Effect of thermal fluctuations on spectra and predictability in compressible decaying isotropic turbulence

TL;DR

This paper explores how molecular thermal fluctuations influence spectra and predictability in compressible decaying isotropic turbulence, revealing their dominance at small scales and their effect on flow predictability.

Contribution

It introduces the USP method to analyze thermal fluctuation effects on turbulence spectra and predictability in 2D and 3D flows, highlighting their significance at small scales.

Findings

Thermal fluctuations follow a ${k}^{(d-1)}$ scaling law at high wavenumbers.

Spectral energy ratios differ between 2D and 3D cases for solenoidal and compressible components.

Thermal fluctuations significantly impact turbulence spectra at the Kolmogorov length scale.

Abstract

This study investigates the impact of molecular thermal fluctuations on compressible decaying isotropic turbulence using the unified stochastic particle (USP) method, encompassing both two-dimensional (2D) and three-dimensional (3D) scenarios. The findings reveal that the turbulent spectra of velocity and thermodynamic variables follow the wavenumber scaling law of ${k}^{(d-1)}$ for different spatial dimensions $d$ within the high wavenumber range, indicating the impact of thermal fluctuations on small-scale turbulent statistics. With the application of Helmholtz decomposition, it is found that the thermal fluctuation spectra of solenoidal and compressible velocity components (${\vec{u}}_{s}$ and ${\vec{u}}_{c}$) follow an energy ratio of 1:1 for 2D cases, while the ratio changes to 2:1 for 3D cases. Comparisons between 3D turbulent spectra obtained through USP simulations and direct numerical simulations of the Navier-Stokes equations demonstrate that thermal fluctuations dominate the spectra at length scales comparable to the Kolmogorov length scale. Additionally, the effect of thermal fluctuations on the spectrum of ${\vec{u}}_{c}$ is significantly influenced by variations in the turbulent Mach number. We further study the impact of thermal fluctuations on the predictability of turbulence. With initial differences caused by thermal fluctuations, different flow realizations display significant disparities in velocity and thermodynamic fields at larger scales after a certain period of time, which can be characterized by "inverse error cascades". Moreover, the results suggest a strong correlation between the predictabilities of thermodynamic fields and the predictability of ${\vec{u}}_{c}$.