Collaborative Research: LES and RANS simulations of estuarine flows: Understanding and parameterizing the role of Langmuir turbulence

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Langmuir circulation is recognized as pairs of parallel counter-rotating vortices (or cells) oriented approximately in the direction of the wind, characterizing the wave- and wind-driven turbulence (i.e. the Langmuir turbulence) within the upper ocean, lakes and estuaries. Its role in lakes and estuaries lacks understanding. The overarching goal of this project is to provide a fine-scale numerical study of Langmuir circulation, its interaction with other coherent flow structures and the resulting influence on vertical mixing and lateral dispersion within estuaries. Estuaries are briefly defined as coastal areas where freshwater and saltwater mix. Estuarine ecosystems are one of the most productive in the world made up of a myriad of habitats including oyster reefs, coral reefs, submerged aquatic vegetation, marshes and mangroves. The proposed numerical simulations will enhance understanding of flow dynamics that have direct impact on estuarine ecosystems. These dynamics govern the spatial distribution of water constituents such as dissolved organic matter, phytoplankton, suspended sediments and spilled contaminants that influence the health of estuarine ecosystems. The proposed studies consist of large-eddy simulations (LES) of the interaction between submesoscale coherent flow structures such as fronts and eddies and the smaller scale wind and wave-driven Langmuir circulation in idealized estuarine settings. LES will be used to investigate the idealized tidal intrusion front entering an estuary and an idealized river plume propagating through an estuary and discharging to a coastal ocean. The LES will allow investigations of (a) the role of turbulent mixing induced by Langmuir circulation in exchanges between the estuary and the coastal ocean via tidal intrusion fronts and river plumes and (b) the role of Langmuir circulation and its interaction with eddies and fronts determining the lateral and vertical mixing throughout the estuary. LES will allow for the development of improved turbulence closures able to account for the vertical mixing of the Langmuir turbulence regime. These closure models will then be used in Reynolds-averaged Navier-Stokes (RANS) simulations of the idealized flows and these results will be compared with the LES. This will allow for assessment of current turbulence closures for the RANS equations in the presence of Langmuir circulation. (3) Turbulence closures will be updated to account for Langmuir turbulence in RANS simulations. To date Langmuir turbulence closures have not been implemented for the three-dimensional RANS equations in coarse-scale estuarine flow simulations.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.