Boundary Condition for the Implementation of Arbitrary Acoustical Modes

摘要

With the increase of bypass ratios of current jet engines, the dominating sound emission sources of aircraft engine noise consist of the tonal components of the fan and the compressor. Hence, the rotor-stator interaction noise is a significant contributor to the overall sound radiation from aircraft engines. The sound field consists of a superposition of various acoustical modes, generated by two effects: Firstly, the interaction of rotor wakes with the stator in each compressor stage and secondly, the relative rotation of potential fields of the cascades. Based on the blade and vane count and on the blade-passing frequency (BPF), the compressor modes that propagate can be estimated. Since the excited modes determine the emitted sound field, the numerical analysis of these acoustic structures is of great importance in understanding jet engine noise emissions. For this purpose, the sponge-layer boundary condition of the CAA-solver PIANO (developed by the German Aerospace Centre, DLR) is extended to implement arbitrary superposed modes. The FORTRAN-based code computes the resulting sound field in a cylindrical geometry, for a given set of azimuthal and radial mode orders, wavenumber and amplitudes of the modes to be excited. The numerically generated sound pressure distribution is validated against an analytical solution.