Flow sensing, estimation and control in turbulent flows

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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.