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Direct Numerical Simulations of Turbulent Channel Flow with Local Forcing at Walls
Guillermo Araya, Stefano Leonardi, Luciano Castillo and Paolo Orlandi

Direct Numerical Simulations (DNS) of the velocity field in a fully developed turbulent channel flow are presented. Periodic normal blowing/suction velocity disturbances at both walls are considered in order to investigate numerically the influence of local forcing on the turbulent structures. Local forcing is modeled as a boundary condition with sinusoidally oscillating normal velocities in thin spanwise slots located cyclically along the streamwise direction. By locally perturbing the flow, a skin friction reduction is attained in a zone situated downstream of the perturbing source. The maximum local skin friction reduction of 30% is accomplished at the maximum forcing amplitude considered, Ao= 0.35, with an intermediate frequency of f = 0.04. A phase-averaging procedure is employed to discriminate between coherent and uncoherent structures, and together with the highest level of information provided by DNS, allow a better understanding of the physical mechanism responsible for drag reduction.

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