High-Resolution CFD Modelling of Scour in the Marine Environment
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Marine structures are vulnerable to scour under extreme weather conditions. Wave interaction with the structures leads to sediment transport locally. This results in the formation of scour holes which can affect the structural stability. While some experimental investigations have been reported in this area, the application of computational fluid dynamics (CFD) modelling can be a suitable alternative for a comprehensive analysis of the scour. This PhD study aims at high-resolution CFD modelling of scour around marine structures. The modelling is based on the solution of the Reynolds-Averaged Navier- Stokes (RANS) equations, the k − ωturbulence model and the sediment transport model. The evolution of the scour profiles with the oscillatory nature of the free surface is modelled as an advancement in the CFD modelling of the scour process. The numerical model incorporates generation of a nonlinear wave field in the numerical wave tank (NWT) with higher-order discretisation schemes. Advanced morphological algorithms are used to account for the complexity of the scour process and accurate prediction of the scour depths. The study highlights the flow hydrodynamics, the transient development of the scour profiles, and the time scale of the scouring process. The factors affecting the scour around different types of marine structures are studied as novel elements of the scour process. The PhD thesis includes many case studies of scour under waves. First, an optimisation study is performed for generation of the wave field and the resulting scour in a reduced-length numerical wave tank (NWT). An appropriate partial decoupling factor for the time scales of the hydrodynamic and morphological model is further investigated. The results demonstrate the applicability of an optimised NWT with a partial decoupling approach to improve the computation time required for the modelling of wave-induced scour. In the following studies, scour around vertical piles is investigated in detail. This includes the scour around a slender pile, piles in a side-by-side arrangement, a large pile, and a jacket structure. The flow hydrodynamics, the variation of the normalised scour depth S/D with the Keulegan-Carpenter number (KC), the effect of the gap to diameter ratio between piles G/D, and the temporal variation of the scour process are presented. The results demonstrate that the maximum scour depth S/D increases with KC number. In the case of the piles in a side-by-side arrangement, the scour is found to be governed by the formation of a flow jet in the gap between the piles. The results describe the variation of S/D and horizontal extents (Lx,y/D) of the scour with G/D. Scour under the combined action of waves and current is more complicated than for waves alone and current alone. The interaction between waves and current produce a distinctive velocity profile, and consequently, the scour mechanism is altered compared to waves alone or current alone. This is investigated through the modelling of pipeline scour under combined waves and current. The scour below the pipeline is thoroughly examined for different combinations of waves and current. The study highlights that the scour below the pipeline changes with KC and the non-dimensional parameter for combined waves and current Ucm. The study also investigates scour under breaking waves on a sloping seabed with a vertical seawall. Results present scour for different scenarios of the seawall location, wave steepness, and seabed slope. It is demonstrated that the wave impact on the seawall governs the maximum scour at the seawall toe. Formation of a standing wave due to the reflected wave from the wall increases the scour seawards. The model is also applied to simulate a field case of erosion by incorporating an almost 500 m long coastline at Svalbard. The results for the coastline erosion are presented along with the free surface profile. Summarizing, the thesis discusses CFD modelling of scour under complex flow conditions of non-breaking waves, breaking waves, and combined waves and current. The simulated case scenarios reflect the scour around different types of marine structures such as slender piles, a large pile, pipelines, a jacket structure and a seawall. Simulated wave hydrodynamics and the resulting scour show a satisfactory match with the wave theory, experimental observations, and available field data. It demonstrates the capabilities of the high-resolution CFD modelling of scouring with free surface capturing. This will contribute towards the analysis of scour under complex wave conditions and sophisticated installations in the marine environment.