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Turbulent Mixing Across a Sharp Density Interface

Author(s): Blair Johnson; Arefe Ghazi

Linked Author(s): Blair Johnson

Keywords: Turbulence; Stratification; Mixing

Abstract: Industrial desalination generates brines that are often released back into the environment. Desalination brines discharged into coastal regions with weak currents or mild bathymetry do not necessarily mix with surrounding waters and can remain stably stratified rather than mixing with ambient waters (Hodges et al. 2011). Dense immobile saline layers from these discharges can cause hypoxia and threaten local ecosystems. There are several hydrodynamic forces at play in the mixing processes associated with brine discharges, including shear imposed by the velocity gradient of the discharge and turbulence at the bore front, among others. We are interested specifically in the role turbulence plays in a low mean shear environment, where ambient turbulence encourages mixing across the density interface. To accomplish this, we are conducting a laboratory based 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 (Reλ ~ 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 entrainment between the two layers. Mixing is quantified using the formulations described in Zhou et al. (2017), in which turbulent diffusivity is measured directly from LIF data. 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 brines into the environment, based on the turbulent structure of the ambient flow.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221238

Year: 2022

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