PROGRAM | Civil Engineering

By: Benjamin Tsai Chair: Tian-Jian Hsu

ABSTRACT

The rise in sea levels and the increase in extreme weather events due to climate change have emerged as significant issues for society. It is imperative to account for their effects when designing coastal structures and formulating resource management strategies. Rapid coastal morphodynamic changes during storms, such as beach erosion and scouring around critical infrastructure like bridge piers, are of particular interest. To address these concerns, this study employs advanced computational fluid dynamics techniques within a multi-phase flow framework, with the aim of deepening our understanding of complex physical processes.

This research first evaluates the capability of turbulence-resolving simulations for a near-prototype scale wave flume experiment under random waves, investigating key processes during wave-swash interactions for two stages of beach profiles observed during a storm event. The findings indicate that large bottom shear stress (represented by the Shields parameter), horizontal pressure gradient (the Sleath parameters), and a robust turbulence-berm interaction (characterized by high turbulent kinetic energy directly contacting the bed) leading to significant sediment transport events are caused by intense interaction between backwash and incoming breaking waves. These results provide valuable insights into the underlying causes of berm erosion.

This study proceeds to utilize the Eulerian two-phase model, SedFoam, to simulate the initiation of scour beneath a 2D pipeline and the subsequent backfilling process. It has been demonstrated via laboratory experiments that the seepage flow underneath the pipeline, called piping, initiates the scouring process. Previous simulations based on single-phase models required artificial adjustments to account for the onset of scour. In contrast, this study demonstrates the capability of the two-phase model in quantitatively simulating piping, validated through comparisons with a series of laboratory measurements. To further confirm the model’s capability to simulate the backfill process, we artificially descent the pipeline into the scour hole. This represents an idealized representation of the complex sinking process that occurs during scour. The resulting burial depth due to backfill aligns with predictions derived from empirical formulas.

SedFoam is further applied to simulate wave-induced scour around a vertical cylinder. Through detailed model validation and grid convergence study, we provide evaluations for future scour simulations concerning the choice of turbulence modeling methods and grid resolutions. Specifically, a higher resolution and turbulence-resolving model, such as large-eddy simulation, is preferred to capture key vortices, namely the horseshoe vortex and lee-wake vortices. A higher resolution and turbulence-resolving model are also suggested for scour simulations, as they can predict a more accurate scour hole in both depth and width. For more efficient Reynolds-averaged simulation, the k-ω-2006 model yields reasonable predictions for scour hole depth development. However, it tends to over-predict the scour hole width.

The present study establishes a simulation framework, based on a multi-phase flow methodology, for selected coastal applications. It is suggested that future research efforts focus on enhancing the modeling of dune erosion by integrating more extensive soil mechanics formulations. Additionally, improvements to wave runup and breaking predictions in regional-scale models are recommended.

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