Opposition control of turbulent wall-bounded flow using upstream sensor

In the present study, opposition control using upstream sensor for the skin-friction drag reduction in turbulent wall-bounded flow is developed and applied to turbulent channel flow at Reτ = 178. The actuation velocity at the wall is determined to be in opposition to the wall-normal velocity at sensing location in fluid flow located upstream of the actuator. Direct numerical simulations are performed with various wall-normal and streamwise locations of sensor. With upstream sensor appropriately chosen, the performance of opposition control is improved compared to the opposition control using sensor directly above the actuator in terms of drag reduction and control efficiency. Furthermore, opposition control with time delay of sensing velocity is constructed and its effect on drag reduction performance is examined. The introduction of time delay to opposition control using upstream sensor, for the cases for which opposition control without time delay is not so successful, successfully reduces the skin-friction.