Lattice Boltzmann modelling of two and three-dimensional flow and scour around offshore pipelines

Muhammad Alam

    Research output: ThesisDoctoral Thesis

    359 Downloads (Pure)

    Abstract

    [Truncated abstract] The hydrodynamic forces on a marine pipeline and the local scour around it are the most serious and important issues in designing and maintaining pipelines. This thesis explores the vortex shedding phenomena for the flow over smooth surface and rough surface isolated cylinders. This thesis also explores the two-dimensional and three-dimensional scour process beneath offshore pipelines numerically. A series of numerical models are proposed in this dissertation for the prediction of flow characteristics and the time development of local scour around pipelines. All the models presented in this thesis are deliberately developed based on novel lattice Boltzmann method (LBM), because in recent years it has been considered as a serious alternative to standard computational fluid dynamics (CFD) as it is ideally suited to massively parallel computations. The lattice Boltzmann method is described in details to reveal how it recovers the Navier- Stokes equations. Various grid refinement schemes available in literature are discussed and a slightly modified new scheme is proposed to remove oscillatory solutions at high velocity change regime. The proposed scheme is then validated against bench mark tests for low Reynolds number flow. A turbulent model based on LBM is developed in order to predict the vortex shedding flow around an isolated square smooth surface cylinder. The various local and global flow parameters and structure of vortices are validated against experimental and numerical data available in literature. The model is then extended to investigate the vortex shedding flow over an isolated rough surface cylinder as it has an engineering significance in the design process of pipelines. The model is employed to investigate the influence of pipe roughness on various local and global parameters of flow. ... Significant part of this thesis is aimed at modelling flow and local scour around pipelines employing LBM and cellular automata (CA) methods. The erosion mechanism of the CA method available in literature for sand particles is improved by defining the threshold of sediment entrainment on bed in a similar manner to that employed in the traditional scour models. The predicted scour profiles for various incoming flow conditions are found to compare well with the experimental results reported in the literature. The existence of lee wake erosion due to continuous generation of vortex shedding in the lee of the pipelines is revealed. The time development of the maximum scour depth below the pipe is also found to be in good agreement with the experimental measurements reported in literature Finally, a three-dimensional flow and scour model is developed in order to explore the scour process beneath pipelines. It is revealed that the three-dimensionality effects are more pronounced near the span shoulder. On the other hand, there exists a two-dimensional scour regime in the vicinity of the middle section of the suspended pipe. It is found that the propagation speed of the scour hole in the sapnwise direction remains almost constant at all stages of scour process.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - 2009

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