Experimentally investigating mixing of a density interface due to mean shear free turbulence

Abstract:

Mixing is an important process in the atmosphere, ocean, and lakes, especially where fluids with different densities constitute a stably stratified flow system. Along with many other factors, turbulence is one of the enhancing factors in mixing across a density interface. To study the effect of turbulence without any mean shear, we designed experiments of a stable two-layer system in which the lower layer is quiescent and the upper layer is continuously forced with turbulence. We use a 16 by 16 randomly actuated synthetic jet array (Variano & Cowen, 2008; Johnson & Cowen, 2018) to produce homogeneous isotropic turbulence with zero mean shear in the upper layer of water. The density difference is generated by dissolving sugar in the lower layer.

Particle image velocimetry (PIV) is used to measure the velocity field and to compute turbulence characteristics such as integral length scale, turbulent kinetic energy, energy spectra, and dissipation. Simultaneously with PIV, laser-induced fluorescence (LIF) is employed to measure the density distribution as the two layers mix. From LIF data, we quantify mixing, entrainment, and erosion of the density interface, by finding the location of isopycnals surrounding the density interface. Since the refractive indices of the two layers are different due to the presence of sugar, the laser beam bends when passing through the interface. This can cause significant inaccuracies in data acquisition and analysis. Thus, a refractive index matching technique is needed. In this study, we use Isopropyl alcohol to increase the refractive index of the upper layer while maintaining the density difference between the two layers. We perform experiments with a wide range of density differences and turbulence levels to explore mixing rates and interfacial dynamics through all possible states of flow.