arXiv subjects

Demei Li

Publications and source records attributed to Demei Li.

8 recordsLinked to original sources

arXiv2026

Discrete Boltzmann model at Burnett level for compressible multicomponent flows under external forces

This work extends the Burnett-level discrete Boltzmann model (DBM) from single-component to multicomponent compressible flows under external forces, building on the fundamental framework of the high-precision discrete kinetic method. A high-isotropy 25-discrete-velocity set is adopted to guarantee numerical stability and spatial symmetry, while a rigorous moment-matching strategy is developed to construct the equilibrium distribution function and external force term. Different from the single-component counterpart, the present model intrinsically incorporates interspecies mass diffusion effects and multi-component thermodynamic nonequilibrium behaviors, which are critical for complex compressible multicomponent systems. The Chapman--Enskog expansion verifies that the proposed model can exactly recover the Burnett-level governing equations for forced multicomponent compressible flows in the continuum limit. Five canonical benchmark cases, including multicomponent mass diffusion, compressible Sod shock tube, thermal Couette flow, Kelvin--Helmholtz instability, and Rayleigh--Taylor instability, are systematically performed. Numerical results demonstrate that the developed Burnett-level multicomponent DBM achieves high accuracy and robustness in capturing both hydrodynamic evolution and multicomponent nonequilibrium characteristics under external forces.

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arXiv2026

A discrete Boltzmann model with state-dependent power-law relaxation time for nonequilibrium transport in compressible flows

Thermodynamic nonequilibrium effects play a central role in momentum and energy transport in compressible flows. In conventional BGK kinetic models, the relaxation time $\tau$ is taken as a constant, which neglects the dependence of the relaxation process on local macroscopic states. To overcome this limitation, we develop a discrete Boltzmann model with a density- and temperature-dependent power-law relaxation time, termed DTRT-DBM, in which $\tau=\tau_0(\rho/\rho_0)^a(T/T_0)^b$. This formulation extends the discrete Boltzmann framework to flows with spatially varying nonequilibrium intensity. The model is validated by the Sod shock tube and by analytical solutions for viscous stress and heat flux, demonstrating accurate recovery of both macroscopic wave structures and nonequilibrium quantities across shock waves, rarefaction waves, and contact discontinuities. On this basis, phase diagrams of viscous stress and heat flux are constructed to examine how these quantities depend on the power-law exponents $a$ and $b$. The extrema of these quantities depend exponentially on the model parameters and exhibit regime-dependent behaviour. The roles of $a$ and $b$ are not symmetric: the nonequilibrium response is more sensitive to $a$ when density gradients dominate, but more sensitive to $b$ when temperature gradients dominate. Within the parameter range and flow configurations examined here, higher-order viscous stress increases the growth rate of the total viscous-stress extremum, whereas higher-order heat flux reduces the growth rate of the total heat-flux extremum. These results show that the proposed model can capture different higher-order nonequilibrium responses in compressible flows and provides a framework for the modelling and analysis of multiscale nonequilibrium processes.

physics.flu-dyn↗

arXiv2025

Kinetic study of compressible Rayleigh-Taylor instability with time-varying acceleration

Rayleigh-Taylor (RT) instability commonly arises in compressible systems with time-dependent acceleration in practical applications. To capture the complex dynamics of such systems, a two-component discrete Boltzmann method is developed to systematically investigate the compressible RT instability driven by variable acceleration. Specifically, the effects of different acceleration periods, amplitudes, and phases are systematically analyzed. The simulation results are interpreted from three key perspectives: the density gradient, which characterizes the spatial variation in density; the thermodynamic non-equilibrium strength, which quantifies the system's deviation from local thermodynamic equilibrium; and the fraction of non-equilibrium regions, which captures the spatial distribution of non-equilibrium behaviors. Notably, the fluid system exhibits rich and diverse dynamic patterns resulting from the interplay of multiple competing physical mechanisms, including time-dependent acceleration, RT instability, diffusion, and dissipation effects. These findings provide deeper insights into the evolution and regulation of compressible RT instability under complex driving conditions.

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arXiv2025

Investigating the Effect of Relaxation Time on Richtmyer-Meshkov Instability under Reshock Impact: A Two-Component Discrete Boltzmann Method Study

The Richtmyer-Meshkov (RM) instability plays an important role in various natural and engineering fields, such as inertial confinement fusion. In this work, the effect of relaxation time on the RM instability under reshock impact is investigated by using a two-component discrete Boltzmann method. The hydrodynamic and thermodynamic characteristics of the fluid system are comprehensively analyzed from the perspectives of the density gradient, vorticity, kinetic energy, mixing degree, mixing width, and non-equilibrium intensity. Simulation results indicate that for larger relaxation time, the diffusion and dissipation are enhanced, the physical gradients decrease, and the growth of the interface is suppressed. Furthermore, the non-equilibrium manifestations show complex patterns, driven by the competitive physical mechanisms of the diffusion, dissipation, shock wave, rarefaction wave, transverse wave, and fluid instabilities. These findings provide valuable insights into the fundamental mechanism of compressible fluid flows.

physics.flu-dyn↗

arXiv2025

Effects of reflection distance on Richtmyer-Meshkov instability in the reshock process: A discrete Boltzmann study

The Richtmyer-Meshkov (RM) instability occurs when a perturbed interface between two fluids undergoes impulsive acceleration due to a shock wave. In this paper, a numerical investigation of the RM instability during the reshock process is conducted using the two-component discrete Boltzmann method. The influence of reflection distance on the RM instability, including both hydrodynamic and thermodynamic non-equilibrium effects, is explored in detail. The interaction time between the reflected shock wave and the material interface varies with different reflection distances. Larger reflection distances lead to a longer evolution time of the material interface before reshock, resulting in more complex effects on the interface deformation, the mixing extent of the fluid system, and non-equilibrium behaviors after reshock. Additionally, while the reflection distance has a minimal impact on mixing entropy before the secondary impact, a significant difference emerges after the secondary impact. This suggests that the secondary impact enhances the evolution of the RM instability. Furthermore, non-equilibrium behaviors or quantities exhibit complex dynamics due to the influence of the transmitted shock wave, transverse waves, rarefaction waves, material interfaces, and dissipation/diffusion processes.

physics.flu-dyn↗

arXiv2025

Burnett-level discrete Boltzmann modeling of compressible flows under force

In this paper, a Burnett-level discrete Boltzmann model (DBM) is proposed for the compressible flow in a force field, and a discrete velocity set with 25 velocities is constructed for the DBM, featuring good spatial symmetry. In the discrete Boltzmann equation, both the discrete equilibrium distribution function and the force term satisfy 25 independent moment relations and are computed with the matrix inversion method. This approach ensures high physical accuracy, computational efficiency, and ease of implementation. Through the Chapman-Enskog expansion analysis, it is demonstrated that the current DBM can recover the Burnett equations for the compressible system under force in the continuum limit. Moreover, the DBM has the capability of capturing essential thermodynamic nonequilibrium behaviors. Finally, the model is validated through five typical benchmarks, including the free falling, Sod shock tube, sound wave, thermal Couette flow, and Rayleigh-Taylor instability.

physics.flu-dyn↗

arXiv2025

Multiscale thermodynamic nonequilibrium effects in Kelvin-Helmholtz instability and their relative importance

This study investigates the complex kinetics of thermodynamic nonequilibrium effects (TNEs) and their relative importance during the development of Kelvin-Helmholtz instability (KHI) using high-order discrete Boltzmann models (DBMs). First, the capabilities and differences among various discrete velocity sets in capturing TNEs and distribution functions are assessed. Practical guidelines for constructing discrete velocity stencils are proposed to enhance phase-space discretization and improve the robustness of high-order DBM simulation. At different stages of KHI and under varying initial conditions, multiscale TNEs, such as viscous stresses of different orders, emerge with distinct dominant roles. Specifically, three scenarios are identified: (i) regimes dominated by first-order TNEs,(ii) alternation between first- and second-order TNEs, and (iii) states where second-order TNEs govern the system's behavior. To quantitatively capture these transitions, criteria for TNE dominance at different orders in KHI evolution are established based on the relative thermodynamic nonequilibrium intensity ($R_{\text{TNE}}$). In scenarios dominated by second-order TNEs, differences between first-order and second-order models are compared in terms of macroscopic quantities, nonequilibrium effects, and kinetic moments, revealing the physical limitations of low-order models in capturing TNEs. Furthermore, the effectiveness, extensibility, and limitations of a representative high-order model are examined under second-order TNE-dominated conditions. To encapsulate these findings, a nonequilibrium phase diagram that visually maps the multiscale characteristics of KHI is constructed. This diagram not only provides intuitive insights into the dynamic interplay of different nonequilibrium effects but also serves as a kinetic roadmap for selecting suitable models under diverse nonequilibrium conditions.

physics.flu-dyn↗

arXiv2021

Specific heat ratio effects of compressible Rayleigh-Taylor instability studied by discrete Boltzmann method

Rayleigh-Taylor (RT) instability widely exists in nature and engineering fields. How to better understand the physical mechanism of RT instability is of great theoretical significance and practical value. At present, abundant results of RT instability have been obtained by traditional macroscopic methods. However, research on the thermodynamic non-equilibrium (TNE) effects in the process of system evolution is relatively scarce. In this paper, the discrete Boltzmann method based on non-equilibrium statistical physics is utilized to study the effects of the specific heat ratio on compressible RT instability. The evolution process of the compressible RT system with different specific heat ratios can be analyzed by the temperature gradient and the proportion of the non-equilibrium region. Firstly, as a result of the competition between the macroscopic magnitude gradient and the non-equilibrium region, the average TNE intensity first increases and then reduces, and it increases with the specific heat ratio decreasing; the specific heat ratio has the same effect on the global strength of the viscous stress tensor. Secondly, the moment when the total temperature gradient in y direction deviates from the fixed value can be regarded as a physical criterion for judging the formation of the vortex structure. Thirdly, under the competition between the temperature gradients and the contact area of the two fluids, the average intensity of the non-equilibrium quantity related to the heat flux shows diversity, and the influence of the specific heat ratio is also quite remarkable.

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