Evolutional Fundamental Study on Active Control of Wall Turbulence

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Facing the global issues such as depletion of energy resources and environmental deterioration, highly advanced technology of turbulence control is ever more needed. Turbulence control opens up new possibilities to achieve far greater efficiency and least environmental impact of various thermal-fluid systems supporting the human society through the manipulation and modification of momentum/heat/mass transfer, noise as well as chemical reaction. In the present study, we focused on one of the most canonical flows, i.e., wall turbulence and associated transport phenomena. We particularly aimed at developing innovative predetermined controls to drastically reduce skin friction drag and enhance heat/mass transfer. Specifically, we introduced new control indices such as the net energy saving rate and the control gain in order to evaluate cost effectiveness of existing control strategies and also a newly proposed traveling wave-like wall blowing/suction. We also applied uniform blowing/suction to a turbulent boundary layer to obtain general control strategies for spatially developing flows. For heat/mass transfer enhancement, we reexamined the governing equations and boundary conditions for the velocity and scalar fields in order to clarify possible scenarios for dissimilar heat transfer enhancement control. By applying the optimal control theory, we demonstrated that the dissimilar control is possible with a simple open-loop control strategy.