Acoustic Resonances and Trapped Modes in Annular Plate Cascades

Abstract

As a stepping stone towards understanding acoustic resonances in axial flow compressors acoustic resonances are computed numerically in fixed single and tandem plate cascades in an infinitely long annular duct. Applying perfectly matched layer absorbing boundary conditions in the form of the complex scaling method of atomic and molecular physics to approximate the radiation condition the resonance problem is transformed into an eigenvalue problem. Of particular interest are resonances with zero radiation damping (trapped modes) or very small radiation damping (nearly trapped modes). Such resonances can be excited by wakes from compressor cascades or struts. If the shedding frequency is sufficiently close to an acoustic resonant frequency the latter may control the vortex shedding causing high intensity tonal noise or occasionally even blade failure. All resonances are computed for zero mean flow approximating low Mach number flows. The influence of various cascade parameters on the resonant frequencies is studied and, whenever possible, our numerical results are compared with published experimental findings.