A new integrated finite volume–finite volume hydrodynamic modelling framework for wave-structure interactions

Rai, Ranjodh Singh (2024) A new integrated finite volume–finite volume hydrodynamic modelling framework for wave-structure interactions. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

This thesis presents a new integrated hydrodynamic modelling framework for and long-time and large-scale wave-structure interaction problems. It is developed by coupling a finite-volumebased fully-nonlinear potential-flow (FNPF) solver with the native OpenFOAM incompressible ‘interFoam’ solver in a numerical wave tank (NWT). This new model, named IntegratedFoam, has the primary advantage that each constituent solver has been developed in the same numerical framework (OpenFOAM), and consequently, are both also based on the same numerical method, i.e., the finite-volume method (FVM). Consequently, the method for transferring information is made simple and the coupling stable. Indeed, the coupling procedure follows a domain decomposition approach in which an overlapping relaxation zone is utilised to implement a oneway coupling. Hence, given that both solvers have been developed in OpenFOAM and are finite-volume-based, only a method to calculate the volume fraction from the free-surface elevation needs to be implemented: the velocity and pressure are already calculated as part of the FNPF solution and can be transferred accordingly in one direction—simplifying things greatly and avoiding unwanted errors. In addition, existing advanced OpenFOAM functionalities can be used for the required interpolation—easily addressing the problem of nonconforming meshes. These functionalities then also allow for the easy implementation of an overlapping relaxation zone which is key to a stable coupling because it ensures that there is a smooth transition from the FNPF to interFoam solution. Without it, there is a danger of there being a lack of continuity between each solution due the underlying physics of each solver being different. This could potentially then lead to errors and subsequently make the coupling unstable. Moreover, this zone also absorbs any reflected waves in the NWT, again aiding stability. In conjunction with the development of this new integrated model, a new stabilisation method for finite-volume or finite-difference FNPF models, motivated by a total variation diminishing (TVD) approach, is also presented. The accuracy and efficiency of the new IntegratedFoam model are then systematically validated through a series of wave propagation and wave-structure interaction test cases. In particular, the sensitivity of the model to its main coupling parameters is first assessed through fifth-order Stokes wave propagation. The model is then applied to a number of test cases involving wave interaction with offshore structures: fifth-order Stokes waves interaction with a 2-D T-shaped floating body acting as simplified midship section with superstructure, focused wave interaction with a fixed 3-D cylinder acting as a simplified monopile foundation, and focused wave interaction with a 3-D wave energy converter (WEC) device. It is shown to produce accurate numerical solutions that agree well with existing theoretical results and experimental data, all whilst significantly improving computational efficiency. Therefore, given that OpenFOAM is open source, the new integrated model can readily be used by researchers as a more efficient model for complex wave-structure interaction problems than interFoam. The new stabilisation method is then also systematically validated through fifth-order Stokes wave propagation, focused wave propagation, and wave shoaling. Again, it is shown to produce accurate numerical solutions that agree well with existing theoretical results and experimental data, all whilst reducing excessive numerical dissipation and thus significantly improving energy conservation.