Floquet and transient growth stability analysis of a flow through a compressor passage

In this paper we investigate the instabilities arising in a flow through a compressor passage using BiGlobal stability analysis. The adopted geometry comes from the results of previous experimental and numerical investigations on a linear low-pressure (LP) compressor cascade [6,19,20]. Specifically, we address the role of laminar separation of the boundary layers at Re=138,500, where such separation effects are enhanced by the strong adverse pressure gradients that the flow experiences, in contrast to the more commonly studied low-pressure (LP) turbines. The vortical structures downstream the separation bubble on the suction surface were recognised to show a well-defined time periodicity, which could be precisely detected. Floquet stability analysis was then used to investigate the response of the flow to infinitesimal perturbations. To overcome the difficulty of performing a Floquet stability analysis when the periodicity is restricted just to a small region of the domain, a phase-averaged base flow was computed, such that only the organised motions are extracted, neglecting all the background unsteadiness. The same technique allowed us to confirm the presence of strong energy transient growth phenomena, which are directly associated with convective instabilities occurring in the region downstream from the separation bubble.