Computational simulation of unsteady flow in transonic compressor rotor

Abstract

The unsteady flow about a section of a modern first stage transonic compressor rotor was simulated using a finite difference approximation to the two-dimensional, Reynolds averaged, unsteady, compressible, viscous Navier-Stokes equations. The computation was performed in both steady state and time-accurate modes, and the results compared. The time-accurate results were analyzed in some detail. Two frequency regimes were observed. High frequency unsteadiness due to vortex shedding was found at frequencies varying between 11 KHz and 19 KHz. A low frequency cycle was also observed at 365 Hz. The low frequency cycle produced significant variations in blade force and moment. It also modulated the strength and frequency of the vortex shedding. Arguments were advanced to explain the mechanics of the vortex street formation in terms of a single free shear layer instability. The variations in shedding strength and frequency were related to movement of the separation point. A wholly satisfactory normalization of the frequencies was not found. The low frequency cycle was analyzed as a quasi-steady sequence of events stemming from movement of a shock wave spanning the blade passage. The possibility was entertained that the cycle was due to purely numerical sources, but no likely mechanism was found.