Characteristic ghost-cell boundary condition

A method is proposed based on the boundary conditions that deals with the variables in ghost cells where the resulting flux differences satisfy the nonreflecting conditions. The boundary conditions were implemented into a compressible mixed finite volume/finite difference code in which the convective terms were discretized with a fifth-order accurate scheme based on a weighted essentially nonoscillatory (WENO) extrapolation of the characteristic variables. The state variables at the boundaries need to be prescribed in ghost cells before the governing equations were advanced in time. Velocities and temperatures were extrapolated at the outflow boundary, assuming zero second derivatives and the static pressure was prescribed. The characteristic boundary condition was found to be more transparent for outgoing waves and vortical structures, resulting in a faster relaxation of the flow to the ambient conditions.