Smart skin for control of wall-bounded turbulent flows

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A boundary layer is a thin layer of fluid in the immediate vicinity of a solid surface where fluid velocity transitions from the velocity of the solid surface to the bulk fluid velocity. The shear stress in the fluid generates this velocity gradient, and projects as skin-friction' on the surface. In most industrial situations, for example, the flow over an aircraft or inside a pipeline, the motions of fluid elements in the boundary layer are chaotic. Such a fluid layer is known as a turbulent boundary layer (TBL), and has a larger velocity gradient that generates greater skin-friction. In addition, when an increasing pressure gradient is imposed on a TBL, like in a diffuser or on an aircraft wing, the TBL may separate from the surface. This generates pressure drag' and strong fluid-structure interactions. As a result, the TBL is the chief source of high fuel consumption, pollution, and structural vibration in aircrafts, submersibles, ships, and pipelines. Due to this integral effect, any progress in the control of TBLs will have a profound impact on our socio-economic and environmental protection needs. The subject of this research program is the control of attached and separated TBLs using a novel smart skin'. To achieve this goal, we will first experimentally characterize the surface pressure of large-scale motions in attached and separated TBLs to develop a predictive model for turbulence control. In parallel, we will develop deformable surfaces that can generate on-demand local surface depression or protrusion. Linear actuators, with a frequency and displacement that matches the pressure footprint, will be integrated under a deformable skin. We will implement an array of active deformable panels and surface pressure sensors into a smart skin'. Finally, the performance of the smart skin' will be evaluated in a closed-loop system for active control of attached and separated TBLs. In the long-term, the proposed program will increase our fundamental knowledge of coherent motions in TBLs and provide guidelines on how to control TBLs. The developed smart skin' will make it possible to manipulate a variety of turbulent flows including TBLs. The smart skin will also become a test-bench for the application of advanced control algorithms, artificial intelligence, and machine learning. Such an approach can ultimately modernize how we handle turbulence. It can make a breakthrough by bringing turbulence control to a variety of industrial sectors to increase efficiency and reduce environmental footprints. In addition, the multidisciplinary nature of this research program will train highly qualified personnel who are trained in a variety of disciplines.