Assessment of suboptimal control for drag reduction in turbulent channel flow

A systematic analysis is made of suboptimal control for drag reduction. The influence of the amplitude of actuation (A) and the timescale of actuation (Deltat(a)(+)) are evaluated. Two wall-sensing variables are employed, (partial derivativew/partial derivativey|(w) and partial derivativep/partial derivativez |(w)), with two actuations (phi(2) and phi(3)). To test the suboptimal control, direct numerical simulations of turbulent channel flow at Re-tau = 100 are performed in a spectral domain. It is found that the effect of A and Deltat(a)(+) on the drag reduction rate (D-r) is significant. The near-wall behaviours of the flow structure are analysed to characterize the drag reduction. An optimal timescale is obtained at Deltat(a)(+) similar or equal to 1 and Deltat(a)(+) should be less than Deltat(a)(+) similar to100. The size effect of the sensor/actuator is examined.