Interaction of a turbulent boundary layer with a free surface

The interaction between a turbulent boundary layer (TBL) and a free surface formed by a relatively thin air layer is experimentally investigated in a recirculating water tunnel with a 152 × 152 mm cross-section. A backward-facing step (BFS) of height, h = 6.35 mm, is used to continuously inject the air layer. Time-resolved particle image velocimetry (PIV), combined with free surface tracking, is employed to examine the relaxation of the TBL beneath the air layer. The furthest PIV measurement station is located approximately 2.5 times the test section height downstream of the BFS. The friction Reynolds number spans 800 to 2200, and the nominal air layer thickness ranges from 2 to 10 mm. Wavenumber–frequency spectra of the free surface deformation show that the free surface is excited by the incoming TBL. When scaled with the outer variables of the boundary layer, the frequency spectrum collapses. Two-point correlations reveal that coherent structures grow to twice their initial no-slip size by the time they reach the furthest PIV station, while turbulence beneath the air layer decays, with peak streamwise stress reduced to one-fourth of its initial no-slip value. Conditional averaging and correlations reveal that flow forms a wave-correlated critical layer where it speeds up beneath the surface bumps and slows down under dimples. This localized mixing may explain why the momentum deficit shown in mean velocity profiles extends deeper than the boundary layer thickness, anchoring the mean flow.