Mixing across a sharp density interface in homogeneous isotropic turbulence

Abstract:

There are many multilayer systems formed by fluids of different densities passing each other, as observed in the atmosphere, oceans, lakes, and others. An example is when brine is released into a relatively quiescent ambient flow forming a two-layer system, with mixing dependent upon ambient turbulence instead of interfacial mean shear stress. Mixing is affected by numerous parameters such as the density gradient, shear across the interface, and turbulence level within each layer or generated between layers. Turbulence can enhance mixing between layers in a stably stratified flow, and, in some situations, turbulence mixes the entire system. In order to quantify the role of turbulence in mixing, we designed experiments to investigate a stably stratified system with a sharp density gradient in which the upper layer is stirred continuously with turbulent forcing.

In laboratory experiments, homogeneous isotropic turbulence with zero mean shear is generated in the upper layer of a water tank using a randomly actuated synthetic jet array located at the top of the tank. This allows us to isolate interfacial mixing as a result of turbulence. The use of particle image velocimetry (PIV) allows us to measure the velocity field and compute turbulence metrics such as the integral length scale, turbulent kinetic energy, energy spectra, and dissipation. Simultaneously, laser -induced fluorescence (LIF) is employed to measure the concentration distribution, from which entrainment and interfacial erosion can be determined. By combining these two methods, a thorough quantification of the instantaneous relationship between mixing and turbulence is possible. Mixing across the interface is affected by the buoyancy force, the turbulence characteristics, and their interaction. Thus, by changing the Richardson number, the turbulent Reynolds number, and geometry of the two layers, we can determine under what conditions different mixing rates and interfacial dynamics occur.