Jed Brown

I'm a week late 🎃 but do any #CFD folks want to see a ghost?

One versatile technique for compressible flow is to implement "characteristic" boundary conditions by using a #RiemannSolver with a freestream solution outside. This nice paper discusses the technique and reports that HLL works as well as the more accurate solvers. But HLL doesn't represent the contact wave and fun things happen after the cold bubble reaches the outflow. https://doi.org/10.2514/6.2014-2923

A Guide to the Implementation of Boundary Conditions in Compact High-Order Methods for Compressible Aerodynamics | AIAA AVIATION Forum

AIAA AVIATION Forum
Nov 8, 2022 at 6:58amWeb
Show threadJed BrownNov 18, 2022
@jedbrown Indeed, upgrading the #RiemannSolver to #HLLC (which includes a contact wave) from #HLL (which only estimates the acoustic waves) fixes this problem. Here's a pair of oblique bubble simulations to campare HLLC (left) with HLL (right). Unlike most cases with finite volume methods (which use Riemann solvers at every grid interface), a high quality solver is important for continuous Galerkin FE boundary conditions.