Large eddy simulation of flow over submerged cylinders and leaky barriers

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

Unsteady vortex shedding dynamics behind a circular cylinder in very shallow free-surface flows

The turbulent wake generated by a horizontal circular cylinder in free-surface flows of increasing shallowness with submergence-to-diameter ratios between 0.5 and 2.1 are investigated using large-eddy simulation. At Froude number ( ) = 0.26, the free-surface deformation is small with little influence on the wake, whereas at = 0.53 there is a drop in the free-surface downstream of the cylinder that impacts the coherence of the vortex shedding. Irrespective to the relative submergence, the close location of the cylinder to the bottom wall generates an asymmetric von-Kármán vortex street. Proper Orthogonal Decomposition (POD) is used to analyse the spatio-temporal coherence of the turbulent structures shed in the cylinder wake. The spatial patterns of the first two POD modes, those containing the most energy, depict the von-Kármán vortices. As increases, the energy content of the first pair of POD modes decreases from 56% at = 0.26 to 26.8% at = 0.53, as large-scale vortices lose coherence more rapidly with shallower conditions. This energy redistribution leads to the smaller flow structures to contain a relatively higher energy when is larger. The frequency of the dominating vortex shedding determined from the spectra of the POD temporal coefficients unveils that the first two coefficients feature a dominant peak at the von-Kármán vortex shedding frequency. At 0.45, the reconstructed flow field using the first 20 POD modes agrees well with the instantaneous velocities from LES, whereas free-surface effects on the wake dynamics at increasing requires more POD modes to reconstruct the flow field with reduced error

Influence of Free-Surface Resolving Techniques on the Wake Characteristics Behind a Circular Cylinder based on POD Analysis

The wake structures generated downstream of a circular cylinder obtained from large-eddy simulations using a level-set method (LSM) and rigid-lid (RL) to represent the air-water interface are studied to assess how the flow dynamics change depending on the numerical treatment of the free-surface. A horizontal cylinder at a gap-to-diameter ratio of 0.5 is considered, with a bulk Froude number equal to 0.45, and Reynolds number based on the cylinder's diameter equal to 13,333. Spanwise vorticity contours show that the coherence of vortical structures is strongly impacted by the water-surface deformation when this is computed with LSM. The coherence of these turbulent structures in the cylinder wake are analysed using Proper Orthogonal Decomposition (POD) based on the instantaneous turbulent velocity fluctuations. The POD analysis indicates that the first two spatial modes contain most of the energy irrespective to the free-surface treatment, which correspond to von-Karman vortices. These have different spatial coherence depending on the air-water surface representation method, as when using the RL these two first modes account for about 67.7 % of the total energy, and are more coherent than in the LSM setup in which they account for 42 % of the total energy. The spectra computed from the temporal coefficients of the first two POD modes feature a dominant peak for both cases, while the energy content of the spectra diminish with increasing frequency for the LSM case. Our study outlines that adopting an accurate free-surface reconstruction method to correctly account for the watersurface deformation and turbulent structures in the flow.</p