Numerical Investigation on the Originating Mechanism of Unsteadiness in Tip Leakage Flow for a Transonic Fan Rotor

Despite the fact that the importance of steady tip leakage flows in rotor efficiency and stability has been long recognized and extensively studied, the unsteadiness of tip leakage flows became an interesting research topic only about 10 years ago. Many issues, such as its onset conditions, its role in compressor instability, etc. need to be further explored. In this paper, we present a numerical investigation on the influences of two important driving “forces”, the incoming main flow and the tip leakage flow, to clarify the originating mechanism of self-induced unsteadiness in transonic compressors. NASA Rotor 67 is chosen as the computational model. It is found that among all the simulated cases, the self-induced unsteadiness exists when the size of the tip clearance equals or larger than design tip clearance of the computational model. The time-dependent flow pattern in the rotor tip region is provided to illustrate that the main unsteady regions are on the blade’s pressure side that happens to be under the alternate influence of tip leakage flow and the incoming main flow. It is found the self-induced unsteady mechanism in the transonic rotor is the same as that in previously studied low-speed rotor. The interaction between shock wave and tip leakage vortex does not initiate the self-induced unsteadiness, but might be the cause of other unsteadiness, such as turbulent unsteadiness. A correlation based on the momentum ratio of tip leakage flow over the main incoming flow at the tip region is used as an indicator for the onset of the self-induced unsteadiness in tip leakage flow.