Effects of the formation of shear-driven liquid-surface waves on the gas-phase turbulent boundary layer

The gas-phase boundary layer evolution following transition from smooth-solid-wall to liquid-surface boundary conditions is characterized and contrasted with the evolution of a turbulent boundary layer over a smooth, solid wall of equivalent length. It was observed that an internal boundary layer forms following the development of stochastic surface waves. The internal boundary layer exhibits displaced momentum away from the surface and increased turbulence intensity compared to smooth-wall boundary layers. Scaling analyses indicate that mean velocity scaling resembles that of rough-walled turbulent boundary layers when scaled with a friction velocity value that was the same as the friction velocity immediately upstream of the liquid surface. Within the internal boundary layer, the streamwise Reynolds stress deviates markedly from the solid-wall behavior. Spectral analysis reveals that turbulence near the liquid surface is generated at wavenumbers corresponding to the surface waves, accompanied by additional small-scale turbulence. These differences are localized near the surface, with spectral characteristics converging toward those of a solid-wall boundary layer as the measurement point moves further from the surface. The turbulent kinetic energy dissipation rate differs significantly between liquid and solid surfaces, reflecting changes to the small-scale structure. A surrogate friction velocity capable of scaling the Kolmogorov length scale near the surface also demonstrates success in scaling the turbulence within the internal boundary layer, revealing that the streamwise Reynolds stress adheres to a logarithmic scaling similar to that observed in the overlap region of high Reynolds number wall-bounded flows.