Hydrodynamic and thermodynamic non-equilibrium characteristics of shock waves: Insights from the discrete Boltzmann method

TL;DR

This paper introduces a discrete Boltzmann method to analyze the complex non-equilibrium effects in shock waves, revealing how Mach number influences these phenomena at multiple scales.

Contribution

It develops a novel discrete Boltzmann approach that captures higher-order non-equilibrium effects and provides analytical solutions, advancing understanding of shock wave dynamics.

Findings

Mach number affects interface smoothness and thickness in shock waves.

Strong compressibility regions shift from outflow to inflow with increasing Mach number.

TNE intensity increases and non-equilibrium regions expand as Mach number rises.

Abstract

Shock waves are typical non-equilibrium phenomena in nature and engineering, driven by hydrodynamic non-equilibrium (HNE) and thermodynamic non-equilibrium (TNE) effects. However, the mechanisms underlying these non-equilibrium effects are not fully understood. This study develops the discrete Boltzmann method (DBM) by directly discretizing velocity space, allowing for the adequate capture of higher-order HNE and TNE effects. To reveal these mechanisms, we derive analytical solutions for distribution functions and TNE quantities at various orders using CE analysis, although DBM simulations do not rely on these theoretical derivations. Using argon shock structures as a case study, DBM simulations of interface profiles and thickness at the macroscopic level agree well with experimental data and direct simulation Monte Carlo results. At the mesoscopic level, DBM-derived distribution functions and TNE measures closely match their corresponding analytical solutions. The effect of Mach number on HNE is analyzed by examining the shape and thickness of density, temperature, and velocity interfaces. Key findings include: (i) Mach number induces a two-stage effect on macroscopic quantities, influencing both interface smoothness and thickness, and (ii) as Mach number increases, the region of strong compressibility shifts from the outflow region to the inflow region. As for TNE characteristics, increasing Mach number significantly amplifies TNE intensity and expands the non-equilibrium region. This research provides kinetic insights into the multiscale nature and effects of non-equilibrium characteristics in shock waves, offering theoretical references for constructing kinetic models that describe different types and orders of non-equilibrium effects.