Collaborative Research: Large Eddy Simulations of Estuarine Mixing

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OCE-0451699/04521740/0452380New Large Eddy Simulations (LES) will be conducted to investigate turbulent mixing processes in estuaries and coastal oceans. Temperature and salinity equations will be added to an advanced LES code that has been validated in engineering flows. The model will include LES equations generalized to incorporate large-scale density and pressure gradients, non-uniform meshes in two directions, allowing for fine resolution of the stratified pycnocline and other regions of interest, and an immersed-boundary method to represent variable-bottom bathymetry and curvy coastlines. This LES model will investigate three important mixing processes in estuaries: (1) asymmetric tidal mixing due to baroclinic pressure gradient and tidal straining; (2) mixing and lateral circulation in a straight, stratified channel with transversely varying bottom depth; (3) topographically-forced, localized mixing in hydraulic transitional flows near channel constrictions. Finally, we shall use the LES model to simulate a segment of the Hudson River estuary and compare the model results with recent dye-release experiments; the velocity and scalar fields obtained from the LES will be used to interpret observations collected in this partially-mixed estuary. The objectives of the proposed research are to elucidate turbulent mixing processes in stratified estuarine flows and to extend LES modeling techniques to horizontally-inhomogeneous estuarine and coastal flows affected by varying bathymetry and shoreline geometry.Intellectual merit: Understanding turbulent mixing in stratified shear flows is a fundamental problem in physical oceanography. Partially-mixed estuaries are an excellent natural laboratory for studying stratified turbulence, and elucidating key turbulent processes affecting salt and momentum transports in estuaries will shed light on the important mixing problem. LES techniques have been used successfully to investigate oceanic boundary layers; however, they are generally limited to horizontally-homogeneous flows in simple geometries. The methodology to be developed in this project can be used to tackle a wide range of turbulent mixing problems in coastal oceanography and generate turbulence data for calibrating and improving turbulence parameterization schemes.Broader impact: The proposed study will lead to improved representations of turbulent mixing processes in regional ocean models, which are used to predict sea level, currents, water quality, contaminant transport, and ecosystem productivity in estuarine and coastal environments of societal concerns. Biogeochemical state variables can be added into the model to investigate ecological hotspots such as harmful algal blooms in estuaries and coastal oceans. The project will provide training for a graduate student and a postdoctoral fellow to acquire advanced numerical skills for solving oceanographic problems.