On the Effect of Stably Stratified Layer to the Development of Free Convection

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Turbulence structure and transport process beneath the gas-liquid interface is important in air-sea interaction and in many industrial operations. As, in stably stratified circumstances (with inversion layer), turbulence parallel to vertical direction is suppressed, turbulence structure is expected to be affected greatly. To clarify the effect, we experimentally and numerically studied the development and statistical properties of a free convective layer under a water surface. Velocity and temperature fluctuations were simultaneously measured by a laser-Doppler velocimeter and a fine thermocouple. Variances and correlations for the fluctuations were normalized by the kinematic heat flux and the thickness of the convective layer, and vertical profiles of those quantities were obtained.Their profiles were compared with those for neutral circumstances (without inversion layer). The variance of the horizontal velocity decrease more slowly near the inversion layer, compared to neutral circumstances. The value of the correlation between temperature and vertical velocity became positive near the inversion layer, whereas the value was always non-positive in neutral circumstances.The situation was numerically simulated by DNS method. The calculated profiles of the variances were consistent with the experimental ones. By the simulation with various boundary conditions at the surface, it was found that stripe structure, which appeared in near the surface, was supposed to be generated by shear stress at the surface. There was no significant difference for the variances by the difference of the boundary conditions.