Freestream Boundary Condition Issues

The freestream boundary, usable with an ideal-gas formulation only, is non-reflective to one-dimensional disturbances incident (that is, normal) to the boundary.

It is based on the one-dimensional method of characteristics, calculating the boundary face fluxes for all the (inviscid) transported properties from the classical “compression”, “expansion” and “convective” waves. These waves have eigenvalues U+a, Ua and U. However, the following limitations exist when using a freestream boundary:

  • This boundary treatment is found to be ill-posed when flow structures cross the boundary. When the flow structure crosses a boundary, spurious compressible pressure waves can emanate from the interaction and propagate upstream to influence the source field.
  • Where the boundary is not normal to the incident compressible waves, errors can occur in the non-normal directions.