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

Abstract

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 approached, enstrophy and viscous dissipation first decrease together, with higher turbulence mixing and production than the airside, and then increase sharply in response to airside stress fluctuations. The waterside turbulence is characterized by interface-connected, hairpin, and quasistreamwise vortices that are closely related to each other in their evolutions. Conditional average using a variable-interval space averaging approach illustrates the strong air-water coupling in splat, hairpin vortex, and quasistreamwise vortex events. It is found that waterside velocity gradients are amplified significantly across the interface, with the presence of horizontal jets at splat and quasistreamwise vortex regions. We also study the transport of passive scalars near the air-water coupled boundary layers. As expected, vertical advection associated with coupling air-water coherent structures greatly enhances interfacial transfer of scalars. By considering scalars of different diffusivity and solubility values, we investigate the partition of interfacial transfer between the waterside and airside. We illustrate statistical characteristics of gaslike and heatlike scalars and perform detailed analysis of the scalar variance budget for these to reveal unique features that are substantially different from wall boundary layer or free-slip surface layer.