Lagrangian Averaging, Nonlinear Waves, and Shock Regularization

In this thesis, we explore various models for the flow of a compressible fluid as well as model equations for shock formation, one of the main features of compressible fluid flows. We begin by reviewing the variational structure of compressible fluid mechanics. We derive the barotropic compressible Euler equations from a variational principle in the material frame. The particle relabeling symmetry of fluid mechanics is explained, and the material-frame Lagrangian is shown to be invariant under this symmetry. We then show how the barotropic compressible Euler equations arise from a variational principle in the spatial frame. Writing the resulting equations of motion requires certain Liealgebraic calculations that we carry out in detail for expository purposes. Next, we extend the derivation of the Lagrangian averaged Euler (LAE-α) equations to the case of barotropic compressible flows. The aim of Lagrangian averaging is to regularize the compressible Euler equations by adding dispersion instead of artificial viscosity. Along the way, the derivation of the isotropic and anisotropic LAE-α equations is simplified and clarified. The derivation in this paper involves averaging over a tube of trajectories η centered around a given Lagrangian flow η. With this tube framework, the LAE-α equations are derived by following a simple procedure: start with a given action, expand via Taylor series in terms of small-scale fluid fluctuations ξ, truncate, average, and then model those terms that are nonlinear functions of ξ. Closure of the equations is provided through the use of flow rules, which prescribe the evolution of the fluctuations along the mean flow.