| Summary: | Direct numerical simulation (DNS) of a feedback-controlled turbulent channel flow at Reτ = 640 is carried out. As an idealized feedback control, we selectively damp either the small scale wall-normal velocity fluctuations (defined as those with the spanwise wavelength smaller than 300 wall units) or the large scale fluctuations (the spanwise wavelength larger than 300 wall units). The present DNS reveals that the control of small scale fluctuations leads to more drag reduction than that of large scale fluctuations. When the small scale fluctuation is damped, the friction drag is reduced by the amount corresponding to the absence of small scale fluctuation. In contrast, when the large scale fluctuation is damped, the friction drag reduction is much less than that expected from the absence of large scale fluctuation. In the latter case, the contribution from the small scale fluctuation to the friction drag is found to be drastically increased due to the reduction of pressure fluctuation and destruction of Reynolds shear stress.
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