Numerical Investigation of Acoustic Resonance in an Axial Multistage Compressor

The unsteady flow structure associated with acoustic resonance in a 4½-stage high-speed axial compressor is investigated through high-fidelity numerical analysis. Acoustic resonance in a multistage compressor is known to be one of the most severe phenomena potentially leading to violent blade vibrations and even structural damage. In this study, the driving mechanisms of acoustic resonance in the compressor tested, including tip leakage fluctuations, are examined using high-fidelity computational fluid dynamics. In addition, the flow structure during acoustic resonance and the relationship between acoustic resonance and stall inception are analyzed. The numerical results obtained using delayed detached eddy simulation show that acoustic resonance is initiated at an off-design operating point and that the resulting pressure wave exhibits a helical pressure mode. The fluctuation in the adverse pressure gradient induced by this helical mode generates a nonaxisymmetric velocity distribution within the rotor blade passages. Three-mode disturbances first appear near the tip region of the third stage rotor and mutually interact with oscillations of the tip leakage vortex at the acoustic resonance frequency. Thus, the driving force of acoustic resonance is the circumferential oscillation of the tip leakage vortex. During stall inception, flow spillage at the first and second rotors, induced by acoustic resonance effects, occurs earlier than in the rear stages. The resulting nonaxisymmetric flow distribution is directly linked to the onset of stall. The compressor thus appears to exhibit a multistage stall inception process that, to the knowledge of the authors, has not been reported yet in the literature.