Abstract
Ideal gas dynamics can develop shock-like singularities with discontinuous density. Viscosity typically regularizes such singularities and leads to a shock structure. On the other hand, in one dimension, singularities in the Hopf equation can be non-dissipatively smoothed via Korteweg–de Vries (KdV) dispersion. In this paper, we develop a minimal conservative regularization of 3D ideal adiabatic flow of a gas with polytropic exponent γ. It is achieved by augmenting the Hamiltonian by a capillarity energy β(ρ)(∇ρ)2. The simplest capillarity coefficient leading to local conservation laws for mass, momentum, energy, and entropy using the standard Poisson brackets is β(ρ) = β*/ρ for constant β*. This leads to a Korteweg-like stress and nonlinear terms in the momentum equation with third derivatives of ρ, which are related to the Bohm potential and Gross quantum pressure. Just like KdV, our equations admit sound waves with a leading cubic dispersion relation, solitary waves, and periodic traveling waves. As with KdV, there are no steady continuous shock-like solutions satisfying the Rankine–Hugoniot conditions. Nevertheless, in one-dimension, for γ = 2, numerical solutions show that the gradient catastrophe is averted through the formation of pairs of solitary waves, which can display approximate phase-shift scattering. Numerics also indicate recurrent behavior in periodic domains. These observations are related to an equivalence between our regularized equations (in the special case of constant specific entropy potential flow in any dimension) and the defocusing nonlinear Schrödinger equation (cubically nonlinear for γ = 2), with β* playing the role of ℏ2. Thus, our regularization of gas dynamics may be viewed as a generalization of both the single field KdV and nonlinear Schrödinger equations to include the adiabatic dynamics of density, velocity, pressure, and entropy in any dimension.
Highlights
Gas dynamics has been an active area of research with applications to high-speed flows, aerodynamics, and astrophysics
Numerics indicate recurrent behavior in periodic domains. These observations are related to an equivalence between our regularized equations and the defocusing nonlinear Schrödinger equation, with β∗ playing the role of h2
Our regularization of gas dynamics may be viewed as a generalization of both the single field Korteweg– de Vries (KdV) and nonlinear Schrödinger equations to include the adiabatic dynamics of density, velocity, pressure, and entropy in any dimension
Summary
Gas dynamics has been an active area of research with applications to high-speed flows, aerodynamics, and astrophysics. We obtain our regularized model by augmenting the Hamiltonian of ideal adiabatic flow of a gas with polytropic exponent γ by a density gradient energy β(ρ)(∇ρ). We obtain our regularized model by augmenting the Hamiltonian of ideal adiabatic flow of a gas with polytropic exponent γ by a density gradient energy β(ρ)(∇ρ)2 Such a term arose in the work of van der Waals and Korteweg in the context of capillarity, but can be important even away from interfaces in any region of rapid density variation, especially when dissipative effects are small, such as in weak shocks, cold atomic gases, superfluids, and collisionless plasmas.
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