The role of turbulent coherent structures on the evolving seabed

Seabed bathymetric features, known as ripples, are responsible for hydrodynamic dissipation, sediment transport, and benthic flux exchange. The three-dimensional low steepness ripples commonly observed in medium and fine sand beaches (median grain size of 0.15-0.3 mm) have received much less attention compared to the high steepness coarse sand ripples, even though the medium and fine sand beaches are more common in nature. Furthermore, the migration of medium and fine sand ripples may contribute to a significant part of the beach profile evolution. The proposed work will apply numerical modeling methods to study medium and fine sand ripples driven by waves and currents. The Large Eddy Simulation (LES) two-phase flow methodology will be applied to understand the geometry and migration of medium-fine ripples in the presence of flow separation vortices, flow instabilities, and turbulent coherent structures in a turbulence-resolving system. Medium and fine sand ripples will be studied using a large-eddy simulation (LES) Eulerian two-phase methodology. The steepness of medium-fine sand ripples is both low and capricious, and it is hypothesized in this project that their evolution is dictated by flow instabilities and coherent structures such that the conventional paradigm which separates the primary vortices from the cascade of turbulent coherent structures is inadequate. The proposed work is expected to gain a new understanding of explaining the geometry and migration of medium and fine sand ripples, deliver an open-source numerical simulation model capable of simulating low steepness ripples, and inform improved parameterizations on transport rate and migration. The project will test the hypothesis that the Reynolds-averaged modeling approach that objectively separates the energy cascade of turbulent coherent structures from the resolved primary vortices becomes inadequate for low steepness ripples. This project will create the first turbulence-resolving (LES) Eulerian two-phase model capable of simulating the geometry and migration of medium and fine sand ripples by extending the existing open-source model, SedFoam. 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.