Turbulence model and immersed boundary method development in TELEMAC-3D for offshore structure modelling

Abstract

In recent years, increased evidences suggest that offshore structures such as wind farms, tidal turbine farms, piles of bridge and breakwater have great impact on the hydrodynamics and hence may have a strong influence on the sediment transport at a site. The open-source hydrodynamic suite of software TELEMAC has been used for the study of such environmental influence around Unite Kingdom. However, the use of the 3-D version of the software, TELEMAC-3D is restricted by how structures are accounted for in the meshes, as water columns have the same number of layers all over the domain. Moreover, a large scale farm has a large impact on turbulence mixing in the coastal regional scale, and this is not properly understood. The PhD project focuses on 3-D hydrodynamics and development of an 3-D unstructured capability using an immersed boundary method to account for obstacles in the flow. Two large eddy simulation models (the 2eddy LES model and the Wall-adapted Large Eddy model) have been incorporated into TELEMAC-3D to get a more realistic and effective representation of the turbulence mixing and to account for the unsteadiness of the flow past the structures. The simulations have been performed using High Performance Computing to enable large scale applications using TELEMAC-3D and fine spatial and temporal resolutions in 3-D. The implementations carried out in the code are fully parallel. The numerical models have been validated for two laboratory scale cases, including the flow around a circular cylinder and the flow over a submerged structure. Then a far-field simulation at the southern North Sea has been carried out, where the hydrodynamics and morphological impacts of the London Array offshore wind turbine farm have been investigated. The numerical results of turbulence model implementation indicate that both turbulence models have good performance in the representation of the flow past a cylinder in laboratory scale. However in the large scale application, only the 2eddy LES model is successfully applied because the WALE model relies on a very fine mesh in the vertical direction. The implementation of IBM suggested that when dealing with an obstacle going from the bottom though the surface of the water, the immersed boundary method offers good accuracy in the prediction of surrounding flow structures. For the submerged obstacles, they can be simulated by TELEMAC-3D by implementing the Immersed Boundary method. Although the accuracy is limited currently, qualitative analysis can still be performed.