and jet structure.

Contribution

The paper presents cuHARM, a novel GPU-based GR-MHD code that significantly accelerates simulations, allowing detailed studies of accretion disks and jets without supercomputers.

Findings

Magnetic field increase does not affect mass accretion rate in SANE disks.

Jet destruction occurs when magnetic flux decreases below a threshold.

Jet width is weakly dependent on the gas's adiabatic index.

Abstract

We introduce a new GPU-accelerated general-relativistic magneto-hydrodynamic (GR-MHD) code based on HARM which we call cuHARM. The code is written in CUDA-C and uses OpenMP to parallelize multi-GPU setups. Our code allows us to run high resolution simulations of accretion disks and the formation and structure of jets without the need of multi-node supercomputer infrastructure. A $256^3$ simulation is well within the reach of an Nvidia DGX-V100 server, with the computation being a factor about 10 times faster if only the CPU was used. We use this code to examine several disk structures all in the SANE state. We find that: (i) increasing the magnetic field, while in the SANE state does not affect the mass accretion rate; (ii) Simultaneous increase of the disk size and the magnetic field, while keeping the ratio of energies fixed, lead to the destruction of the jet once the magnetic flux through the horizon decrease below a certain limit. This demonstrates that the existence of the jet is not a linear function of the initial magnetic field strength; (iii) the structure of the jet is a weak function of the adiabatic index of the gas, with relativistic gas tend to have a wider jet.