Turbulent transport of algae in stably stratified rivers: a computational fluid dynamics study


In stably stratified shear flows, turbulent mixing is suppressed by the background stratification. In rivers, these conditions allow buoyant cyanobacteria to float to the surface and grow to bloom conditions. In this study, computational fluid dynamics will be used to identify the critical conditions that lead to blooms and understand when these conditions arise.


Dr Nicholas Williamson, Professor Steven Armfield

Research Location

Aerospace, Mechanical and Mechatronic Engineering

Program Type



Stable stratified shear flows occur in numerous environmental settings such as in rivers, estuaries and in the atmospheric boundary layer. These conditions are also common in industrial flows such as within heat exchangers and building and ventilation flows. There is a strong motivation to understand the behaviour of these flows. One critical aspect addressed in this study is how turbulent mixing is damped by stable stratification. In river environments, turbulent mixing is a critical process which transports heat, oxygen and nutrients. When turbulence is damped there can be adverse consequences for aquatic life. A particular motivation of this study is how this damped state of mixing encourages algal blooms in Australian river systems. This project will investigate the fundamental fluid mechanics of mixing in these scenarios using direct numerical simulations to examine how buoyant algae particles interact with turbulence to float near the surface where there is greater exposure to light.

Additional Information

This project is based in the School of Aerospace, Mechanical and Mechatronic Engineering and is supported by the ARC Discovery Project Grant: `Thermal stratification, overturning and mixing in riverine environments’. We have industry partners: Murray Darling Basin Authority, Water NSW and Hunter Water corporation and further collaboration with freshwater ecologists outside our school of engineering. The outcomes of this work are both fundamental and applied. The direct numerical simulations of stably stratified flow will be unique and provide insights into turbulent mixing that are relevant to a wide range of environmental and industrial flows. The work is also focused on laying the foundations of a hydraulic model to predict the onset of algal blooms, which is the primary interest of the industrial partners.  In this project you will work is other research students, research fellows and academic staff in our very capable fluid mechanics laboratory and be part of a large fluid mechanics community at USYD. On a day to day basis you will be running very large scale numerical simulations of well resolved turbulent flow on large super computers using our in-house computational fluid dynamics code written in Fortran 90 and parallelized using MPI. You will analyse the results to obtain scaling relationships of critical flow behaviors. You will be working in a team which is also using laboratory and field based measurements to examine this problem. You will also be able to use laboratory experiments to obtain the buoyancy and drag characteristics of algae particles using Particle Image Velocimetry.Eligibility: International students will be asked to apply for `University of Sydney International Scholarship’. Domestic students with Australian permanent residency or New Zealand Citizenship will be asked to apply for `Australian Government Research Training Program (RTP) scholarships’. Further competitive Faculty of Engineering funding may be available if these scholarship applications are unsuccessful.

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fluid mechanics, turbulence, mixing, stratified flow, diapycnal mixing, turbulence modelling, environmental fluid mechanics, Computational Fluid Dynamics, algal blooms

Opportunity ID

The opportunity ID for this research opportunity is: 2328

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