Collaborative Res: Physics of lutoclines and laminarization extracted from turbulence-resolved numerical investigations on sediment transport in wave-current bottom boundary layer

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Recent numerical investigations reveal the existence of four distinct regimes of wave-induced fine sediment transport ranging from well-mixed transport, to the formation of a lutocline, and eventually a complete flow laminarization over a range of sediment availabilities and settling velocities. The numerical model is based on an Eulerian-Eulerian two-phase formulation simplified for fine sediment (small particle response time) while resolving all the scales of turbulence-sediment interactions. This project will further investigate four critical science issues related to these regimes via numerical simulations. Firstly, a complete phase map will be constructed of flow regimes as a function of wave Reynolds number, bulk Richardson number (sediment availability) and nondimensional settling velocity with a series of carefully designed simulations. The results will highlight the major differences between the tidal and wave boundary layers in response to sediments. Secondly, with a better understanding of the onset of laminarization, the model will be enhanced to support non-Newtonian rheology in order to study the interplay between rheological stress and turbulence modulation in determining the transition of flow regimes and hydrodynamic dissipation. Thirdly, mean current will be added to the simulations. Wave-current interaction may enhance the mud layer thickness and transport, as observed in a recent field study at the shelf of Waiapu River (New Zealand). However, if the current is too strong, sediments can be re-entrained and become well-mixed and hence the formation of gravity flow is prevented. Finally, the model will be expanded for transport of coarser grains. Concurrent transport of clay and silt due to decreasing wave Reynolds number will be first studied. Next step will be to simulate a complete polydispersed system using a direct quadrature method of moments approach. Polydispersed simulation efforts allow insights into the processes causing the observed microstratigraphy in mud-dominant environments.Several prior field observations on continental shelves reveal a variety of seabed states due to wave-current driven sediment transport. The occurrences of these seabed states have several critical implications. For example, the formation of a lutocline indicates trapping of fine sediments near the bed and the resulting large density anomaly may yield significant offshore sediment transport on the shelf through wave-supported gravity flows. When surface waves propagate over a muddy seabed, high wave dissipation rate is often observed during the waning stage of a storm as the fluid mud layer becomes laminarized. A recent microstratigraphy study of mud deposits suggests a three-part sedimentary microfabric that can be associated by processes occur during wave-supported gravity flow events. The main challenges of modeling wave-induced fluid mud transport are the coupling between sediment and turbulence, the transitional nature of turbulent flow, rheology and the polydispersed nature of transport. This research addresses these challenges and the results will be valuable in further interpreting critical processes observed in the mud-dominant coastal environment.The project will improve our understanding of the resuspension and delivery of fine sediment across the continental margin, which is a critical element of the sediment source to sink study. This study will also improve the ability to predict the surface layer properties of the seabed which is critical to underwater exploration and wave prediction. Using wave tanks already available, a hands-on laboratory experiment to visualize the existence of wave boundary layer and the intermittent nature of the mixing process near the bed will be developed by undergraduate students. This newly-designed experiment will be used in Engineering outreach activities taking place annually during the summer session of each institution. In Year 3, this experiment will be added to the curriculum in the undergraduate fluid mechanics laboratory at U. Delaware.