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Abstract
Topographic complexity on continental shelves is the catalyst that transforms the barotropic tide into the secondary and residual circulations that dominate vertical and crossshelf mixing processes. Island wakes are one such example that are observed to significantly influence the transport and distribution of biological and physical scalars. Despite the importance of island wakes, to date, no sufficient, mechanistic description of the physical processes governing their development exists for the general case of unsteady tidal forcing. Controlled laboratory experiments are necessary for the understanding of this complex flow phenomenon. Here, threedimensional velocity field measurements of cylinder wakes in shallowwater oscillatory flow are conducted across a parameter space that is typical of tidal flow around shallow islands. The wake form in steady flows is typically described in terms of the stability parameter (where is the island diameter, is the water depth and is the bottom boundary friction coefficient); in tidal flows, there is an additional dependence on the KeuleganCarpenter number (where is the tidal velocity amplitude and is the tidal period). In this study we demonstrate that when the influence of bottom friction is confined to a Stokes boundary layer the stability parameter is given by where is the ratio of the wavelength of the Stokes bottom boundary layer to the depth. Three classes of wake form are observed with decreasing wake stability: (i) steady bubble for ; (ii) unsteady bubble for ; and (iii) vortex shedding for . Transitions in wake form and wake stability are shown to depend on the magnitude and temporal evolution of the wake return flow. Scaling laws are developed to allow upscaling of the laboratory results to island wakes. Vertical and lateral transport depend on three parameters: (i) the flow aspect ratio ; (ii) the amplitude of tidal motion relative to the island size, given by ; and (iii) the relative influence of bottom friction to the flow depth, given by . A model of wake upwelling based on Ekman pumping from the bottom boundary layer demonstrates that upwelling in the nearwake region of an island scales with and is independent of the wake form. Finally, we demonstrate an intrinsic link between the dynamical eddy scales, predicted by the Ekman pumping model, and the island wake form and stability.
Original language  English 

Pages (fromto)  158184 
Number of pages  27 
Journal  Journal of Fluid Mechanics 
Volume  874 
DOIs  
Publication status  Published  10 Sept 2019 
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 1 Finished

Extreme Tidal Forcing of a Topographically Complex Coastal Region  the Kimberley Western Australia
Ivey, G., Jones, N., Lowe, R., Ghisalberti, M., Meuleners, M., Brinkman, R. & Koseff, J.
1/01/10 → 31/12/14
Project: Research