Turbulent Erosion of a Sharp Density Interface

Desalination, often used in cooling systems and potable water production, generates brines that are ultimately released back into the environment. Desalination brines discharged into coastal regions with weak currents and mild bathymetry, such as the Gulf of Mexico, do not necessarily mix with surrounding waters and can remain stably stratified (Hodges et al. 2011). Dense immobile saline layers from these discharges can cause hypoxia and threaten local ecosystems. Although there are several hydrodynamic forces at play in the mixing processes associated with brine discharges, we are interested specifically in the role turbulence plays in a low mean shear environment. To accomplish this, we are conducting an experimental study to investigate the effect of homogeneous isotropic turbulence on a sharp density interface and identify the flow mechanisms that promote and/or inhibit interfacial erosion.

We use randomly actuated synthetic jet arrays (RASJA – Variano & Cowen 2008) to generate high Reynolds number (Re_λ ~ 300) horizontally homogeneous isotropic turbulence with negligible secondary mean flows of fresh water above a dense layer. Stereo particle image velocimetry (PIV) measurements are collected for turbulence analysis in the forced upper layer. Statistical metrics include turbulent kinetic energy, dissipation, spectra, and integral scales. Simultaneous laser induced fluorescence (LIF) measurements are used to visualize the instantaneous density of the saline layer and quantify entrainment between the two layers.

Mixing is quantified using the formulation described in Zhou et al. (2017), (see image) in which b denotes buoyancy, z_* describes a reference point along the isopycnals, κ denotes molecular diffusivity, and κ_e is an effective diffusivity enhanced by turbulence. Using this method gives a refined metric of buoyancy gradients across the spatio-temporally varying two-dimensional concentration record and allows for a full exploration into what initial conditions (i.e. relative density between layers, turbulent forcing) encourage mixing. By quantifying the interplay between mean shear free homogeneous isotropic turbulence and a sharp density gradient, we aim to deduce under what environmental conditions it is sustainable to discharge brine into relatively quiescent flows.