Experimental Investigation of Turbulence Influence of Wake Passing on the Boundary Layer Development of Highly Loaded Turbine Cascade Blades

Abstract

Experimental investigations regarding the effects of wake passing on boundary layer development were performed on a highly loaded low pressure (LP) turbine cascade blade. Data was obtained for various flow conditions and is intended to be used for the validation of numerical models dealing with periodic unsteady transition. A phase shift between turbulence and velocity fluctuations in the wake path was observed in the near-wall boundary layer, which might have considerable impact on the use of integral-type transition criteria. The experimental data set is already publicly available for download on the web page of the institute. A moving bar type wake generator was employed to simulate the upstream blade row. Tests were performed at turbomachinery-like Mach and Reynolds number conditions in the High-Speed Cascade Wind Tunnel for different bar speeds, bar spacings and background turbulence levels. This paper focuses on the exemplary description of flow phenomena characteristic for unsteady flow for a single exit Reynolds number of Re2th = 200,000. Conventional measurement techniques like static pressure tappings on the suction and pressure side as well as five hole probe traverses were complemented with fast-response pressure transducers embedded into and hot film sensors glued onto the suction surface to study the time-resolved boundary layer development in detail. Additional studies using single hot wire traverses in the suction side boundary layer and triple hot wire probes upstream of the cascade inlet plane were deployed to complete the data sets. The results indicate that for the LP turbine the transition point moves periodically when subjected to wake passing, which greatly affects the loss generation in the suction side boundary layer. Laminar separation is suppressed or reduced whenever the wake is present.