Direct Numerical Simulations of Interfacial Turbulence at Low Froude and Weber Numbers
Sea surface temperature accessible through use of remote sensing techniques (IR imaging, etc.) suggests abundant flow and thermal field information at the ocean surface that is closely related to subsurface turbulent activities. The suggested information includes wind stress, surface dissipation, underneath velocity and vorticity, and heat and gas transportation. Due to the constantly outgoing interfacial latent and sensible heat flux, the very surface of the ocean is often cooler than the bulk. This so called ?cool skin layer? below the very surface is greatly involved in the underlying interfacial turbulence and is the primary support of using sea surface temperature imaging to detect the subsurface activities. In addition, studies have shown that for this detection method the effects of ubiquitous surfactants (surface free agents) to the subsurface turbulence should also be considered.
In the case when the wind stress at the surface is far less significant than the buoyancy force in the water phase, the cool skin layer accumulates and triggers free convection. A series of numerical simulations is conducted to reproduce such a free convection flow to obtain detailed statistics and structural features in order to investigate the correlation between the surface temperature and the subsurface activities of the flow. The simulations are also aimed at the quantitative evaluation of the surfactant effects on the flow. The results of the simulations demonstrate that the surface temperature is statistically and structurally correlated to the subsurface activities in various patterns, and that surfactant has a certain influence to the subsurface turbulence with an overall effect of reducing the average surface temperature.
Based upon the framework of the controlled flux method, a novel approach to actively determine the interfacial gas transfer velocity at the free convection surface is proposed and numerically investigated. The proposed and simulated approach employs a temporal volumetric heating source to suppress the free convection. The heating source is defined and parameterized with respect to the physical properties of radiation absorption in water phase. Observation and interpretation of the surface temperature evolution and the flow features during and after the heating suggest the effective suppression of the free convection, the onset of the Rayleigh instability and the re-establishment of the free convection. Based on that, an analytical conduction model is formulated to obtain the heat transfer velocity at the free surface from the surface temperature. The gas transfer velocity is then inferred through similarity.