An explicit high-order single-stage single-step positivity-preserving finite difference WENO method for the compressible Euler equations

In this work we construct a high-order, single-stage, single-step positivity-preserving method for the compressible Euler equations. Space is discretized with the finite difference weighted essentially non-oscillatory (WENO) method. Time is discretized through a Lax-Wendroff procedure that is constructed from the Picard integral formulation (PIF) of the partial differential equation. The method can be viewed as a modified flux approach, where a linear combination of a low- and high-order flux defines the numerical flux used for a single-step update. The coefficients of the linear combination are constructed by solving a simple optimization problem at each time step. The high-order flux itself is constructed through the use of Taylor series and the Cauchy-Kowalewski procedure that incorporates higher-order terms. Numerical results in one- and two-dimensions are presented

A Space-time Smooth Artificial Viscosity Method For Nonlinear Conservation Laws

We introduce a new methodology for adding localized, space-time smooth, artificial viscosity to nonlinear systems of conservation laws which propagate shock waves, rarefactions, and contact discontinuities, which we call the $C$-method. We shall focus our attention on the compressible Euler equations in one space dimension. The novel feature of our approach involves the coupling of a linear scalar reaction-diffusion equation to our system of conservation laws, whose solution $C(x,t)$ is the coefficient to an additional (and artificial) term added to the flux, which determines the location, localization, and strength of the artificial viscosity. Near shock discontinuities, $C(x,t)$ is large and localized, and transitions smoothly in space-time to zero away from discontinuities. Our approach is a provably convergent, spacetime-regularized variant of the original idea of Richtmeyer and Von Neumann, and is provided at the level of the PDE, thus allowing a host of numerical discretization schemes to be employed. We demonstrate the effectiveness of the $C$-method with three different numerical implementations and apply these to a collection of classical problems: the Sod shock-tube, the Osher-Shu shock-tube, the Woodward-Colella blast wave and the Leblanc shock-tube. First, we use a classical continuous finite-element implementation using second-order discretization in both space and time, FEM-C. Second, we use a simplified WENO scheme within our $C$-method framework, WENO-C. Third, we use WENO with the Lax-Friedrichs flux together with the $C$-equation, and call this WENO-LF-C. All three schemes yield higher-order discretization strategies, which provide sharp shock resolution with minimal overshoot and noise, and compare well with higher-order WENO schemes that employ approximate Riemann solvers, outperforming them for the difficult Leblanc shock tube experiment.Comment: 34 pages, 27 figure

High-order Approximate Lax-Wendroff methods for systems of conservation laws

En esta tesis se introduce una nueva familia de métodos de alto orden: Los métodos Aproximadores Compactos Taylor para leyes de conservación (CAT por sus siglas en ingles). En estos métodos numéricos el proceso de Cauchy-Kovalevsky se evita al aplicar aproximaciones en forma recursiva. La diferencia a otros métodos aproximadores radica en que aquí se usan cómputos de los flujos numéricos de forma local, lo cual nos permite que los métodos tengan (2p+1) puntos en su esténcil y un orden de precisión 2p, donde p es un numero arbitrario entero. Aun mas, cuando el flujo es lineal estos métodos se reducen a los ya conocidos métodos de alto orden Lax-Wendroff y además son L2-estables table bajo la condición usual CFL. Sin embargo, los métodos CAT presentan un costo computacional extra por su carácter local, aunque este costo es compensado por el hecho que siguen dando buenos resultados aun con valores del CFL próximos a 1. Para evitar las oscilaciones que aparecen cerca de las discontinuidades se consideran aquí dos técnicas shock-capturing: la primera una nueva familia de métodos de alto orden, los métodos adaptativos compactos Taylor (ACAT), basados en la adaptación del orden del esquema acorde a una nueva familia de indicadores de suavidad. La segunda técnica es la combinación de los métodos CAT con una variante original de los métodos WENO, nombrada : Approximate Taylor methods with fast and optimized weighted essentially nonoscillatory reconstructions (FOWENO-CAT)