Testing the effect of resolution on gravitational fragmentation with Lagrangian hydrodynamic schemes

TL;DR

This study compares Lagrangian hydrodynamic schemes, MFV and MFM, with SPH in simulating gravitational fragmentation, revealing convergence criteria, differences in artificial fragmentation, and the impact of kernel choice on resolution.

Contribution

It provides a detailed comparison of Lagrangian schemes for gravitational fragmentation, identifying resolution requirements and kernel effects, which were previously not well understood.

Findings

All schemes converge to the analytic solution when fluid element diameter is less than a quarter of the Jeans wavelength.

None of the Lagrangian schemes exhibit artificial fragmentation at wavelengths shorter than the Jeans wavelength.

Kernel standard deviation is a better measure of fluid element size than support radius.

Abstract

To study the resolution required for simulating gravitational fragmentation with newly developed Lagrangian hydrodynamic schemes, Meshless Finite Volume method (MFV) and Meshless Finite Mass method (MFM), we have performed a number of simulations of the Jeans test and compared the results with both the expected analytic solution and results from the more standard Lagrangian approach: Smoothed Particle Hydrodynamics (SPH). We find that the different schemes converge to the analytic solution when the diameter of a fluid element is smaller than a quarter of the Jeans wavelength, $\lambda_\mathrm{J}$. Among the three schemes, SPH/MFV shows the fastest/slowest convergence to the analytic solution. Unlike the well-known behaviour of Eulerian schemes, none of the Lagrangian schemes investigated displays artificial fragmentation when the perturbation wavelength, $\lambda$, is shorter than $\lambda_\mathrm{J}$, even at low numerical resolution. For larger wavelengths ($\lambda > \lambda_\mathrm{J}$) the growth of the perturbation is delayed when it is not well resolved. Furthermore, with poor resolution, the fragmentation seen with the MFV scheme proceeds very differently compared to the converged solution. All these results suggest that, when unresolved, the ratio of the magnitude of hydrodynamic force to that of self-gravity at the sub-resolution scale is the largest/smallest in MFV/SPH, the reasons for which we discussed in detail. These tests are repeated to investigate the effect of kernels of higher-order than the fiducial cubic spline. Our results indicate that the standard deviation of the kernel is a more appropriate definition of the size of a fluid element than its compact support radius.