Studies on Wake-Induced Bypass Transition of Flat-Plate Boundary Layers under Pressure Gradients and Free-Stream Turbulence

摘要

This study investigates wake-induced bypass transition of a flat-plate boundary layer that experiences favorable and adverse pressure gradients. The effect of free-stream turbulence is also examined. This paper mainly focuses on how the pressure gradients affect the transitional behavior of the wake-disturbed boundary layer with and without the influence of free-stream turbulence. A wall-contouring of the test duct generates the flow acceleration and deceleration on the flat plate, emulating aft-loading or front-loading pressure distributions of actual turbine blades. A spoked-wheel type wake generator is used to produce wakes moving over the boundary layer. Detailed boundary layer measurements are performed by use of a hot-wire anemometer. In addition to velocity fluctuations, which clearly indicate transition process, intermittency factors are obtained and compared with the results given by a transition model. Noticeable differences in transitional behavior are observed between the cases with and without the enhanced free-stream turbulence. It is also confirmed that the wake width as well as the direction of the bar movement are influential factors to the bypass transition.