A Full Hydro- and Morphodynamic Description of Breaker Bar Development

Publication: ResearchPh.D. thesis – Annual report year: 2011

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The present thesis considers a coupled modelling approach for hydro- and morphodynamics in the surf zone, which is based on a solution to the Reynolds Averaged Navier-Stokes (RANS) equations with a Volume of Fluid (VOF) closure for the surf tracking. The basis for the numerical approach is the surface tracking method in the open-source CFD toolbox OpenFoam\textregistered{}, which is released by OpenCFD\textregistered{}. This basic version has been extended with the ability for modelling surface water waves and sediment transport in the surf zone. The validation of these functionalities are described as part of the project. The inequilibrium in the sediment transport field leads to a morphological change in the bed level, which is incorporated through a movement of the computational mesh. This allows for a integrated coupling with the hydrodynamics. The morphological module is also developed as part of this work. The numerical model is applied onto several physical settings. Firstly, the morphological response is turned off, and the hydrodynamics and the sediment transport patterns in the surf zone are described. The description considers these processes as a function of several non-dimensional variables, namely the surf similarity parameter, $\zeta_0$, and Dean's parameter in various forms, $\Omega$ and $\Omega_{HK}$. This investigation has an emphasis on (i) the spatial and temporal lag-effects in the hydrodynamics, and between the hydrodynamics and the sediment transport and (ii) the integrated net cross shore suspended sediment transport flux as a function of either of the variables $\zeta_0$, $\Omega$, or $\Omega_{HK}$. Secondly, the bed is allowed to evolve under the influence of the sediment transport processes. The development of breaker bars in both laboratory scale settings and prototype scale settings is considered. The temporal development of the cross shore profile is simulated for several combinations of wave forcing and sediment grain diameters. The variation is described with emphasis on the development of the crest level of the breaker bar, the variation in the bed shear stress on the crest of the breaker bar, and its migration speed. Additionally, a net onshore current over a breaker bar is considered, where this current mimics the presence of a horizontal circulation cell. The development of the breaker bar is described for different values of the net onshore current speed. This description is undertaken with and without a coupling to the morphology.
Original languageEnglish
Publication dateJun 2011
PublisherDTU Mechanical Engineering
Number of pages174
ISBN (print)978-87-90416-64-5
StatePublished
NameDCAMM Special Report
NumberS136

Keywords

  • Breaker bars, RANS equations, Wave breaking, VOF method, Free surface modelling, Morphological development, Cross shore sediment transport
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