PERIODIC UNSTEADINESS OF TIP CLEARANCE VORTEX IN AN AXIAL COMPRESSOR ROTOR

A series of unsteady simulations, supported by experimental data, are used to characterize the periodic unsteadiness of the tip clearance vortex in an axial compressor rotor. The numerical probes detect significant periodic fluctuations in the blade tip region at near stall conditions. A reduced frequency at different condition is limited to a small range although there exist a large difference on the natural frequency. Physical explanations of the periodic fluctuations are made in terms of vortex-core identification, contour, etc. The nature of the periodic unsteadiness in the tip region is the periodic bubble-type breakdown of the tip leakage vortex induced by the broken vortex core generated by the previous breakdown. The life cycle of the broken vortex core can be summarized as three processes, generation, propagation and inducing breakdown of tip leakage vortex. The broken vortex core arrives at mid-chord of the adjacent blade, resulting in change of momentum in the tip clearance and pressure in the leading edge of the adjacent blade. The flow in this blade tip region is similarly affected by another adjacent blade. The tip leakage vortex core is bent, then the breakdown of tip clearance happens and a new broken vortex core appears accompanied by a back flow region.