TURBULENT MIXING AND FRACTAL STRUCTURE AT A DENSITY INTERFACE IN A STABLY STRATIFIED FLUID

Laboratory mixing box experiments were undertaken to examine turbulent mixing and fractal structure at a density interface in a stably stratified two-component fluid (fresh on salt), subjected to shear-free turbulence induced by an oscillating grid within the salt water layer. The density interface was visualized by adding fluorescein and dye into the salt water layer. A total of 12 runs were made. Measurements were made of (a) the height of the mean density interface above the bottom of the mixing box; (b) the densities of the fresh water and salt water layers. Photographs and videos were made of the two-dimensional and three-dimensional density interfaces. Calculations were made for (1) the entrainment velocity; (2) the overall Richardson number (Ri₀); and (3) the fractal dimension of the two-dimensional and three-dimensional density interfaces. The entrainment rate decreases with an increasing overall Richardson number. It can be expressed as Ri_o to the -3/2 or -7/4 power. This indicates that the rate of mixing deceases with decreasing turbulence intensity. Fractal dimension of the two-dimensional density interface is larger than 1, while that of the three-dimensional density interface is larger than 2. Fractal structures are present on the two-dimensional and three-dimensional density interfaces. Fractal dimension decreases with an increasing overall Richardson number. This suggests that the density interfaces become smoother with decreasing turbulence intensity.