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

本研究分为两部分，即：旁路转捩机理的阐明和旁路转捩的控制研究的第一部分旨在阐明无压力梯度平板边界层尾迹诱导的旁路转捩过程。特别注意的是成立以及成长的湍流点所产生的传入的唤醒作为一个外部干扰。为了实现这一目标，发明了一种独特的尾流发生器来产生孤立的湍流点。采用一种由七根单热线探针组成的多探针传感器测量尾流影响边界层。尾流发生器由圆盘、圆柱体和一根带有小球的细线组成，细线上的小球通过试验平板前的主流时，在其后面产生周期性的尾流。这些球体尾流以空间和时间局部化的方式撞击平板，使得尾流周期性地在边界层内留下狭窄的受影响区域。观测结果证实，一个孤立的湍流点出现在每个受尾流影响的区域。研究还发现，在本研究中观察到的湍流斑与传统的箭头状湍流斑非常相似。第二部分研究了尾迹诱导旁路转捩的控制方法。在这种情况下，采用来自壁的流动抽吸。尽管为了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，壁面抽吸可以成为延迟尾流诱导边界层旁路转捩的一种可能方法

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