This work was carried out in the framework of the EC project TurboNoiseCFD and the DFG SWING+ project. The first author (X. D. Li) was partially supported by the NSFC-50076002 and the ASFC-01C51036. The authors would like to thank Rienstra and Eversman for their kind permission to use their MS and FEM solutions for comparison. The authors are grateful to the referees for carefully reviewing the manuscript and providing valuable comments.