CFD Simulation of Wind Flow over Vegetated Coastal Sand Dunes

Abstract

This research applies Computational Fluid Dynamics (CFD) to modelling flow over a vegetated foredune in the coastal zones. Morphological changes resulting from wind flow and sedimentation have direct impacts on plant and animal species, and people living nearby. Information on natural processes such as foredune formation have remained poorly understood owing to the complex interaction of a range of biogeomorphic factors. CFD can be employed in order to investigate complex flows over a coastal foredune. Extending CFD to modelling flow over a vegetated foredune is challenging as the combined natural processes need to be modelled. This thesis aims to test the efficacy of CFD in accurately predicting i) wind flow over a vegetated complex foredune, ii) associated sand transport and iii) to determine whether CFD can be employed for use to predict complex flows in real circumstances. In two-dimensional computations, the RANS-based turbulence models were optimised with the wall functions to model a wall bounded flow with a vegetation cover. The turubence model: the renormalisation group (RNG), the realisable κ — є model as well as the SST κ — ω model were optimised with the wall functions (the standard, the non-equilibrium and the enhanced wall function). The optimised models were then applied to model flow over a ve-getation cover in 2D. The effects of vegetation cover was modelled by using two approaches: i) using the roughness parameter (κѕ) in the wall function; and ii) adding the source/sink terms into the momentum equation for flow (to account for plant drag). The models for sand transport were investigated and the Volume of Fluid (VOF) model was selected, modified and verified with field data from the literature. The verified models for turbulence, vegetation covers and and transport were then used in three dimensional simulations. A more complex turbulence model the detached eddy simulation (DES) model was tested in addition to the RNG model in three dimensional computations. The DES model performed better than the RNG model in predicitng 3D flow, and was used with the source/sink term models (for vegetation cover), and the VOF model (for sand transport) to model the 3D-flow over a vegetated foredune at Mason Bay, Stewart Island, New Zealand. In 2D, the combination of the RNG and the non-equilibrium wall function returned the most accurate predictions particularly for the streamwise mean velocity. Modelling vegetation cover with the source/sink term in the momen¬tum equation returned more accurate predictions than by using the roughness parameter in the wall function particularly in the canopy regions. The modi¬fied VOF model predicted realistic sand bed profiles but under-predicted the velocity magnitude near the surface. In 3D, the model combination of the DES, the source/sink term model, and the VOF model successfully predicted the pattern of sand transport over the foredune at Mason Bay. The results given by CFD simulations provided new information on natural processes. The simulation results in this research showed that sand trapped on a flat vegetated surface has a certain pattern. More sand is trapped at the front and rear side of the vegetation covered surface area, which agree with wind tunnel data collected at Oregon State Univerisity. In real circumstances, at the foredune system at Mason Bay, the pattern of sand distributed on the foredune between two cases; with- and without grass cover, are only different in terms of magnitude. The distribution patterns of sand on the foredune's surface between the two cases are similar. The maximum height of foredunes can be predicted by looking at the wind speed in vegetation cover layers at the foredune crest. If the wind speed is below a threshold velocity for sand transport, more sand can be trapped by the vegetation, resulting in an increase in dune height. The pattern of sand deposition on a foredune can also be predicted by determining the surface shear velocity, the higher shear velocity the less sand trapped on the surface. CFD simulations even without the models for sand transportation can be used to predict the maximum foredune height when the topographic and wind data are available. In conclusion, CFD can be an effective tool for modelling complex flows in coastal zones, as long as numerical errors, and modifying assumptions are clearly recognised. Accuracy of the predictions can be improved, and possible solutions were provided in this research.