Flow separation and vertical motions in a tidal flow interacting with a shallow-water island (original) (raw)
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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.
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-topography interaction in the vicinity of an isolated, deep ocean island
Deep Sea Research Part I: Oceanographic Research Papers, 1999
The results of two oceanographic surveys designed to delineate the #ow response near Cato Island (155332E, 23315S) in the Western Coral Sea are presented. The surveys were conducted in October 1992 and February 1993 and coincided with conditions of strong, steady incident currents and relatively weak, variable currents, respectively. For the strong in#ow case study, a surface-intensi"ed cyclonic eddy observed in the wake of the island was co-incident with a zone of strong upwelling. Isotherm displacements within the eddy were in excess of 90 m. The lee side response was strongly depth dependent, with recirculation con"ned to the upper 120 m. A dynamical systems approach incorporating ADCP data was used to compute Lagrangian trajectories numerically for particles released at various locations in the wake zone. There was no evidence of enhanced chlorophyll concentrations downstream of the island. Comparisons with other dynamically similar studies indicate that eddy shedding is likely during periods of steady incident currents. During the second survey, weaker incident currents resulted in a less pronounced #ow disturbance. Small isothermal displacements were capped beneath the strong seasonal thermocline. Lee side currents were weak and variable, with recirculation con"ned to the upper 50 m. A strong biological response was observed downstream, with increased integrated chlorophyll content and zooplankton biomass in the lee providing evidence of the island mass e!ect.
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
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.
Continental Shelf Research, 2007
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
Island wakes in shallow coastal waters
Journal of Geophysical Research, 1984
wide, and lies in well-mixed water approximately 25 m deep. Its long axis is inclined at about 60 ø into the direction of the dominant semidiurnal tidal current. The length of the wake in the lee of the island, as documented by aerial photographs and satellite imagery, appears to equal that of the wake behind a flat plate in a two-dimensional flow at a Reynolds number of about 10. However, current metering, drogues measurements, and temperature mapping indicate internal wake velocities much greater than would be consistent with such a simple low Reynolds number model. Further, estimates of the turbulent eddy coefficient suggest an effective Reynolds number more in the vicinity of 10 3. To reconcile these observational differences and to explain the observed upwelling in the core of the wake, an Ekman pumping model is proposed. It is postulated that the Ekman benthic boundary layer driven by rotation in the wake allows the vertical vorticity introduced into the water at the tip of the island at the point of separation, to be negated by the vorticity of opposite sign introduced at the bottom. Further, it is shown that a large fraction of the kinetic energy of the upstream flow facing the island is used to drive the wake eddies, leading to the conclusions that the trapping of water in the lee of islands greatly increases head losses on continental shelves with numerous islands, coral reefs, and rock outcrops. 1. INTRODUCTION There is, at present, considerable interest in understanding the details of the flows in the wake of headlands and islands. The eddies formed in these wakes trap water and particulates behind the island. This has important implications in the determination of the location of fisheries and in the siting of waste outfalls. These eddies are distinct from topographic gyres or the eddies induced by phase lags in an oscillating flow. The eddies in the lee of islands and headlands find their origin in the separation mechanism at the perimeter of the island. Little work seems to be available that deals with wake formation in shallow waters. Laboratory experiments [Batchelor, 1967; Gerrard, 1978] suggest that a two-dimensional flow around a plate can be parameterized by the Reynolds number UD Re-(1) where U is the unperturbed velocity, D the dimension of the body, and v the kinematic viscosity. If Re < 1, flow separation is minimal. If Re > 1, a steady wake is formed comprising a vortex pair and a central return flow. If the plate in inclined into the flow, one eddy dominates. At Re > 10, "gathers" (waves) develop on the interface between wake waters and surrounding waters. At Re > 20 the "gathers" generate waves farther downstream. At even higher values of Re, Karman vortices are formed moving downstream at a speed of order 0.8 U. The flow around a cylinder produces similar flow pat-Copyright 1984 by the American Geophysical Union. Paper number 4C0959. 0148-0227/84/004C-0959505.00 terns, although the various transitions occur at somewhat higher values of Re, and the wakes are shorter. The flow around an island similar in shape to a vertical flat plate or cylinder in a uniformly deep ocean may be expected to behave similarly to a two-dimensional flow. However, as the depth of water is decreased the vertical shear will distort the vertical vortex lines formed at the point of separation until at extremely shallow depths the bottom friction will completely dominate. This implies that the depth-averaged velocity at a point in the flow is proportional to the gradient of the water surface. Such flows have streamlines conforming to those of a potential flow with no separation at all anywhere in the flow. Coastal flows appear to operate somewhere between these two extremes, and the existence of flow separation phenomena and eddy generation in the lee of islands or headlands has been noted for some time [
2009
Initial numerical modelling results of the study that is currently being undertaken with the overarching aim of determining morphodynamic processes governing flood-delta/bay-margin shoreline dynamics are presented. The first part of the study focussed on investigating the relative importance of tide, wind and wave driven currents along the northern bay-margin shoreline (Jimmy's Beach) within a large estuary system located along the SE coast of Australia (Port Stephens, New South Wales ). A process -based modelling study was carried out by applying a depth-averaged flow module (MIKE 21 HD) and a spectral wave model (MIKE 21 SW). The models were calibrated and successfully validated against recently acquired hydrodynamic data. Various combinations of tide, wind and wave forcing were applied in numerical experiments to distinguish between the relative role that each forcing mechanism plays in the generation of nearshore currents, which ultimately drive nearshore sediment transport and associated erosion/accretion patterns. The model results indicate a significant spatial variability in flow patterns, depending on the site-specific characteristics of the beach -shoreline orientation; proximity to the mouth of the estuary and sheltering influences of adjacent headlands. The results suggest that va rious combinations of forcing dominate nearshore circulation in different parts of Jimmy's Beach. Under moderate conditions, nearshore flows along Jimmy's Beach appear to be dominated by mean currents induced by ocean-waves and tides. Currents driven by wind shear and locally generated wind waves appear to be relatively insignificant in comparison to tide induced and ocean wave-induced mean currents.
Coastal Engineering, 2007
Currumbin Creek on the Australian Gold Coast is a wave-dominated tidal inlet which exhibits a particularly active morphology. The recent history of Currumbin Creek entrance has seen rapid growth of the entrance for access to the ocean by fishermen, as a world class surfing site, and as a recreational area. Before the construction of two groynes in the 70's, Currumbin Creek entrance was highly variable in terms of inlet location and sand bar characteristics due to a cyclical behaviour of spit migration. Nowadays, the entrance is stabilised. However, natural processes continue with the entrance infilling causing flood and navigation issues, resulting in a regular dredging program to maintain an open entrance and for regular beach nourishment plans.