Large scale motions in a manipulated turbulent boundary layer

Abstract Experiments were performed to examine structural changes that exist in a turbulent boundary layer with measured 2–10% net drag reduction obtained using a tandem arranged parallel plate manipulator (hereafter abbreviated TAPPM) . Conditionally sampled and space-time correlation measurements support a picture of approximately equal reductions in the streamwise and normal components of the large scale fluctuations, both in the boundary layer and in the irrotational ambient flow in the valleys between the large scale motions (LSMs) at 20δ o . However, by 51 δ o , both the LSMs and the response of the irrotational ambient flow relax to approximately their unmanipulated state. Simple modeling suggests that in the valleys of irrotational fluid the motion would reestablish itself in 5 δ , and it is suggested that the presence of the TAPPMs wake in the outer region inhibits the reformation of these large scale wallward sweeps sustaining the lower values of the local skin friction, C f . Because of the thick boundary layer in the present experiment, at 20 δ o the TAPPMs wake has not entered the wall region, in contrast to many other investigations. At streamwise locations just before the main body of the wake intersects the wall (56 δ o ) , a rapid rise in C f is observed. Afterwards, a small reduction in C f (confirmed by three independent methods of measurements) is maintained resulting from the remaining defect in the wake, which lasts until the wake is fully mixed.