Density stratification effects on turbulent diffusion on the liquid side near the gas-liquid interface-Ocean flax of greenhouse gases-

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Turbulence structure and transport mechanism across the gas-liquid interface was investigated theoretically and experimentally. A special attention was made on the density stratification effects on the liquid side behavior, presuming that its strong dependence on the ocean fluxes of greenhouse gases.Experiments were made for unstable and stably stratified conditions in the water channel flow (0.6mWx0.6mHx10mL) with heat transfer. Turbulent velocity components and temperature were measured by laser Doppler velocimeter and cold wire meth\ods, respectively.It was found that the vertical motions of turbulence near the gas-liquid interface induces extra pressure and then surface waves. As a result, the correlation between vertical velocity fluctuations and temperature or concentration ones becomes zero at the interface and so does the turbulence components of momentum, heat and mass fluxes. However, since the vertical component of turbulent velocity fluctuation is converted into horizontal ones at the interface and so the turbulence intensity remains at high levels, exchange coefficients of heat and mass across the interface are high, compared with those across the solid boundary. For example, in the strongly unstable condition, i.e., in the thermal convection beneath the interface, the exchange coefficient is twice larger than that in heat transfer over a heated horizontal solid plate. In stably stratified conditions, the stratification depresses the turbulent motion into the internal waves and causes low exchange coefficient.A turbulence closure model for stratified flow was deduced, based on the Reynolds stress and turbulent heat flux equations and a theoretical prediction of the stratification effects on the exchange coefficient was presented.