Research opportunities supervised by Dr Mathieu Sellier
Email supervisor: mathieu.sellier@canterbury.ac.nz
Potential applicants, please note:
- A PhD scholarship is available for Projects 1 and 2.
- In the absence of specific funding for a research project, scholarships are available for outstanding students; see http://www.mech.canterbury.ac.nz/courseadvice/PGFeesFundingAdvice.shtml and self-funded students are welcome to apply.
- Instructions on the application procedure can be found at: http://www.mech.canterbury.ac.nz/courseadvice/PGApplicationAdvice.shtml
Project 1: Self-propelling, Coalescing Droplets
Degree: PhD
Supervisors: Dr Mathieu Sellier, Dr Volker Nock (Electrical and Computer Engineering)
Project description:
Digital microfluidic devices play an ever increasing role in nano- and biotechnologies. These rely on the micromanipulation of discrete droplets which are transported, stored, mixed, reacted, or analyzed in a discrete manner. One of the key challenges is to transport them in an efficient and reliable way. This research proposes to investigate experimentally and numerically a previously unexplored propulsion mechanism which relies on the induction of a surface tension gradient in the droplet by mixing droplets of different substances having a large surface tension contrast. We have recently proven the feasibility of this new mechanism in “proof of concept” experiment (see http://www.scivee.tv/node/26233). One of the key advantage of this new droplet propulsion mechanism is that it does not rely on high-tech, high-cost micro-fabrication techniques. The experiment raised a number of fundamental questions such as what is the role of the thin film connecting both droplets? What is the role of the surrounding atmosphere in the generation of the surface energy gradient? Can the coalescence enhance fluid mixing, a difficult task in microfluidic applications? The project aims to understand the underlying physics of this phenomenon and assess its potential in engineering applications.
Relevant literature:
Sellier, M., Nock, V. and Verdier, C. (2011) Self-propelling coalescing droplets. Int. J. Multiphase Flow, 37, 462-468.
Funding available (fees and living expenses)
Project 2: Control of Bitumen Spraying
Degree: Masters or PhD
Supervisors: Dr Mathieu Sellier and Associate Professor Ken Morison (Chemical and Process Engineering)
Project Description:
Road pavements that are created by spraying bitumen onto them, followed by a covering of aggregate, over time develop ‘fatty’ or ‘bleeding’, or ‘flushed’ areas where the bitumen is at, or near to, the surface of the aggregate.
These flushed areas are dangerous (e.g., aquaplaning) and difficult to fix – usually the pavement needs to be milled off. Subsequent applications of the sprayed bitumen simply replicate the problem. This is a worldwide problem.
The research project is to design a novel system for applying bitumen to road surfaces, in a manner in which the application of the bitumen is variable, across the width and length of the pavement – less bitumen on the flushed areas and more bitumen on the areas where the aggregate is well above the level of the bitumen. The system would need to have graduated applications, from light coatings to heavy coatings, to be applied in real time as the bitumen application equipment moves along a pavement.
The research is likely to consist of the following parts:
1.Physical and chemical properties of bitumen spray (density, viscosity, temperature effects, composition, age changes in composition)
2.Physical properties of bitumen at road temperature.
3.Definitions and quantification of satisfactory, ideal, “fatty”, “bleeding” and “flushed” areas.
4.Application systems, especially spraying equipment, spraying coverage, effects of physical properties and pressure on flow rate and drop size.
5.Coverage of surfaces: spreading dynamics, film flow, cooling.
6.Imaging of surfaces to detect size distribution, roughness and presence of bitumen.
7.Potential means of controlling coverage (i.e. the manipulated variable): vehicle speed, pressure, positive flow control, temperature, dilution.
8.Control algorithms to relate the measured variables and manipulated variables.
The study will involve an understanding of fluid mechanics, control, and mechatronics.
Funding: Financial support by TECHNIX INDUSTRIES LIMITED (Fees and living expenses).
Project 3: What Interfaces Hide?
Degree: Masters or PhD
Supervisors: Dr Mathieu Sellier
Project Description:
Free surface flows occur in wide range of context. They arise, for example, in the form of water droplets when we take a shower, in the form of a thin liquid film when we apply a paint layer on a wall. They are also prevalent in geophysics where they appear as river or glacier flows to name a few. The recent development of numerical simulation tools has tremendously expanded our understanding of such flows but to date the focus has mostly been on "what if" scenarios. For example, how does the free surface of a river respond to an increase of the flow rate? How does the free surface of a glacier respond to bedrock variations? This viewpoint, referred to as the direct problem, consists in finding the observable consequences of a set of causes and conditions. The proposed research focuses on the inverse problem for which the causes and conditions of the flow are reconstructed from the knowledge of observable consequences. In this new paradigm, the free surface is a "signature" of the flow which can be related to unknown flow quantities. The proposed research will develop a theoretical framework and numerical tools to solve such inverse problems. More specifically, the research program will enable the reconstruction of the bedrock profile from free surface data in geophysical flows such as river or glacier flows. It will allow the reconstruction of the surface tension distribution in Marangoni driven flows, i.e. flows driven by surface tension gradients, thereby shedding light on phenomena such as the formation of a coffee stain or the transport of surface active agent (surfactant) at interfaces. Finally, the research program will pave the way to a new methodology to characterize the rheology of fluids based on the free surface response to prescribed perturbations.
Relevant literature:
Gessese, A.F., Sellier, M., Van Houten, E. And Smart, G. (2011) Reconstruction of river bed topography from free surface data using a direct numerical approach in one-dimensional shallow water flow. Inverse Problems, 27, 025001.
Funding: self-funded or through University scholarship. Funding is sought from the Royal Society.
Project 4: Numerical Techniques in Optimal Shape Design
Degree: Masters or PhD
Supervisors: Dr Mathieu Sellier, Prof XiaoQi Chen
Project Description:
Using numerical simulation to compute the dynamics of flows or the response of structures to external loads is nowadays routine practice in engineering and science. The development of sophisticated numerical techniques has allowed substantial progress in the solution of direct problems which consists in finding the effects of a set of causes. For example, how are the aerodynamics coefficients of a body immersed in a flow affected by its shape? We are now in a position to tackle more challenging problems where the effect is known (or desired) and the cause is sought. For example, an engineer might want to find the shape of a body which maximizes its lift, reduces its drag, or prevent flow separation. Such problems are called optimal shape design problems and they are particularly difficult. Typically, addressing such problems first involves the definition of an objective function which measures the performance of the current shape and constraints. This objective function may, for example, be the total lift generated by the body and the constraint may be the area of the cross section. The evaluation of the objective function typically requires a numerical simulation using Computational Fluid Dynamics. The next steps involve parameterizing the body shape and evaluating the sensitivities. The sensitivities give a measure of how the objective function varies with elementary variations of the body shape. This step is particularly difficult as an explicit relationship between the body shape and the objective function is usually unavailable. Once the sensitivities are known, it is possible to infer a new estimate of the shape closer to the optimal solution. The process is repeated iteratively until a extremum in the objective function is obtained. A difficulty associated with this process relates to the fact that after every iteration in the optimization process, a new shape is generated. Consequently, a new mesh needs to be generated in order to compute the objective function. Also, there is no explicit relationship between the objective function and the body shape. In order to address these issues we propose to use the Boundary Element technique to discretize the problem and compute the objective function. The main feature of the boundary element technique is that only the contour of the body is discretized instead of the entire flow domain. This is a significant advantage because the required remeshing after each iteration of the optimization procedure is considerably simplified. Also, the Boundary Element technique opens up the prospect of finding an explicit expression for the sensitivity thereby considerably enhancing the convergence. The particular problem we propose to focus on relates to the optimal design a Non-Contact Adhesion Pad (NCAP) for robotic pick-and-place applications, often referred to as Bernoulli grippers. Such a pad recently developed at the University of Canterbury has recently been shown to hold great promise, see Reference 1.
Relevant literature:
[1] Journee, M., Chen, X., Robertson, J., Jermy, M. and Sellier, M. “An investigation into improved non-contact adhesion mechanism for wall climbing robotic application” in Proceedings of the 2011 IEEE International Conference on Robotics and Automation.
Funding: self-funded or through University scholarship.
Project 5: Bedrock Reconstruction in Glacier Flows
Degree: Masters or PhD
Supervisors: Dr Mathieu Sellier, Dr Wolfgang Rack (Gateway Antarctica), Dr Christian Heining (University of Bayreuth, Germany)
Project Description:
The melting of ice-sheets and glaciers is stigmatic of global warming and climate change. Because the ice-mass is such a good indicator of climatic changes, it has been under intense scrutiny in the recent past. As counter-intuitive as it sounds, solid ice tends to flow under its own weight like a “very thick” liquid would and the mathematical description of its dynamics bears many similarities with that of highly viscous flows. One of the major challenges geologists face when it comes to understanding ice flows is that while information at the surface of the ice-sheet or glacier is easily accessible, the base is notoriously difficult to access and assess [1]. Current techniques to indirectly infer the bedrock topography rely on radar measurements from an aircraft, a costly operation.
In order to circumvent this difficulty and cost, the aim of the proposed work is to use information from the surface of the ice mass such as the free surface elevation and/or the free surface velocity to infer unknown basal conditions such as the bedrock elevation and/or the basal slip, i.e. the amount by which the ice mass slips on the bedrock. Such problems are often referred to as “inverse problems” since one tries to infer the unknown causes of observed consequences.
The proposed research builds on parallel efforts from the proposed supervisory team to solve similar inverse problems in a different context. For example, Sellier [2] and Sellier & Panda [3] proposed a simple technique to reconstruct the topography of a substrate from the knowledge of the free surface variation in the context of thin liquid films such as coatings. Gessese et al. [4] applied the same idea to river flows, i.e. the authors showed that it is possible to reconstruct the riverbed topography from the knowledge of the free surface elevation or the free surface velocity. Heining & Aksel generalized the results of [5] to include the effects of inertia [4] and showed that the full velocity field could be reconstructed [6].
A preliminary step to solve this inverse problem is to understand and describe in mathematical terms glacier dynamics. To do so, we propose to use the Shallow-Ice-Approximation developed by Hutter in the 80’s [7] which express the evolution of the glacier free surface as a function of the bedrock profile, the ice properties, and the rate of ice accumulation/ablation. However, to model realistic glaciers or ice-sheets, a large computational domain with a sufficient mesh resolution combined with a long simulation span is required. In order to tackle this simulation challenge, we propose to develop and implement a numerical technique known for its optimal convergence rate, the Multigrid technique with adaptive time-stepping and local mesh refinement similar to the one developed by the senior supervisor in the context of creeping flows, [8]. A key advantage of this numerical technique is that it easily lends itself to parallel computation, [9]. We will use here a geometric decomposition of the domain assigning each subdomain to a single parallel processor. Each processor is then responsible for implementing the Multigrid algorithm on its own subdomain. The Message Passing Interface framework which facilitates portability across different (distributed and shared memory) high performance computing platforms will be used.
With the participation of Dr W Rack from Gateway Antarctica, we will have access to the necessary field data to calibrate and validate the implementation of the forward Multigrid solver and solution methodology for the inverse problem.
The student participating in this project will gain valuable experience in scientific computing, numerical techniques, parallel computing/programming, and inverse problem theory. He will be involved in a project with a multi-disciplinary research team. It is hope that the student will be able to visit the University of Bayreuth to interact with a proposed member of the supervisory team, Dr Christian Heining.
Relevant literature:
[1] Maxwell D., Truffer M., Avdonin S., Stuefer M., 2008, “An iterative scheme for determining glacier velocities and stresses”, J. Glaciology 54, 888-898.
[2] Sellier M., 2008, “Substrate design or reconstruction from free surface data for thin film flows” Phys. Fluids 20, 062106.
[3] Sellier M. and Panda S., 2010, “Beating capillarity in thin film flows”, Int. J. Numer. Meth. Fluids 63, 431-448.
[4] Gessese A.F., Sellier M., Van Houten E., Smart G. „Reconstruction of river bed topography from free surface data using direct numerical approach in one dimensional shallow water flow“, Inverse Problem 27, 025001
[5] Heining C., Aksel N., 2009. “Bottom reconstruction in thin-film flow over topography: Steady solution and linear stability”, Phys. Fluids 21, 083605.
[6] Heining C., Aksel N. “Velocity field reconstruction in gravity-driven flow over unknown topography”, to appear in Physics of Fluids.
[7] Hutter, K., 1983, “Theoretical glaciology; material science of ice and the mechanics of glaciers and ice-sheets”, Dordrect, etc., D. Reidel Publishing Co./Tokyo, Terra Scientific.
[8] Gaskell, P.H., Jimack, P.K., Sellier, M., Thompson, H.M., 2004, “Efficient and accurate time adaptive multigrid simulations of droplet spreading”, Int. J. Num. Meth. Fluids 45, 1161-1186.
[9] Gaskell, P.H., Jimack, P.K., Koh, Y.-Y., Thompson, H.M., 2008, “Development and application of a parallel multigrid solver for the simulation of spreading droplets”, Int. J. Num. Meth. Fluids 56, 979-989.
Funding: self-funded, through University scholarships, or HPC scholarship.
Project 6: Understanding Blood Droplet Formation
Degree: Masters or PhD
Supervisors: Dr Mathieu Sellier, Dr Mark Jermy (Mechanical Engineering), Dr Michael Taylor (ESR)
Project Description:
Bloodstain pattern analysis (BPA) is a method applied by trained investigators in their efforts to solve serious crimes. It is based on the study of the shapes and distribution of individual blood splashes. It has the potential to incriminate or exonerate a suspect and, as such, has life-changing consequences. To date, it relies extensively on the experience of forensic scientists and a range of empirical laws. It is vital that this experience is complemented with the development of a sound scientific basis for the opinions offered in courts of laws.
We propose to unravel the underlying fundamental mechanisms involved in the formation of blood droplets, subsequent flight, and the resulting bloodstain pattern during the rapid motion of a weapon covered with a thin layer of blood. The better understanding of how the droplets are formed and how the size distribution depends on the weapon motion is expected to help forensic scientists in their endeavour to solve crimes.
This study will have a computational and an experimental part. For the computational part, we intend to use the Gerris flow solver developed by Dr Stephane Popinet which is ideally suited to model complex interfacial flow phenomena such as atomization processes. A key feature of this CFD program is that it is well-suited to run on parallel CPUs, a feature which will be necessary for the type of simulation we propose to perform. Indeed, the droplet formation during the atomization process requires high grid resolution thus increasing the computational cost several folds. Paralellism is achieved in Gerris through domain decomposition: the global simulation mesh is split into as many subdomains as processors and each processor performs the same instructions as the others but only on its subdomain. As a benchmark problem for this study, we will consider the generation of blood droplets resulting from a bat being swung. This kind of interfacial flow problem is a precisely what Gerris was developed for and we are therefore confident that we will be able to perform these simulations.
The experimental part will build on an ongoing effort by Dr Mark Jermy to study experimentally droplet formation, transport, and deposition using high-speed imaging and particles tracking. Experiments will allow the validation of the numerical simulation while the simulation will provide an insight otherwise unachievable into fluid dynamics of the droplet break-up and subsequent flight.
The student will be exposed to both computational and experimental techniques, an ideal combination for future engineers or academics alike. The student will work in a multi-disciplinary project with a supervisory team covering a wide range of knowledge and expertise and he/she will learn a lot about interfacial fluid mechanics, parallel computing, and flow visualization. The student will have an opportunity to make an important contribution to a much applied field, forensic science, which recently acknowledge its need of a more fundamental understanding of the underlying science.
Funding: self-funded, through University scholarships, HPC scholarship, or ESR scholarship.
Project 7: Contact line dynamics in the presence of solid occlusions
Degree: Masters or PhD
Supervisors: Dr Mathieu Sellier
Project Description:
Anyone who has ever painted a wall understands the challenges of producing a defect-free finish when the surface on which the paint is deposited is imperfect in some ways. For example, when the paint layer encounters an occlusion such as a nail, a dry patch may develop downstream of the occlusion. The appearance of not of this defect in the paint layer is a result of capillary and wetting phenomena. As the wetting front, also known as the contact line, passes over the occlusion it may find energetically favourable to detach from the occlusion and form a downstream dry patch. It can be anticipated that the contact line behaviour as it flows past the occlusion is dependent on several parameters such as its velocity, the nature of the fluid, or the surface properties of the occlusion and the substrate. In spite of its obvious practical relevance, this problem has not to date been investigated in a rigorous and systematic way. The proposed project consists in studying experimentally the effect of an occlusion on the contact line and comparing the results with a theoretical model developed by the project leader. The envisaged experimental rig is rather simple. It consists of a plane which can easily be inclined at a desired angle to the horizontal and on which fluid can released at a desired flow rate or a desired volume. A simple mechanism will allow the clamping of occlusions of different sizes, shapes and materials on the inclined plane. The contact line dynamics past the occlusion will be monitored using a simple video camera but high-speed imaging is also available if deemed necessary. The surface properties such as the surface wettability which determines the tendency of the surface to attract the fluid or repel it will be measured using a purposely purchased surface force measurement device known as a goniometer. Part of the project will also involve reviewing the underlying theory, running the simulations in the commercial Finite Element Package COMSOL based on the model developed by the project leader. As a nice addition to the project and depending on the available time, it may be possible to quantify the free surface elevation in the flow field which would be another very useful outcome of the project to validate the numerical model.
Funding: self-funded or through University scholarship.
Project 8: Modelling silica scale deposition in geothermal systems
Degree: Masters or PhD
Supervisors: Dr Mathieu Sellier, Dr Mark Jermy
Project Description:
The problem of silica scale deposition occurs in geothermal power stations when the working geothermal fluid is deprived of most of its thermal energy in power generation process. At this point due to the changes in thermodynamic parameters and loss of steam, initially dissolved in geothermal fluid minerals, especially silica, became oversaturated and start to precipitate. They may then deposit on the internal surfaces of the power plant equipment decreasing its efficiency and causing high maintenance costs and equipment over sizing.
The costs incurred by this problem are significant. For example, the replacement of a reinjection well costs at least 10 million dollars. The maintenance involving mechanical clean, chemical clean or process clean is also a costly process and results in loss of power generation during downturn time.
To date, this problem is mitigated by injecting acid which has an associated cost and possible negative environmental impacts. Injecting acid into the brine tends to limit silica polymerisation and deposition.
Silica scale deposition is a complex phenomenon which involves several strongly coupled mechanisms such as heat transfer, chemistry, and hydrodynamics. The current knowledge on silica scale deposition is incomplete limiting the range of possible mitigation strategy.
The research we propose to undertake aims to narrow that knowledge gap to enable the development of innovative, cost-effective, and environmentally friendly mitigation strategies.
Funding: self-funded or through University scholarship. Funding currently being sought from Mighty River Power and the Ministry for Science and Innovation.