Computational Fluid Dynamics and Visualisation of Coastal Flows in Tidal Channels Supporting Ocean Energy Development

Flow characteristics in coastal regions are strongly influenced by the topography of the seabed and understanding the fluid dynamics is necessary before installation of tidal stream turbines (TST). In this paper, the bathymetry of a potential TST deployment site is used in the development of the a CFD (Computational Fluid Dynamics) model. The steady state k-ϵ and transient Large Eddy Simulation (LES) turbulence methods are employed and compared. The simulations are conducted with a fixed representation of the ocean surface, i.e., a rigid lid representation. In the vicinity of Horse Rock a study of the pressure difference shows that the small change in height of the water column is negligible, providing confidence in the simulation results. The stream surface method employed to visualise the results has important inherent characteristics that can enhance the visual perception of complex flow structures. The results of all cases are compared with the flow data transect gathered by an Acoustic Doppler Current Profiler (ADCP). It has been understood that the k-ϵ method can predict the flow pattern relatively well near the main features of the domain and the LES model has the ability to simulate some important flow patterns caused by the bathymetry

Aeolian dynamics of beach scraped ridge and dyke structures

Where urban areas are situated close to a beach, sand dunes act as protection from flooding and erosion.When a dune has been removed or damaged by erosion, dune, ridge or dyke re-building using heavymachinery, a process known as beach scraping, is a common method of restoration. Following construction, natural accretion of sediment on the backshore is preferable as it facilitates sustained natural dune building, growth of vegetation, and habitat creation and reduces the need for further beach scraping. This study investigates the near surface flowand transport potential for three artificial structure designs: a single ridge, a double ridge and a dyke. The three shapes contained an identical volume of sand and were preceded by 50mof beach at an angle of 3°. A computational fluid dynamic model (CFD)was created for each scenario to calculatewind flowand shear velocity from 4 differentwind directions at 22.5° intervals from 0° (onshore) to 67.5°. From this data sediment flux was predicted along a two dimensional transect for each of the scenarios. For all structures, shear velocity on the beach and stoss slope decreased as incident wind direction became more oblique; conversely shear velocity in the lee of the crest increased. A reduction in shear velocity at the foot of each structure also occurred and appears related to stoss slope,with the greatest reduction at the toe of the dyke structure (stoss slope 34°) and the least before the single ridge (stoss slope 17°). Specifically the results suggest that the double ridge structure is the most resilient to aeolian erosion. Shear velocity reduction on the back beach is comparable to the dyke and sediment flux fromthe stoss slope of the double ridge structure may become trapped in the swale between the two ridges encouraging sediment deposition, thus reducing sediment transport beyond the dunes and backshore. Although the dyke structure underwent the greatest reduction in shear velocity on the back beach it experienced substantial sediment flux at the crest and along the top of the structure, making it susceptible to erosion during a strongwind event. The highest sediment transport rate was calculated at the crest of the single ridge, and the single ridge structure also created the smallest reduction of shear velocity on the back beach, thus making it less desirable than the double ridge

CFD Simulation of Wind Flow over Vegetated Coastal Sand Dunes

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

CFD Simulation of Wind Flow over Vegetated Coastal Sand Dunes

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

Modeling of Surface Roughness for Flow Over a Complex Vegetated Surface

Abstract—Turbulence modeling of large-scale flow over a vegetated surface is complex. Such problems involve large scale computational domains, while the characteristics of flow near the surface are also involved. In modeling large scale flow, surface roughness including vegetation is generally taken into account by mean of roughness parameters in the modified law of the wall. However, the turbulence structure within the canopy region cannot be captured with this method, another method which applies source/sink terms to model plant drag can be used. These models have been developed and tested intensively but with a simple surface geometry. This paper aims to compare the use of roughness parameter, and additional source/sink terms in modeling the effect of plant drag on wind flow over a complex vegetated surface. The RNG k-ε turbulence model with the non-equilibrium wall function was tested with both cases. In addition, the k-ω turbulence model, which is claimed to be computationally stable, was also investigated with the source/sink terms. All numerical results were compared to the experimental results obtained at the study site Mason Bay, Stewart Island, New Zealand. In the near-surface region, it is found that the results obtained by using the source/sink term are more accurate than those using roughness parameters. The k-ω turbulence model with source/sink term is more appropriate as it is more accurate and more computationally stable than the RNG k-ε turbulence model. At higher region, there is no significant difference amongst the results obtained from all simulations. Keywords—CFD, canopy flow, surface roughness, turbulence models

Flow deflection over a foredune

Flow deflection of surface winds is common across coastal foredunes and blowouts. Incident winds approaching obliquely to the dune toe and crestline tend to be deflected towards a more crest-normal orientation across the stoss slope of the foredune. This paper examines field measurements for obliquely incidentwinds, and compares them to computational fluid dynamics (CFD)modelling of flow deflection in 10° increments fromonshore (0°) to alongshore (90°) wind approach angles. The mechanics of flow deflection are discussed, followed by a comparative analysis of measured and modelled flow deflection data that shows strong agreement. CFD modelling of the full range of onshore to alongshore incidentwinds reveals that deflection of the incident wind flowis minimal at 0° and gradually increases as the incidentwind turns towards 30° to the dune crest. The greatest deflection occurs between 30° and 70° incident to the dune crest. The degree of flow deflection depends secondarily on height above the dune surface, with the greatest effect near the surface and toward the dune crest. Topographically forced flow acceleration (“speed-up”) across the stoss slope of the foredune is greatest for winds less than 30° (i.e., roughly perpendicular) and declines significantly for winds with more oblique approach angles. There is less lateral uniformity in the wind field when the incident wind approaches from N60° because the effect of aspect ratio on topographic forcing and streamline convergence is less pronounced