Studies on Mechanism of Boundary Layer Bypass Transition over Airfoils and its Control

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This research consists of two parts ; i.e., clarification of bypass transition mechanism and control of the bypass transition.The first part of the study aims at clarification of wake-induced bypass transition process of a boundary layer on a flat plate with no pressure gradient. Special attention is paid to inception as well as growth of a turbulent spot created by the incoming wake as an external disturbance. To meet this goal a unique wake generator is invented to create an isolated turbulent spot. A multi-probe sensor with seven single-hot-wire probes is used to measure wake-affected boundary layer. The wake generator consists of a disk, pillars and a very thin wire with a small sphere on it. The sphere on the wire generates periodic wakes behind it when it passes across the main flow in front of the test flat plate. These sphere wakes impinge the flat plate in a spatially and timewisely localized manner so that the wakes periodically leave narrow affected zones inside the boundary layer. The observations confirm that an isolated turbulence spot emerges from each of those wake-affected zones. It is also found that the turbulent spot observed in this study bears a close resemblance to the conventional turbulent spot that takes a shape of arrowhead pointing downstream.The second part of the study is devoted to the development of controlling methodology of wake-induced bypass transition. In this case flow suction from the wall is employed. Although there remain a lot of things to do in order to improve the control method, the detailed boundary layer measurements have revealed that the wall suction can be a possible method to delay wake-induced boundary layer bypass transition