Investigation of coupled air-water turbulent boundary layers using direct numerical simulations

We perform direct numerical simulation of air and water turbulence in a Couette flow. The air-water interface is kept flat, and the coupling between the two fluids is through continuity of velocity and shear stress at the interface. True air-to-water ratios of density and viscosity are used in the simulation to illustrate features of air-water coupled boundary layers. Our analysis of statistics of the velocity, vorticity, and turbulent kinetic energy budget confirms known features, notably the similarity of the airside interface boundary layer to wall boundary layer. Our study obtains new insights on the characteristics of the waterside motions. Compared to the airside, waterside turbulence structures are more persistent and larger in scale, which dominate the interface signatures. The interface boundary layer on the waterside possesses unique features that are intermediate between but qualitatively different from wall boundary layer and free-slip surface layer. On the waterside, as the interface is appro...