Research

We are going to have a seminar on 'Modelling study of flushing of trapped salt water from a blocked tidal estuary' on 1st May 2014, Thursday, given by Dr Yakun Guo, Reader of the Environmental and Industrial Fluid Mechanics research group at the University of Aberdeen.

Abstract and biography are given below.

Date & Time: 1:00-2:00 PM, 1st May 2014, Thursday

Venue: Room 355 (J10), James Watt Building South

Tea/coffee/biscuits before the start.

 

 

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Abstract

The complicated purging of saline water trapped in deeper bed regions by an overflow of turbulent freshwater has been widely studied, partly due to the serious environmental implications (Anderson and Morison 1989; Kurup et al. 1998). Here we present the results from a laboratory model studies and numerical simulations of the flushing of saline water from a partially- or fully-closed estuary. The first part is the experiments which have been carried out to determine quantitatively the response of the trapped saline volume to fresh water flushing discharges Q for different values of the estuary bed slope α and the density difference (Δρ) between the saline and fresh water. The trapped saline water forms a wedge within the estuary and for maintained steady discharges, flow visualisation and density profile data confirm that its response to the imposition of the freshwater purging flow occurs in two stages, namely (i) an initial phase characterised by intense shear-induced mixing at the nose of the wedge and (ii) a relatively quiescent second phase where the mixing is significantly reduced and the wedge is forced relatively slowly down and along the bed slope. Scaling based upon simple energy balance considerations are shown to be successful in (i) describing the time-dependent wedge behaviour and (ii) quantifying the proportion of input kinetic energy converted into increasing the potential energy of the wedge/river system. Measurements show that the asymptotic value of the energy conversion factor increases with increasing value of the river Froude number Fr at small values of Fr, thereafter reaching a maximum value and a gradual decrease at the highest values of Fr. Experiments with multiple, intermittent periodic flushing flows confirm the importance of the starting phase of each flushing event for the time dependent behaviour of the saline wedge after reaching equilibrium in the intervals between such events. For the parameter ranges investigated and for otherwise-identical external conditions, no significant differences are found in the position of the wedge between cases of sequential multiple flushing flows and steady single discharges of the same total duration. The second part presents the results of a numerical simulation investigating this complicated flushing process. A multiphase model is applied. The time-dependent salt-wedge position, vertical-density distribution and proportion of total input kinetic energy converted into potential energy are simulated for various incoming flow densimetric Froude number and estuary bed slope. The vertical position and thickness of the interfacial mixed layer between freshwater and salt water as well as the local gradient Richardson number are determined from simulated density profiles and velocity fields. The good agreement between the simulated and measured results is obtained.

Biography

Dr Yakun Guo is a Reader of the Environmental and Industrial Fluid Mechanics research group at the University of Aberdeen. He has research interests in buoyancy-driven and free-surface flows, coastal and estuary hydrodynamics, sediment transport and water quality simulation, open channel flows, fluid-structure-interaction in hydro turbines and has worked extensively on numerical modelling of water flows. He recently develops research interests in marine renewable energy (e.g. stability of offshore wind/tidal energy system and their influence on marine environment (sediment transport and local scour). He currently holds grants from the EPSRC, KTP DTI/EPSRC projects, China National Natural Science Foundation (as Co-I).  Dr Guo has published over 160 papers including about 90 papers in peer-reviewed journals.

First published: 24 October 2013