Measurements of Separated Flow Transition on a Moderately-Loaded Turbine Airfoil

Abstract

This paper describes the measurement of separation, transition and re-attachment of a boundary layer on a moderately-loaded turbine blade at low Reynolds number. Under the conditions of the measurements, the flow on the aft portion of the suction surface experiences laminar separation and transition in a separated shear-layer. As the free shear layer becomes turbulent, the increased entrainment results in reattachment of the boundary layer. This paper compares three different methods of identifying separation, transition and re-attachment of boundary layers. It offers a time-average comparison between pressure, hot-film and hot-wire based methods. The results show that all three methods provide useful results, but that the hot-wire data is superior to the other two methods in providing a picture of the blade-surface flow field. Finally, in the past, hot-films located on the surface of the blade had been used to estimate the location of transition in separated shear-layers. This paper evaluates this practice and highlights its potential and pitfalls through a comparison between hot-wire and hot-film signals. For this comparison, a procedure based on a crosscorrelation parameter relating the velocity field to the quasi-wall-shear-stress values on the surface of the blade was applied. The resulting correlation parameter provides new data that attempts to shed some light on the capacity of laminar, transitional and turbulent boundary layer to propagate information from the external flow to the wall region.