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

本研究包括两个部分，即旁路转变机理的阐明和旁路转变的控制。研究的第一部分旨在阐明无压力梯度平板上边界层尾迹诱导的旁路转变过程。特别注意由即将到来的尾流作为外部扰动而产生的湍流斑点的开始和增长。为了实现这一目标，发明了一种独特的尾迹发生器来创建一个孤立的湍流点。采用具有7个单热线探头的多探头传感器测量尾迹影响的边界层。尾流发生器由圆盘、柱子和一根上面有一个小球体的非常细的金属丝组成。当导线上的球体穿过测试平板前面的主流时，它会在其后面产生周期性的尾迹。这些球体尾迹以空间和时间局部化的方式撞击平板，从而使尾迹周期性地在边界层内离开狭窄的受影响区域。观测证实，每个受尾迹影响的区域都出现了一个孤立的湍流点。研究还发现，本研究中观察到的湍斑与传统的指向下游的箭头状湍斑非常相似。研究的第二部分致力于尾迹诱导旁路转变控制方法的发展。在这种情况下，使用的是从壁面的流动吸力。虽然要改进控制方法还有很多工作要做，但详细的边界层测量表明，壁面吸力可以作为延迟尾迹诱导的边界层旁路转变的一种可能的方法。

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