On the universality of supersonic turbulence

TL;DR

This study presents the largest simulations of supersonic turbulence, revealing how different driving mechanisms affect the density distribution and power spectra, challenging the notion of universal turbulence scaling.

Contribution

It provides high-resolution simulations comparing solenoidal and compressive driving, demonstrating non-universality in the turbulence power spectrum and supporting recent theoretical models.

Findings

Compressive driving results in a wider, more intermittent density PDF.

Velocity spectrum follows Burgers turbulence with P(v) ~ k^(-2).

Density-weighted velocity spectrum varies with driving mode, aligning with recent theories.

Abstract

Compressible turbulence shapes the structure of the interstellar medium of our Galaxy and likely plays an important role also during structure formation in the early Universe. The density PDF and the power spectrum of such compressible, supersonic turbulence are the key ingredients for theories of star formation. However, both the PDF and the spectrum are still a matter of debate, because theoretical predictions are limited and simulations of supersonic turbulence require enormous resolutions to capture the inertial-range scaling. To advance our limited knowledge of compressible turbulence, we here present and analyse the world's largest simulations of supersonic turbulence. We compare hydrodynamic models with numerical resolutions of 256^3-4096^3 mesh points and with two distinct driving mechanisms, solenoidal (divergence-free) driving and compressive (curl-free) driving. We find convergence of the density PDF, with compressive driving exhibiting a much wider and more intermittent density distribution than solenoidal driving. Analysing the power spectrum of the turbulence, we find a pure velocity scaling close to Burgers turbulence with P(v) k^(-2) for both driving modes in our hydrodynamical simulations with Mach = 17. The spectrum of the density-weighted velocity rho^(1/3)v, however, does not provide the previously suggested universal scaling for supersonic turbulence. We find that the power spectrum P(rho^(1/3)v) scales with wavenumber as k^(-1.74) for solenoidal driving, close to incompressible Kolmogorov turbulence, k^(-5/3), but is significantly steeper with k^(-2.10) for compressive driving. We show that this is consistent with a recent theoretical model for compressible turbulence that predicts P(rho^(1/3)v) k^(-19/9) in the presence of a strong div(v) component as is produced by compressive driving and remains remarkably constant throughout the supersonic turbulent cascade.