レゾルベント解析の示唆に基づく乱流摩擦抵抗低減手法の提案

抄録

Suboptimal control proposed by Lee et al. (J. Fluid Mech., vol. 358, pp. 245–258, 1998) is one of the effective feedback control schemes for turbulent skin friction reduction. In 2017, Nakashima et al. (J. Fluid Mech., vol. 828, pp. 496–526, 2017) elucidated the suboptimal control effect by using resolvent analysis. Building on the implication obtained from their result, we try to improve the performance of the control law developed by Lee et al.. However, this attempt does not result in drag reduction. Examination for spectral budget equation attributes this lack of efficacy to nonlinear energy transport between different scales or wall-normal locations. Then, we propose a new feedback control law via resolvent analysis and assess its effect by direct numerical simulation at the friction Reynolds number of Re_τ = 180. As a result, about 10% drag reduction is obtained by the new law. Conditional averaging method reveals the control successfully suppresses near-wall coherent structures. Physical meaning of the control law is finally derived from the theoretical viewpoint.