Focusing schlieren visualization of transonic turbine tip-leakage flows

This paper presents a focusing schlieren system designed for the investigation of transonic turbine tip-clearance flows. In the first part, the functional principle and the design of the system are presented. Major design considerations and necessary trade-offs are discussed. The key optical properties, e.g depth of focus, are verified by means of a simple bench test. In the second part, results of an idealized tip gap model as well as linear cascade tests at engine representative Reynolds and Mach numbers are presented and discussed. The focusing schlieren system, designed for minimum depth of focus, has been found to be well suited for the investigation of three-dimensional transonic flow fields in turbomachinery applications. The schlieren images show the origin and growth of the tip-gap vortex on the blade suction side. A complex shock system was observed in the tip region and the tip-gap vortex was found to interact with the suction side part of the trailing edge shock system. The results indicate that transonic vortex shedding is suppressed in the tip region.