Turbulent boundary layer relaxation over an air layer

The relaxation of turbulent boundary layers (TBLs) exposed to a free surface formed by a thin air layer is investigated. A backward facing step of height, h = 6.35 mm, is used to facilitate air layer formation and fix the flow separation downstream of which liquid TBL goes from a solid wall to a free surface. Experiments are conducted in a recirculating water tunnel with a 1830 mm long, (152.4 mm)² cross-section test section and a gas-liquid separation tank. Laser Doppler Velocimetry (LDV) is employed to measure boundary layer relaxation and Reynolds stresses as a function of free stream velocity, U_∞, and streamwise direction, x. Incoming TBL ranges between 200 < Re_τ < 300, and nominal air layer thickness is kept constant at t_AL/h = 1.5. Nearest to the air-water interface, TBL is found to relax more rapidly than in other regions, resulting in an inflection point that moves toward the centerline as the streamwise distance evolves from x/h = 1 to 74. Reynolds stress peaks are found to diffuse and move away from the interface, approaching the centerline with increasing streamwise distance. Time-resolved PIV is employed to quantify diffusion (ν∇²Ω) and stretching/tilting (Ω⋅∇V) evolution of initially wall-attached eddies.