Cohesive Sediment Dynamics in Turbulent Flow

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Cohesive sediment refers to particles below approximately 63 microns in size. It is present in ecologically sensitive environments such as rivers, lakes, estuaries, and fisheries. Reliable prediction of contaminant and nutrient transport in these environments requires accurate models of cohesive sediment dynamics. We currently lack such models. For the small particles in cohesive sediments, attractive forces between particles due to electric charges frequently dominate hydrodynamic and gravitational forces. While these attractive forces can cause the particles to form larger aggregates or "flocs", turbulent fluid stresses tend to break up the flocs. Hence, the size distribution of cohesive sediment flocs is governed by a delicate balance of interparticle and turbulent stresses, which affects their transport rates. The attractive forces between particles also strongly affect the erodibility of sediment deposits on the seafloor, which influences sediment transport processes in rivers and oceans. This research will explore cohesive sediment dynamics in turbulent environments via a series of computer models and simulations. The aim is to develop reliable, predictive tools for the transport of nutrients and contaminants in the environment. The results will also improve predictions related to other technologies, including deep sea hydrocarbon exploration. The research will educate and train a doctoral student, as well as undergraduate and high school students, in computational modeling, fluid dynamics, sediment transport, and high-performance computing.The proposed research explores the dynamics of cohesive sediment in turbulent environments via a series of increasingly complex computational investigations, based on a hierarchy of approaches ranging from one-way coupled, reduced-order Lagrangian point particle models to grain-resolving direct numerical simulations (DNS) that are fully `four-way' coupled. It will address a broad range of fundamentally important questions, among them: a) how do the turbulence properties affect the equilibrium balance between sediment flocculation/coalescence and break-up?, b) how does the floc size distribution vary as a function of the turbulence and sediment properties?, c) how does the effective settling velocity of the cohesive sediment depend on the turbulence and sediment properties?, d) how are the turbulence properties altered by the sediment?, e) how is the effect of cohesive sediment on turbulence different from that of non-cohesive sediment?, and f) how is the erodibility of a sediment bed affected by cohesive forces? The envisioned computational simulations will serve as basis for formulating scaling laws that capture the dynamics of cohesive sediment in turbulence, and which are suitable for implementation into existing larger-scale sediment transport models.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.