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，克服车辆阻力所需的燃料或将流体从一地泵到另一地所需的电力。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，或连贯的结构，对许多工程兴趣至关重要，如传热，阻力或噪音产生。目前的研究计划旨在提高我们对如何操纵这些连贯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的相干结构的认识，即三维壁面射流和向后台阶的流动。每当射流切向一个表面时，就会形成一个三维的壁面射流。三维壁面射流经常出现在加热和冷却应用、燃烧器、通风应用以及涂层过程中。三维壁面射流强烈地“粘”在表面上，导致射流沿壁面向外扩散和横向增长，其增长速度是垂直于壁面的5倍。我以前的大部分研究工作都是为了理解壁面射流中的湍流相干结构是如何引起这种行为的。提出的研究目标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。增强的表面覆盖在许多加热和冷却应用中都是有益的，可以用来防止流动分离和增加飞机机翼的升力（因为一架飞机“粘”在表面上的效果更好，就不容易分离）。另一方面，减少表面覆盖在需要更高混合水平的通风和燃烧应用中是有用的。后向台阶上的流动是流动控制技术发展的第二个目标。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。能够控制流动分离将减少对车辆的阻力，从而提高燃油经济性并减少有害排放。由于流动分离过程产生的湍流是车辆内噪声的来源，控制流动分离也将创造更安静的车辆。提出的研究将通过首先开发工具来估计基于壁面传感器的瞬时湍流速度场，以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.