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

对气液界面湍流结构和输运机理进行了理论和实验研究。在0.6mWx0.6mHx10mL的水槽流动中，进行了不稳定和稳定分层条件下的传热实验。用激光多普勒测速仪和冷丝法分别测量了湍流速度分量和温度，发现湍流在气液界面附近的垂直运动产生了附加压力，进而产生表面波。其结果是，垂直速度波动和温度或浓度之间的相关性在界面处变为零，动量，热量和质量通量的湍流分量也是如此。然而，由于湍流速度脉动的垂直分量在界面处被转换成水平分量，因此湍流强度保持在高水平，与穿过固体边界的热交换系数和质量交换系数相比，穿过界面的热交换系数和质量交换系数较高。例如，在强不稳定条件下，即，在界面下的热对流中，交换系数比在加热的水平固体平板上的热交换系数大一倍。在稳定层结条件下，层结将湍流运动压制为内波，导致交换系数降低，基于雷诺应力方程和湍流热流方程，推导了层结湍流闭合模型，并对层结对交换系数的影响进行了理论预测。

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.