Temporal and Spatial Evolution of a Turbulent Boundary Layer from a Drag-Reduced to a Canonical State

Experimental results are presented that are designed to shed light on the long-held questions regarding the causal direction and importance of interactions between outer and near-wall vortical motions in turbulent boundary layers that are associated with turbulence production. This makes use of our approach (Duong, et al, JFM, 2021) to produce up to 80% viscous drag reduction in turbulent boundary layers, with scaling covering a full decade range of Mach numbers. The drag-reduced boundary layers exhibit reduction in all Reynold stress components, turbulence production and the frequency of discrete turbulence producing events in proportion to the degree of drag reduction. The experiments document the temporal and spatial development of the turbulent boundary layer when the actuation that produces drag reduction is impulsively ceased, and the boundary layer evolves back to the canonical state. The measurements include all three velocity components in 3-D space, and 3-D spatial reconstructions of conditionally-averaged velocity components based on the detection of Reynolds stress-producing "burst-sweep" events. The ensemble-averaged velocity components are used to document the evolution of 3-D vortical structures associated with the turbulence production in the wall layer.