Takahide Endo

Feedback control of wall turbulence with wall deformation

Abstract Direct numerical simulation of turbulent channel flow was made in order to evaluate feedback control with deformable walls. When the local wall velocity is determined by an active cancellation control scheme similar to that of Choi et al. (Choi, H., Moin, P., Kim, J., 1994. J. Fluid Mech. 262, 75–110), the drag is decreased by about 12% with the wall deformation of the magnitude on the order of one viscous length. On the basis of the typical dimensions of the wall deformation thus obtained, a novel array of deformable actuators elongated in the streamwise direction is proposed. A new realizable control scheme by using wall information is developed based on physical arguments of the near-wall coherent structures. The location of quasi-streamwise vortices accompanied with streak meandering is successfully detected about 50 viscous lengths downstream from the sensing location, at which the spanwise gradients of wall shear stresses are measured. The wall velocity of each actuator is determined to counteract the wall-normal velocity induced by the streamwise vortices. By the present control scheme with the arrayed sensors and actuators, 10% drag reduction is achieved through selective manipulation of the streamwise vortices and streak meandering. It is also found that the energy input of the present control is one order of magnitude smaller than the pumping power saved.

Numerical Analysis of Flow around a Sphere with Active Dimples

Numerical simulations of flow around a smooth sphere and a sphere with temporally deforming dimples (active dimples) in critical Reynolds number regime, have been conducted in order to investigate the influence of deformation of active dimples on surrounding boundary layer. It is observed that the drag coefficient is decreased when the Reynolds number is greater than Re> 105 by the transition of the boundary layer for the smooth sphere, as is reported by many experiments in the literature. It is shown the drag coefficient is decreased up to 40% when appropriate dimple scales and period of deformation are given. The active dimple promotes transition of boundary layer in the vicinity of the sphere, and restraints the detachment of flow. It is expected the applicability of active dimples displayed to a flow control device which is attached the surface of transportation body such as an airplane, a ship and so on.