Direct numerical simulation of feedback-controlled turbulent channel flow is carried out. Two cases of damping of the wall-normal velocity in the entire channel and of control with blowing and suction from the walls are examined. We define two distinct scale components, i.e., the viscous scale fluctuations with their spanwise wavelength smaller than 300 wall units, and the large ones of more than 300 wall units. Whilst the damping of the viscous scale fluctuations does not significantly affect the large scale ones, the damping of the large scale fluctuations results in increase of the contribution of viscous scale fluctuations to the friction drag. These results imply that controlling the viscous scale fluctuations is more effective. Moreover, in the case of wall blowing and suction, little drag reduction is attained by suppressing the large scale fluctuations.
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