Origins and Structure of Rotating Instability: Part 2 — Numerical Observations in a Transonic Axial Compressor Rotor

Abstract

Rotating instability (RI) is an obvious unsteady flow phenomenon occurring in the tip region of compressors, which is potentially linked to tip clearance flow noise, blade vibration and rotating stall/surge. The existing investigations for RI indicate the origins of RI are closely related to unsteady flow behaviors in given blade passages in the rotating reference frame, which depend on the design specifics of axial machines. However, no efforts are made to set up a quantitative link between the time scale of unsteady behavior in a given passage and the characteristic parameters of RI, let alone to define the fluid dynamic processes/events which are causally linked with the RI inception. This is the motivation for the current investigations. For the case shown in Part I, a quantitative link between the time scale of tip flow unsteadiness and the characteristic parameters of RI has been set up. The consistency between experimental and numerical results also demonstrates that multi-passage computations with the circumferential extent larger than the length scale of RI could shed light on the flow mechanisms relevant to the emergence of RI. In this part, systematic multi-passage simulations were thus laid out for a tip-critical transonic rotor, NASA Rotor 35, to identify the fluid dynamic processes for the inception of RI in presence of shock wave. It is found that the flow field becomes unsteady when the mass flow rate is below a critical value close to the stability limit. The flow unsteadiness in specific passages originates from the rotor tip region, but it is not synchronous with the occurrence of the circumferential travelling wave similar to RI as is the case in Part I. The origin of flow unsteadiness attributes to the periodic oscillation of a new vortex structure is termed as tip secondary vortex (TSV). The TSV is essentially a vortex segment arising from the spiral-type breakdown of TLV. The underlying flow mechanism for the inception of a rotating wave similar to RI is that the periodic oscillation of TSV is capable of inducing the flow blockage transfer against the rotor rotation direction. In conjunction with the investigation results reported in Part 1, a local inception criterion for RI is thus identified, which could be depicted as the initiation of the flow blockage transfer induced by the unsteady flow behavior in blade passages near tip.