Large-scale control strategy for drag reduction in turbulent channel flows

In a recent article, Canton et al. [J. Canton et al., Phys. Rev. Fluids 1, 081501(R) (2016)] reported significant drag reduction in turbulent channel flow by using large-scale, near-wall streamwise swirls following the control strategy of Schoppa and Hussain [W. Schoppa and F. Hussain, Phys. Fluids 10, 1049 (1998)] for low Reynolds numbers only, but found no drag reduction at high friction Reynolds numbers (Re-tau = 550). Here we show that the lack of drag reduction at high Re observed by Canton et al. is remedied by the proper choice of the large-scale control flow. In this study, we apply near-wall opposed wall-jet forcing to achieve drag reduction at the same (high) Reynolds number where Canton et al. found no drag reduction. The steady excitation is characterized by three control parameters, namely, the wall-jet-forcing amplitude A(+), the spanwise spacing Lambda(+), and the wall jet height y(c)(+) (+ indicates viscous scaling); the primary difference between Schoppa and Hussain's work (also that of Canton et al.) and this Rapid Communication is the emphasis on the explicit choice of y(c)(+) here. We show as an example that with a choice of A(+) approximate to 0.015, Lambda(+) approximate to 1200, and y(c)(+) approximate to 30 the flow control definitely suppresses the wall shear stress at a series of Reynolds numbers, namely, 19%, 14%, and 12% drag reductions at Re-tau = 180, 395, and 550, respectively. Further study should explore optimization of these parameter values.