Entrainment across a density-stratified interface via isotropic turbulence mixing*

The present laboratory experiments are motivated to further our understanding of how turbulence generated by wind on the surface of a river or an estuary leads to entrainment and mixing of a stably stratified water column. The experiments are conducted in a transparent double glass tank, with turbulence produced using an oscillating grid arrangement. The inner and outer tanks have dimensions of 24.5 cm x 24.5 cm x 35 cm and 35 cm x 35 cm x 50 cm, respectively. The main purpose of using an inner tank is to minimize the effect of secondary flows in achieving isotropic turbulence. By varying the grid Reynolds number (Re_g = f_gS²/υ, where f_g is the gird frequency, S is the stroke length, and υ is the kinematic viscosity of water) from 1510 to 3025, we achieve a Taylor Reynolds number of 120 to 150 and a Froude number of 0.09 to 0.14. A stable step density of 1% is established in the inner tank using a double-bucket method, with fresh water above saline water. The instantaneous velocity field is measured using time-resolved Particle Image Velocimetry (PIV), and the results indicate the presence of reasonably isotropic conditions in the constant density fluid region, whereas turbulence becomes anisotropic near the interface. We also find that the decay of vertical velocity follows the expected linear rate of z^-1 in the constant-density fluid, but there is a greater decay when entering the denser fluid.