To apply the FWH method, the entire flow field including mean flow and acoustic perturbations on a surface surrounding the source regions must be known, as well as the mean flow in the far-field. The sound sources for the FWH are obtained on an interface plane in the near-field where they are known from the CAA calculations. The mean flow in the far-field is assumed as constant, which is an adequate assumption for an aircraft in flight.
The FWH-solver implements a retarded-time algorithm as well as a source-time-dominant algorithm as proposed by Brentner in 1996. In both cases the required time interpolation and derivation is done applying second order schemes. The solver has been validated against analytical solutions of the linearized wave equation, and has already been applied in combination with CAA-results [Schönwald et al. (2002)], where comparisons between the FWH-solution and a propagation simulation showed a good agreement of the two solutions.