Comparison of Nonreflecting Outlet Boundary Conditions for Compressible Solvers on Unstructured Grids

This paper describes extensions and tests of characteristic methods for outlet boundary conditions in compressible solvers. Three methods based on the specification of incoming waves using one- and multidimensional approximations are extended to unstructured grids. They are first compared for weak to strong vortices propagating on low-to high-speed mean flows through outlet sections. A major issue is to determine the Mach number to be used in the specification of the transverse terms that must be taken into account in the incoming wave amplitude specifications. For the vortex computations, results show that the averaged Mach number leads to better results than its local value. The boundary conditions are then tested in a more complex case: the flow around a turbine blade. A reference solution using a long distance between the blade trailing edge and the outlet plane is first computed. For this solution, outlet boundary conditions have almost no effect on the flow around the blade. The distance between the trailing edge and the outlet plane is then shortened and the various characteristic treatments are compared, in which intense vortices cross the outlet plane. Results confirm the conclusions obtained on the simple vortex test case. [PUBLICATION ABSTRACT]