The Classical Field Theories

Weak solutions of nonlinear hyperbolic equations and their numerical computation

The Riemann problem for fluid flow of real materials is examined. An arbitrary equation of state is allowed, subject only to the physical requirements of thermodynamics. The properties of the isentropes and the shock Hugoniot loci that follow from conditions imposed on the equation of state are reviewed systematically. Important properties of these wave curves are determined by three dimensionless variables characterizing the equation of state: the adiabatic exponent $\ensuremath{\gamma}$, the Gr\"uneisen coefficient $\ensuremath{\Gamma}$, and the fundamental derivative $\mathcal{G}$. Standard assumptions on these variables break down near phase transitions. The result is an anomalous wave structure: either shock waves split into multiple waves, or composite waves form. Additional questions related to shock stability and nonuniqueness of the solution of the Riemann problem are discussed.

The Riemann problem for fluid flow of real materials

The Riemann problem for general systems of conservation laws

Introduction The physical problem of the propagation of small disturbances (sound waves) corresponds to linearization of the equations in the appropriate mathematical formulation. In an inviscid fluid this leads to plane waves which can be either compressive or rarefactive and which propagate at a constant speed (the sound speed) which is a property of the medium. Since differential equations inherently assume the differentiability of solutions, the propagation of discontinuous wave fronts requires special treatment. This is accomplished by the simple expedient of approximating an initially discontinuous wave front by a sequence of continuous initial functions. The corresponding sequence of continuous solutions of the differential equations is observed to converge to a discontinuous wave front propagating at the same sound speed. This serves to extend the manifold of solutions of the differential equations to include discontinuities which propagate unaltered whether they are compressive or rarefactive. If dissipation is introduced into the mathematical formulation, it is observed that initial discontinuities are wiped out, and all disturbances tend to broaden with time. On the other hand, it is a classical result that the propagation of non-dissipative waves of finite amplitude is no longer symmetric, compressive fronts tending to steepen and rarefactive ones to broaden. In particular, a compressive front will steepen until it attains a vertical slope, after which it becomes multiple-valued and can no longer represent a physical quantity. Furthermore, if it is attempted to approximate an initially discontinuous compressive wave by a sequence of smooth initial functions, the time of breakdown of each succeeding smooth wave will approach the initial instant, so that no limit exists which could be termed propagation of the initial discontinuity. By contrast, successive approximations of this sort do converge if the initial discontinuity is rarefactive, but it is found that the initial discontinuity is immediately wiped out and propagates as a broadening rarefaction wave. 1

The profile of a steady plane shock wave

The following investigation tries to clear up the general hydrodynamical and thermodynamical foundations of the shock phenomenon.1 The first part, Sections 2–5, answers the question: What are the conditions for the equation of state of a fluid under which shocks with their distinctive qualitative features may be produced. These conditions, enumerated in Section 3, are partly of differential, partly of global nature. The second part, Sections 6–7, investigates the physical structure of the shock layer whose “infinitesimal” width is of the order of magnitude e provided heat conductivity and viscosity are small of the same order. Initial state and final state are singular points for the differential equations of the shock layer, and it is shown that they are of such a nature as to make one expect the problem to have a unique solution.

Hermann Weyl

Papers

Shock waves in arbitrary fluids