Stability of Correction Procedure via Reconstruction With Summation-by-Parts Operators for Burgers' Equation Using a Polynomial Chaos Approach

In this paper, we consider Burgers' equation with uncertain boundary and initial conditions. The polynomial chaos (PC) approach yields a hyperbolic system of deterministic equations, which can be solved by several numerical methods. Here, we apply the correction procedure via reconstruction (CPR) using summation-by-parts operators. We focus especially on stability, which is proven for CPR methods and the systems arising from the PC approach. Due to the usage of split-forms, the major challenge is to construct entropy stable numerical fluxes. For the first time, such numerical fluxes are constructed for all systems resulting from the PC approach for Burgers' equation. In numerical tests, we verify our results and show also the advantage of the given ansatz using CPR methods. Moreover, one of the simulations, i.e. Burgers' equation equipped with an initial shock, demonstrates quite fascinating observations. The behaviour of the numerical solutions from several methods (finite volume, finite difference, CPR) differ significantly from each other. Through careful investigations, we conclude that the reason for this is the high sensitivity of the system to varying dissipation. Furthermore, it should be stressed that the system is not strictly hyperbolic with genuinely nonlinear or linearly degenerate fields

Entropy stable wall boundary conditions for the three-dimensional compressible Navier-Stokes equations

Non-linear entropy stability and a summation-by-parts framework are used to derive entropy stable wall boundary conditions for the three-dimensional compressible Navier--Stokes equations. A semi-discrete entropy estimate for the entire domain is achieved when the new boundary conditions are coupled with an entropy stable discrete interior operator. The data at the boundary are weakly imposed using a penalty flux approach and a simultaneous-approximation-term penalty technique. Although discontinuous spectral collocation operators on unstructured grids are used herein for the purpose of demonstrating their robustness and efficacy, the new boundary conditions are compatible with any diagonal norm summation-by-parts spatial operator, including finite element, finite difference, finite volume, discontinuous Galerkin, and flux reconstruction/correction procedure via reconstruction schemes. The proposed boundary treatment is tested for three-dimensional subsonic and supersonic flows. The numerical computations corroborate the non-linear stability (entropy stability) and accuracy of the boundary conditions.Comment: 43 page

SBP operators for CPR methods: Master's thesis

Summation-by-parts (SBP) operators have been used in the finite difference framework, providing means to prove conservation and discrete stability by the energy method, predominantly for linear (or linearised) equations. Recently, there have been some approaches to generalise the notion of SBP operators and to apply these ideas to other methods. The correction procedure via reconstruction (CPR), also known as flux reconstruction (FR) or lifting collocation penalty (LCP), is a unifying framework of high order methods for conservation laws, recovering some discontinuous Galerkin, spectral difference and spectral volume methods. Using a reformulation of CPR methods relying on SBP operators and simultaneous approximation terms (SATs), conservation and stability are investigated, recovering the linearly stable CPR schemes of Vincent et al. (2011, 2015). Extensions of SBP methods with diagonal-norm operators to Burgers’ equation are possible by a skew-symmetric form and the introduction of additional correction terms. An analytical setting allowing a generalised notion of SBP methods including modal bases is described and applied to Burgers’ equation, resulting in an extension of the previously mentioned skew-symmetric form. Finally, an extension of the results to multiple space dimensions is presented

Explicit reconstruction of the entanglement wedge

The problem of how the boundary encodes the bulk in AdS/CFT is still a subject of study today. One of the major issues that needs more elucidation is the problem of subregion duality; what information of the bulk a given boundary subregion encodes. Although the proof given by Dong, Harlow, and Wall states that the entanglement wedge of the bulk should be encoded in boundary subregions, no explicit procedure for reconstructing the entanglement wedge was given so far. In this paper, mode sum approach to obtaining smearing functions for a single bulk scalar is generalised to include bulk reconstruction in the entanglement wedge of boundary subregions. It is generally expectated that solutions to the wave equation on a complicated coordinate patch are needed, but this hard problem has been transferred to a less hard but tractable problem of matrix inversion.Comment: version accepted by JHEP; added references and discussions on covarianc