Boundary Layer Transition Induced by Periodic Wake Passage. Effect of Velocity Fluctuation Caused by Wake Passage.

Abstract

Detailed measurements are performed on boundary layers on a flat plate subjected to incident periodic wakes. The objective of this study is to examine whether the velocity fluctuation associated with the wake passage, which is due to a so-called negative jet, might have any effects on the wake-induced boundary layer transition. A spoked-wheel-type wake generator is used to simulate the unsteady flowfield over the suction surface of a turbine blade or a compressor blade by changing the direction of rotation of the wake generator. Wake-affected heat transfer distributions on the flat plate indicate that the wake passage promotes the boundary layer transition ; however, appreciable difference in the transition onset point appears between the two unsteady flow conditions with the normal rotation and the reverse rotation of the wake generator, emulating a turbine blade case and a compressor blade case, respectively. This difference is also confirmed by the boundary layer measurement using a hot-wire probe, which shows that a turbulent region induced within the boundary layer by the wake passage for the reverse rotation case evolves downstream at a consider-ably slower speed compared to that of the normal rotation case.