Influence of the turbulence closure scheme on the finite-element simulation of the upwelling in the wake of a shallow-water island (original) (raw)
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A three-dimensional finite-element model is used to investigate the tidal flow around Rattray Island, Great Barrier Reef, Australia. Field measurements and visual observations show both stable eddies developing at rising and falling tide in the wake of the island. The water turbidity suggests intense upwelling able to carry bed sediments upwards. Based on previous numerical studies, it remains unclear at this point whether the most intense upwelling occurs near the centre of the eddies or off the island's tips, closer to the island. All these studies resorted to a very simple turbulence closure, with a zeroequation model whereby the coefficient of vertical viscosity is computed via an algebraic expression. In this work, we aim at studying the influence of the turbulence closure on model results, with emphasis on the prediction of vertical motions. The Mellor and Yamada level 2.5 closure scheme is used and an increase in the intensity of vertical transport is observed. This increase is partly explained by the fact that the Mellor and Yamada model takes into account the hysteresis effect in the time variation of turbulence variables. The influence of the advection of turbulence variables is estimated to be negligible. By a better representation of transient coastal phenomena, the Mellor and Yamada level 2.5 turbulence closure improves the model to a significant degree. r
Estuarine, Coastal and Shelf Science, 2007
A three-dimensional finite element model is used to investigate the formation of shallow-water eddies in the wake of Rattray Island (Great Barrier Reef, Australia). Field measurements and visual observations show that stable eddies develop in the lee of the island at rising and falling tides. The water turbidity downstream of the island suggests the existence of strong upwelling that would be responsible for carrying bed sediments up to the sea surface. We first propose to look at the upwelling velocity and then use the theory of the age to diagnose vertical transport. The water age is defined as the time elapsed since particles of water left the sea bottom, where the age is prescribed to be zero. Two versions of this diagnosis are considered. Although the model predicts upwelling within the eddies, it is not sufficiently intense to account for vertical transport throughout the water column during the life span of the eddies. As mesh resolution increases, this upwelling does not intensify. However, strong upwelling is then resolved off the island's tips, which is confirmed by the results obtained with the age. This study also shows that the finite element method, together with unstructured meshes, performs well for representing three-dimensional flow past an island.
Flow separation and vertical motions in a tidal flow interacting with a shallow-water island
Estuarine, Coastal and Shelf Science, 2008
This paper reports on the case study of Rattray Island (Great Barrier Reef, northeast Australia), lying perpendicular to tidal flow in shallow waters. At ebb and flood, attached (stable) eddies develop in the wake where swirls of turbidity suggest that sediment-laden waters are brought to the surface as a result of vertical transport. Both eddy and tip upwellings are encountered in the tidal flow around Rattray Island but there is currently no clear-cut answer as to which secondary flow generates upwelling with the largest intensity. This paper addresses this specific issue through idealized and realistic high-resolution numerical experiments. The analysis is supported by physical arguments based on the theory of flow separation. Given Rattray's geometry and surrounding bathymetry, the mechanism of flow separation in shallow waters helps explain the asymmetry in size of the eddies and their intensity. The results of idealized numerical experiments also suggest that eddy and tip upwellings may be of similar intensity at Rattray Island.
Modelling tidal circulation in an island’s wake
Details of a numerical model study to predict the tidal circulation observed and measured in the lee of Rattray Island, Australia are presented. The hydrodynamic model is of the two‐dimensional depth integrated type, with particular emphasis being placed on the modeling of the advective accelerations and the lateral mixing in the free shear layer in the island's wake. The component of free shear layer turbulence includes a constant eddy viscosity approach in the mixing zone and the use of a semi‐empirical lateral velocity distribution. The numerically predicted circulatory velocity fields have been compared with field measurements, taken at 26 sites, aerial observations and Landsat imagery. The comparisons between both predicted and measured results are encouraging, with the eddy dimensions and circulation strength being similar for all tidal phases. Various other simulations are described, which chiefly suggest that, for this study: (1) The eddy characteristics are reduced when the lateral shear stress is neglected; (2) no eddy is reproduced when the advective accelerations are excluded; (3) bathymetric effects are significant; and (4) geostrophic effects are important at water elevation boundaries.
Modeling Tidal Circulation in an Island's Wake
Journal of Waterway, Port, Coastal, and Ocean Engineering, 1986
Details of a numerical model study to predict the tidal circulation observed and measured in the lee of Rattray Island, Australia are presented. The hydrodynamic model is of the two-dimensional depth integrated type, with particular emphasis being placed on the modeling of the advective accelerations and the lateral mixing in the free shear layer in the island's wake. The component of free shear layer turbulence includes a constant eddy viscosity approach in the mixing zone and the use of a semi-empirical lateral velocity distribution. The numerically predicted circulatory velocity fields have been compared with field measurements, taken at 26 sites, aerial observations and Landsat imagery. The comparisons between both predicted and measured results are encouraging, with the eddy dimensions and circulation strength being similar for all tidal phases. Various other simulations are described, which chiefly suggest that, for this study: (1) The eddy characteristics are reduced when the lateral shear stress is neglected; (2) no eddy is reproduced when the advective ' accelerations are excluded; (3) bathymetric effects are significant; and (4) geostrophic effects are important at water elevation boundaries.
Numerical simulation of the summer wake of Rottnest Island, Western Australia
Dynamics of Atmospheres and Oceans, 2007
During the summer, a northward, wind-driven current dominates the Rottnest Island region in southwestern Australia. Field studies have shown that the interaction between Rottnest Island and the northward current generates upwelling at the western end of the island, which is advected downstream, resulting in isotherm doming in the wake region. Flow curvature-induced secondary circulation has been proposed as the dominant mechanism responsible for this upwelling. Here, a three-dimensional numerical model, together with field observations, was used to undertake a detailed investigation of the three-dimensional flow structure in the wake region. Comparison of the observed upwelling pattern and the simulated flows revealed the island's dominant role in generating upwelling. This result was confirmed with the use of idealized numerical experiments. The modeling results confirmed the presence of secondary circulation, generated as a result of flow curvature at the western end of the island, which caused strong upwelling and extended downstream.
Three-dimensional island wakes in the field, laboratory experiments and numerical models
Continental Shelf Research, 1996
Results of field, laboratory and numerical studies are used to describe the threedimensional circulation in a barotropic island wake in shallow waters. Bottom friction generates a closed circulation characterized by a strong upwelling (typically 10-20 m h-~ ) in the bulk of the eddy and an even larger downwelling velocity in a narrow zone along the edges of the eddy. This downwelling exists at the solid boundaries of the island and also all along the separation streamline. This circulation can be realistically modeled numerically provided the intense turbulence in the free shear layers is explicitly parameterized. This secondary circulation aggregates buoyant material along the edges of the eddy even in well-mixed coastal waters.
2017
The success of many coastal management projects hinges on the ability to predict the dispersal and settling of sediment particles. Hydrodynamic models have enabled the efficient simulation of sediment transport scenarios at large spatial scales and long time scales. However, these models have limited predictive capacity owing to an incomplete understanding of the processes involved. Turbulence has been shown to have a substantial influence on sediment transport by influencing flocculation (i.e. aggregation of particles), hence driving the behaviour of particles (e.g. deposition, erosion, mixing). Turbulence tends to promote aggregation at low shear stresses and cause floc breakups at high shear stresses. However, despite the key role of turbulence in coastal modelling, there is not a unique approach but several methods to describe turbulence, each based on a different combination of assumptions. We present modelling results exploring the performance of one closure scheme implemented...
Numerical Modelling of Turbulent Coastal Processes
A numerical model HYDROTAM-3, is developed that simulates turbulent coastal transport processes due to tidal or nontidal forcing. Model includes four sub model components; hydrodynamic, turbulence, salinity and temperature transport, suspended sediment transport. The only simplifying assumption used in the model is Boussinesq approximation, i.e. the density differences are neglected unless the differences are multiplied by the gravity. It is a composite finite difference finite element model. Model has been applied to Fethiye Bay located at the Mediterrenean Sea. At three Stations in the Bay, continuous measurements of velocity throughout the water depth and water level, were taken for 27 days. Model well simulates the measurements.