Large Eddy Simulation of Flow Over Submerged Cylinders and Leaky Barriers

Abstract

Extreme weather events are increasing their frequency due to climate change, leading to more recurrent destructive flooding incidents over the recent years, which require the development of potential solutions. For this, leaky barriers are a natural-based flood mitigation solution that can reduce and delay peak flow events. Understanding the local hydrodynamics involved in the flow around these mostly-submerged hydraulic structures is essential to enhance their performance in retaining flood events but also to ensure their structural integrity. Numerical methods arise as a complementary tool to experimental approaches that enable a further understanding of the fluid dynamics around submerged cylinders used in these leaky barriers. This thesis adopts a large-eddy simulation (LES) computational approach, incorporating the level-set method (LSM) to capture free-surface deformation. The hydrodynamics around a single cylinder are investigated, finding a critical Froude number threshold when free-surface effects become pronounced and influence the hydrodynamic coefficients, vortex shedding patterns, and wake structures downstream of the cylinder. Proper-orthogonal decomposition (POD) is employed to quantify and analyse energetic coherent structures developed behind the cylinder, revealing redistribution in the energy contribution as flow conditions approach shallower conditions. Furthermore, POD is used to compare flow pattern predictions from two separate LESs of flow past a single horizontal cylinder in very shallow conditions, highlighting the limitations of traditional rigid-lid modelling and emphasising the importance of adopting LSM for accurate free surface and flow dynamics. The hydrodynamics of leaky barriers are simulated and analysed with LES to investigate the impact of barrier’s inclination and length on the flow. Results reveal configurations with flatter inclinations or shorter barrier lengths lead to reduced bed scour risk and improved performance. Two novel methodologies for estimating water depths and velocities around leaky barriers have been proposed and validated using experimental and simulation datasets, providing an easy-to-use design tool for eco-friendly wood structures in future flood management. This thesis seeks to enhance the current understanding of the complex hydrodynamic phenomena developed in the flow around fully-submerged horizontal circular cylinders and leaky barriers, providing essential insights for practical flood management strategies and environmental conservation efforts.