Flow sensing, estimation and control in turbulent flows

湍流是人类能源消耗的重要组成部分，例如，克服车辆阻力的燃料或从一个地方到另一个地方泵送液体所需的电力。现在人们认识到，在看似随机的湍流中存在有组织的运动；这些有组织的运动或相干结构对许多工程量的关注至关重要，如热传递、阻力或噪声产生。目前的研究计划旨在提高我们对如何操纵这些相干结构以产生有用的工程结果的理解，如增强传热、减少车辆阻力或减少噪音排放。提出的研究计划旨在提高我们对如何操纵两种流动--三维壁面喷流和后向台阶流动--相干结构的了解。 当喷流切向指向某一表面时，就形成了三维壁射流。三维壁面射流经常出现在加热和冷却应用、燃烧室、通风应用以及涂层工艺中。三维壁面喷流强烈地“粘住”表面，导致喷流沿壁面向外扩散并向外扩展，其大小是垂直于壁面的5倍。我之前的大部分研究工作都是为了理解墙喷流中的湍流相干结构是如何导致这种行为的。这项研究的目的之一是改变壁面射流中的相干结构，从而改变流动的表面覆盖率。增加表面覆盖率将有利于许多加热和冷却应用，并可用于防止气流分离和增加飞机机翼的升力(因为更好地粘合在表面上的喷气式飞机不太容易分离)。另一方面，减少表面覆盖率在需要更高水平混合的通风和燃烧应用中将是有用的。 后向步骤上的流是用于开发流控制技术的第二个流。后向台阶上的流动是车辆尾部经常发生的流动分离过程的简化模型。能够控制气流分离将减少对车辆的阻力，从而提高燃油经济性，减少有害排放。由于分流过程产生的湍流是车辆内的噪声源，控制分流也会产生更安静的车辆。这项研究将首先开发工具，利用基于壁面的传感器估计瞬时湍流速度场，从而实现分离控制的目标。

Turbulence is responsible for a significant portion of human energy consumption, for example, in fuel to overcome the drag on vehicles or the electricity required to pump fluids from place to place. It is now recognized that there are organized motions in seemingly random turbulent flows; these organized motions, or coherent structures, are critical to many quantities of engineering interest, like heat-transfer, drag, or noise production. The present research program aims to improve our understanding of how to manipulate these coherent structures to produce useful engineering results like enhanced heat-transfer, drag-reduction on vehicles, or reduced noise emissions. The proposed research program aims to improve our knowledge of how to manipulate the coherent structures in two flows, the three-dimensional wall jet and the flow over a backwards facing step. Whenever a jet is directed tangentially at a surface, a three-dimensional wall jet is formed. Three-dimensional wall jets occur frequently in heating and cooling applications, combustors, ventilation applications, as well as in coating processes. The three-dimensional wall jet strongly “sticks” to surfaces which causes the jet to spread and grow laterally outward along the wall 5 times greater than it grows perpendicular to the wall. Much of my previous research effort has gone into understanding how the turbulent coherent structures in the wall jet cause this behaviour. One of the goals of the proposed research is modifying the coherent structures in the wall jet so as to alter the surface coverage of the flow. Enhanced surface coverage would be beneficial in numerous heating and cooling applications and could be used to prevent flow separation and augment lift on aircraft wings (as a jet that “sticks” better to surfaces would be less prone to separate). Reduced surface coverage, on the other hand, would be useful in ventilation and combustion applications where higher levels of mixing are desired. The flow over a backwards facing step is the second flow targeted for developing flow control techniques. The flow over a backwards facing step is a simplified model of the flow separation process which occurs frequently on the rear of vehicles. Being able to control flow separation will reduce the drag on vehicles, thereby increasing fuel economy and reducing harmful emissions. As the turbulence created by flow separation processes is a source of noise within the vehicle, controlling flow separation will also create quieter vehicles. The proposed research will work towards the goal of separation control by first developing tools to estimate the instantaneous turbulent velocity field using wall based sensors.