Associate ProfessorNicholas Williamson

Project Opportunities

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

Summary of opportunity:

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.

Opportunity synopsis:

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.

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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Project Opportunities

Title: Turbulent mixing in stably stratified flow: a laboratory and field based study

Summary of opportunity:

Stably stratified shear flows, turbulent mixing is suppressed by the background stratification. This project will examine the dynamics of this flow using laboratory experiments and field measurements in strongly thermally stratified Australian rivers.

Opportunity synopsis:

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 both field measurements supported by a large multidisciplinary team and also with controlled laboratory experiments. Associated work using large scale direct numerical simulations is also ongoing.

This project is based in the School of Aerospace, Mechanical and Mechatronic Engineering and is supported by the ARC Discovery Project Grant: `The Dynamics of Suppressed Mixing Regimes in Australian rivers'. 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 field measurements of stably stratified flow will be unique and provide insights into turbulent mixing at large Reynolds numbers that is 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 and low dissolved oxygen events, 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 designing and performing laboratory experiments to study stratified flow using particle image velocimetry and laser induced fluorescence (PIV/LIF). You will also play a large part in designing and deploying lab scale instrumentation in field investigations such as on the Murray River. In particular, you will be working with Acoustic Doppler Current Profilers and a new high speed micro-PIV system to measure properties of turbulence in the field concurrently with high-speed temperature measurements.

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Project Opportunities

Title: Dissolved Oxygen Transfer by Convective Cooling in Lakes and Rivers

Summary of opportunity:

Gaseous Oxygen is dissolved into the surface waters of rivers, lakes oceans and then mixed by turbulence within the water column. The turbulent mixing which controls this transfer rate is poorly understood in conditions of low wind and strong thermal stratification where convective cooling and other mechanisms are important. This project will investigate these effects through both experimental and numerical approaches.

Opportunity synopsis:

Large scale fish kill events are an increasingly common occurrence in inland Australian rivers as a result of low dissolved oxygen (DO) levels. Rivers have significant DO demand for fish respiration and biological activity in the sediments and water column. Under normal conditions this DO demand is met by the continual transfer of gaseous oxygen from the atmosphere into the water surface, where it is then mixed through the water column by turbulence. When river flow is low however the water column can become thermally stratified and buoyancy effects damp out turbulent mixing thereby reducing DO transport from the surface. These conditions lead to DO depletion and fish kills. The gas transport process is very complex. Studies across a wide range of flows (lakes, oceans, rivers) have shown that gas transfer from the surface is usually controlled by small scale mixing at the surface driven by larger scale turbulence within the water column. However, the dynamics of the turbulence, including how the turbulence is generated have a strong influence on the process and require separate parameterisations. The strongly stratified flow regimes seen leading up to fish kill events in Australian rivers are very different from conditions examined in most early investigations, so we are unable to predict DO supply into a water body before, during or after a fish kill event.

This project comprised of two main lines of investigation:

• Numerical research program which will use highly resolved direct numerical simulations to investigate  the near surface mixing in strongly stratified river/lake flow under light wind forcing. The dynamics of convective cooling on gas transfer and the dynamics of non-breaking surface waves will investigated.
• A field program which will involve deploying high accuracy eddy covariance instruments to capture the turbulent oxygen and heat flux near the surface. Our well equipment fluid mechanics laboratory will be used for testing and development of instrumentation.

 PhD projects are available on both topics.

The research will be conducted within a large multi-disciplinary team including engineers and freshwater ecologists.

Successful candidates must:

• Have a strong interest in fundamental fluid mechanics, heat and mass transfer and numerical or experimental methods for fluid mechanics.

How to Apply: To apply, please email nicholas.williamson@sydney.edu.au the following:

• CV
• Transcripts

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Project Opportunities

Title: Turbulent mixing in urban environments - towards trusted simulations under strongly stable and unstable conditions

Summary of opportunity:

Using high resolution numerical simulations this project will advance our understanding of the effects of buoyant convection on turbulent mixing in urban environments (i.e. within city streets) and will develop new modelling approaches for large eddy simulations of these flows.

Opportunity synopsis:

The modelling and simulation of urban environments is undergoing an extraordinary revolution. The increase in computational power currently available means that computational fluid dynamics simulations of an urban city block can be performed at ∼ 5m resolution in near real time using Large Eddy Simulation (LES). This advancement has the capacity to revolutionise local weather prediction, decision making in urban planning and responses to natural and anthropogenic disasters, as well as address critical challenges such as urbanisation, urban heat islands and climate change. There are key deficiencies in current urban flow models that must be addressed however.

This project focuses on understanding the effect of buoyancy on street level mixing process. These process include convective boundary layers formed on streets and buildings and thermal plumes that discharge into the urban roughness layer. These processes are poorly represented in course resolution models. This project will use high resolution direct numerical simulation and high resolution large eddy simulations to understand the flow physics and develop new modelling strategies for them. 

Successful candidates must:

• Have a strong interest in fundamental fluid mechanics and numerical methods
• Have a background in Engineering or Science/Mathematics is preferred

How to Apply: To apply, please email nicholas.williamson@sydney.edu.au the following:

• CV
• Transcripts