The Role of Turbulent Coherent Structures on the Evolving Seabed
a Warren Distinguished Lecture with Tian-Jian Hsu, Civil and Environmental Engineering, University of Delaware
ABSTRACT
In the estuarine and coastal modeling community, the Reynolds-averaged methodology has been the most popular and affordable approach. The main assumption of a Reynolds-averaged methodology is to model the effect of turbulent fluctuations via a “diffusion” and hope that the ensemble averaged mean flow structure can be well-resolved. This methodology is proven to be useful for turbulent shear flows in simple flow configuration. However, the Reynolds-averaged closure assumption becomes ambiguous when the flow of interest involves instabilities, generation of primary vortices (e.g., flow separation), and 3D configurations as it forces a separation between mean flow structure and turbulent coherent structure.
Computational fluid dynamics (CFD) for multiphase flows are applied in this study to investigate coastal processes, particularly coastal sediment transport problems that require resolving turbulent coherent structures. SedFoam is a Eulerian two-phase model for sediment transport applications. It is based on solving the mass and momentum equations for the water phase and fluid phase with closure on turbulence, particle stresses, and interphase momentum coupling. Therefore, SedFoam is capable of modeling the full dynamics of sediment transport without the need to artificially separate transport into bedload and suspended load layers. To resolve turbulent coherent structures, SedFoam has recently been extended for 3D large-eddy simulation capability. In this talk Tian-Jian Hsu will discuss three applications. The first application demonstrates how flow instabilities and turbulent coherent structures drive the observation of large sheet flow layer thickness and resulting unexpected offshore transport unique for fine sand in onshore velocity-skewness waves. The second application further investigates the importance of resolving turbulent coherent structures in the generation of primary vortices and the resulting scour development around a vertical pile. In particular, Hsu demonstrates that the conventional Reynolds-averaged two-equation closure significantly under-predicts the vortices’ intensity and, hence, the predicted scour depth. Finally, Hsu extends the investigation to understand the significance of resolving turbulent coherent structures in the formation of bedforms. Preliminary results will be presented.
SPEAKER
Dr. Tian-Jian Hsu (Tom) is currently Professor of Civil and Environmental Engineering at the University of Delaware (UD) and the Director of the Center for Applied Coastal Research. He earned a bachelor’s degree in Ocean Engineering from National Taiwan University in 1994 and Ph.D. degree in Civil Engineering from Cornell University in 2002. Before joining UD, he was a Postdoctoral Scholar and Assistant Scientists at Woods Hole Oceanographic Institution and an Assistant Professor at the University of Florida. He received an NSF Early Career Development (CAREER) Award in 2007. He was also the recipient of the 2021 Hans Albert Einstein Award from ASCE. Hsu has published about 100 peer-reviewed journal papers and book chapters. His main research covers numerical modeling/simulation of various sediment transport problems, including wave-driven sediment transport, beach erosion/recovery, scour, flocculation of cohesive sediments and interaction of sediment and spilt oil. Hsu’s research team devoted major efforts in the past several years to create open-source numerical modeling tools for fine-scale nearshore processes and sediment transport in the OpenFOAM framework.
