School of Civil, Environmental and Mining Engineering

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Contact

Paul Branson

Phone: (+61 8) 6488 1683
Fax: (+61 8) 6488 1015


Start date

Mar 2011

Submission date

Mar 2019

Paul Branson

Thesis

Laboratory investigation of the formation and evolution of shallow island wakes in oscillatory flow

Summary

This study will investigate the dynamics of island wake generation and evolution in oscillatory shallow flow through a series of laboratory scale experiments. The PhD forms part of an ARC funded project (DP1095294) designed to study the dynamics of the tidally driven flow through the topographically complex Kimberley coastal region in northern Western Australia. In particular, the ARC project aim is to quantify the tidally driven and topographically influenced circulation and mixing that controls the vertical and cross-shelf mass exchange and coastal zone flushing.

Whilst there is some understanding of island wake formation in steady flows the influence of flow unsteadiness and reversal on secondary circulation and wakes is an area that is very poorly addressed in the literature, particularly in shallow water environments. There exists only a handful of papers directly addressing oscillatory flow around islands in shallow water where the influence of bed friction plays an important role in governing wake dynamics. Of these only one (Lloyd, 2001) undertook laboratory investigations and they were limited to the measurement of surface velocities. The work of Lloyd (2001) suggested a model of shallow island wake form through dimensionless parameters that took account of the relative length scales of flow oscillation and island size, however the study ignored the influence of time dependent bed friction, an essential element of unsteady flows. The present study will extend previous work by establishing, for the first time, a predictive model of wake form based on dimensionless parameters that incorporate the key aspects of unsteady flows, in particular the influence of oscillation frequency and bed friction, on both the vertical and horizontal length scales relevant to oscillatory flow around cylinders in shallow water.

Why my research is important

The ecosystem role that shallow island wakes play in the transport and distribution of biological and physical scalars is yet to be accurately quantified but is thought to be highly important. Characterisation of island wakes in the field is limited to observation by aerial photography and point source measurements. Whilst numerical simulations provide quantitative insight into full field wake dynamics, assumptions underpinning the parametisation of bottom friction and the diffusion of momentum, in addition to the computational constraints imposed by spatial and temporal discretisation put practical limits on the range of their application and resolving power.

Laboratory investigations are necessary to support and inform the findings of field and numerical studies. This study will be the first laboratory investigation into shallow island wakes in over a decade and will make a fundamental and original contribution to scholarship in the field of geophysical fluid dynamics through the development and application of an innovative laboratory velocimetry technique.


Vorticies shed from a cylinder in shallow oscillatory flow
 

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