3D unsteady flow in oscillating compressor cascade

Abstract

An experimental and computational study has been carried out to enhance current understanding of three dimensional (3D) cascade aeroelastic mechanisms. 3D unsteady pressure data produced during executing this project is the first-of-its-kind, which can be directly used for validation of advanced 3D numerical methods for the prediction of aeroelastic problems in turbomachines. A new, low speed flutter test rig with a linear compressor cascade consisting of seven Controlled-Diffusion Blades has been commissioned. The unsteady aerodynamics of the oscillating cascade is investigated using the Influence Coefficient Method, by which the middle blade is mechanically driven to oscillate in a 3D bending mode. Off-board pressure transducers are utilized to allow detailed measurement of the unsteady blade surface pressures in conjunction with a Tubing Transfer Function (TTF) method to correct tubing distortion errors. The linearity of the unsteady aerodynamic response is confirmed by tests with different oscillation amplitudes, which enables unsteady results of a tuned cascade to be constructed by using the Influence Coefficient Method at various inter-blade phase angles. An examination of the techniques adopted and experimental errors indicates a good level of accuracy and repeatability to be attained in the measurement of unsteady pressure. A detailed set of steady flow is obtained from the middle three blades, which demonstrates a reasonable blade-to-blade periodicity. At a nominal steady flow condition unsteady pressure measurements were performed at six spanwise sections between 20% and 98% span for three different reduced frequencies. The 2D laminar bubble-type separation around middle chord on the suction surface is identified to have a local effect on the unsteady flow. The measured results illustrate the fully 3D unsteady flow