Intermittent breakdown of the tip leakage vortex and the resultant flow unsteadiness in the tip-region of a subsonic compressor cascade

Abstract

In axial-flow compressor rotors, the breakdown of the tip leakage vortex (TLV) occurs in an intermittent manner under some conditions. In order to gain better knowledge of this unsteady phenomenon, a numerical investigation of the tip-region flow field in a subsonic compressor cascade has been conducted using unsteady Reynold-averaged Navier-Stokes simulations. First of all, the reason why TLV breakdown exhibits intermittency has been clarified through the analysis of time-resolved flow fields. It was found that the switch between the breakdown and no-breakdown states is triggered by a feedback mechanism: when the TLV breaks down, either an unsteady induced vortex or a backflow vortex forms and induces low-pressure on the pressure surface of the adjacent blade, which unloads the blade tip and subsequently suppresses the TLV breakdown; as the unsteady induced vortex or backflow vortex migrates downstream, the pressure on the pressure surface increases again so that the loading at the blade tip is increased again and the TLV breakdown is re-established. The amplitude and frequency characteristics of the unsteadiness associated with the intermittent TLV breakdown have been analyzed and presented. It was found that the backflow vortex-dominant breakdown induces much stronger flow fluctuation and has a much lower characteristic frequency than the unsteady induced vortex-dominant breakdown. The underlying flow physics responsible for this difference have been discussed based on the understanding of the feedback mechanism for the intermittent TLV breakdown.