Effects of Periodic Unsteadiness, Free-Stream Turbulence, and Flow Reynolds Number on Separation-Bubble Transition

Abstract

This paper presents measurements of the combined effects of free-stream turbulence and periodic streamwise velocity variations on separation-bubble transition. The measurements were performed on a flat plate at two values of flow Reynolds number, with a streamwise pressure distribution similar to those encountered on the suction side of axial turbine blades. The experiment was designed to facilitate independent control of turbulence and periodic velocity fluctuations in the free-stream. The free-stream turbulence intensity was varied from 0.4% to 4.5%, and the periodic unsteadiness corresponded to Strouhal numbers of 0.0, 2.4 and 4.0. Based on the results, the relative importance of free-stream turbulence and periodic unsteadiness on the streamwise locations of separation, transition and reattachment points are quantified. Existing mathematical models for predicting separated-flow transition and reattachment are then evaluated in this context.