Extreme phenomena of turbulence under strongly stable stratification conditions

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In turbulent flows under extremely strong stably straified conditions flow pattern and diffusion and mixing mechanisms are drastically changed, and transition of turbulent eddy motion into wavy motion plays a substantial role and causes such drastic changes. From this point of view, this research project was aimed to investigate experimentally the characteristics of turbulence and intemal wave and the evolution of internal wavy motion in the stratified turbulent flow and to derive a unified explanation on the extreme phenomena which appear under strong stratification conditions. Extreme phenomena includes counter-gradient diffusion in the vertical direction, and horizontal diffusion and difference between active and passive scalar diffusion mechanisms in the counter-gradient diffusion field.It was found that the counter gradient diffusion and the associated phenomena under strongly stable stratification for zero shear cases was explained quite well by a liner theory. In addition, the theory was extended for the cases when both the shear and stratification exist, and explained such extreme phenomena successfully. It indicates that quite a large part of the extreme phenomena can be considered to be linear processes, although their time evolution is influenced by the initial conditions.Basing upon the new concept mentioned above, a turbulence closure model for the stratified flows was developed in which fluid motion was divided into mean, wave and turbulence components and nonlinear interactions between them were modeled, and the model parameters were determined by the experimental results.