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Fluid Dynamics Research Group

The Fluid Dynamics Research Group serves to advance and disseminate understanding of the fundamental fluid dynamics that underpins novel applications-driven research. Our scientific interests lie in the study of vortex dynamics, wave propagation, hydrodynamic stability, shear-layer phenomena, mixing, free-surface flows, multiphase physics and reacting flows using theoretical, computational and experimental methods.

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Research

The research conducted by members of the Fluid Dynamics Research Group covers a wide range of topics. Details of projects in each of the research areas are available from the following links:

Heat transfer

Jet impingement heat transfer

High-speed fluid jets are widely being considered for cooling of electronic components in modern instruments due to their inherently high heat transfer characteristics. The research in this area is aimed at examining the fundamental mechanisms of heat and fluid flow in continuous and interrupted jet flows with emphasis on the future application to real-world heat transfer problems.

Modelling thermal dynamics to predict coral bleaching

CFD, is potentially valuable in computing the thermal stress to which corals might be subjected if they had certain morphologies, occurred in different flow environments, and/or had different skeletal porosities or pigmentation. The technique is highly relevant to the ecophysiology of corals as corals live near their upper thermal limits, and thus what would otherwise be viewed as small temperature changes, could have physiological impacts on a coral such as bleaching, or death.It is hoped our work will be useful for coupling coral bleaching predictions to oceanic and climate models which can predict statistical information about the effects of climate change on coral thermal microenvironment.

Boiling and single-phase pulsed jet impingement heat transfer

Heat transfer techniques involving phase-change have been employed widely for cooling of components dissipating very large heat fluxes (over 100 W/cm2), primarily due to the inadequacy of standard (steady) single-phase cooling techniques. Jet impingement boiling has been found to be a potential technique for applications involving cooling of such large heat fluxes concentrated at discrete locations, such as in power electronics, synchrotron X-ray, fusion, and semiconductor laser systems. Literature also suggests that the introduction of jet pulsations in the form of self-oscillating jets, pulsating jets and synthetic jets provide substantial enhancement over traditional steady jet impingement; however, the studies are mostly limited to air impingement cooling applications. The present research aims to investigate the flow and heat transfer characteristics of steady-state and pulsed single phase and boiling heat transfer due to an impinging liquid jet on a heated surface. Both, computational and experimental approaches are employed for the study.

Bio-fluid mechanics

Simulation of the lungs

The simulation of lungs is of significant interest, as it offers a means to observe flow patterns, pressure fields within the lungs. However, most of the modelling work carried out thus far has used models with a fixed geometry and therefore has not considered lungs in motion (i.e. breathing) and is of limited applicability. Here we are developing a model of the lungs that simulates the expansion and contraction that occurs during breathing. Such a model will allow us to realistically simulate the behaviour of inhaled particles, such as pollutants or inhalable medications.

Syringomyelia

Syringomyelia is a disease in which fluid-filled cavities, referred to as syrinxes,form within the spinal cord. The progressive expansion of syrinxes over many years compresses the surrounding nerve fibres and blood vessels, which is associated with neurological damage.

Open questions:

  • What is the syrinx fluid source?
  • Is there a cerebrospinal fluid (CSF) pumping mechanism?
  • What causes syrinxes to be so localised?
  • How is syrinx fluid maintained?
  • Do congenital and post-traumatic syringomyelia have the same syrinx filling mechanism?
  • What is the relative importance of cardiovascular (cardiac cycle) and percussive (coughs/sneezes)?

Mechanically we have approached this problem from several standpoints.Wave propagation in the spinal canal has been investigated analytically using asymptotic analysis of 1-d collapsible tube models, and numerically by treating the wave dispersion as a harmonic eigenvalue problem. Using a lumped-parameter model the phasic pumping of CSF into the spinal cord has also been investigated.

A cantilevered plate in a viscous channel flow as an analogue of human snoring

Snoring noise is generated by vibration of the soft tissues of the upper airway, principally those that form the back of the roof of the mouth (the soft palate) and its conical extension (the uvula). In addition to discord with bed partners, snorers are at much greater risk of obstructive sleep apnoea. The instability that leads to flow-induced oscillations characteristic of inspiratory snoring in the human upper airway may be modelled using a cantilevered flexible plate in a mean channel flow.

Open questions:

  • What anatomical features of the airway make it prone to collapse?
  • Can we predict the site of collapse?
  • When in the respiratory cycle will collapse occur?
  • Is there a mechanical relation between the flow-induced oscillations typical of snoring and the airway?
  • Can we predict which treatment will be the most successful for a given patient?

The fluid and solid mechanics are modelled using an open source finite element code written in c++ (oomph-lib). Preliminary results show that there is a critical uvula-length fraction that determines whether the uvula stabilises or destabilises the system. Increasing the thickness, hence inertia and flexural rigidity, of a ‘short’ uvula, e.g., by oedema, makes the fluid-structure system more unstable. In this case if the oedema were aggravated by the vibratory mechanical insult then it would be self-sustaining and imply a bidirectional relationship between snoring and oedema of the uvula.

Upper airway flow modelling from patient-specific data

Obstructive Sleep Apnoea (OSA) is a common respiratory disorder characterised by the partial or complete blockage of the airway during sleep.The repetition of airway obstructions during the night induces hypoxemia and leads to the fragmentation of the normal sleep pattern. This disorder causes sleepiness and concentration loss during daytime and increases the risk of cardiovascular disease and hypertension as well as mental health problems.

The modelling of the airflow in the upper respiratory system intends to provide a better understanding of the influence of the physiological properties of the airway in the flow pattern. As a first step, measurements on patients with medical imaging procedures, such as computed tomography (CT scan), are used to simulate flow fields within particular geometric configurations of the airway and thus determine the potential sites of airway collapse. Taking into account the mechanical properties of the airway tissues, the fluid-structure interaction modelling gives a prediction of the static and dynamic deformation of the airway during the respiratory cycle.

The purpose of this study is to understand the mechanisms and the parameters leading to the airway obstruction. Since surgery, and in particular maxillomandibular advancement (MMA), is a developing treatment modality for OSA patients, this research seeks to assist in the diagnosis of OSA and the surgery planning by providing an objective evaluation of the disorder and a prediction of the surgery result.

General fluid structure interaction

Modelling and Analysis of the Fluid-structure Interaction of a T-Foil

This research will model and analyse the T-foil subjected to stress, vibration and instability due to FSI and the changes of the medium in relation to time. Mathematical analysis and computational methods will be the two primary methods employed in this study. The cross comparison of these results will also facilitate an ongoing validation. This analysis will lead to designs that minimise vibration and improve stability in applications where T-foils are exposed to FSI.

The most significant requirements when using T-foils in marine High Speed Craft (HSC) are reducing vibration and resistance, damping vertical wave induced ship motions at high speed and improving sea keeping performance T-foils are attached to HSC such as Navy units, fast ferries, and yachts.Therefore, in this research the T-foil will be treated as a cantilevered beam attached to the hull of a ship assumed rigid member. Theory of wings will be applied to the T-foil. The cross section of a T-foil is usually a NACA section (National Advisory Committee for Aeronautics). The fluid around T-foil is assumed to be in ideal flow by assuming inviscid irrotational, incompressible fluid flow. This research will investigate and analyse the influence of fluid structure interaction on a T-Foil. It will clarify the behaviour of T-foil when it is fully or partially immersed in water. It will help and widen the scope of marine design and hydrodynamic factors when the design of high speed craft is in its early stages.

Flutter power

We study a new system in fluid-structure interaction (FSI) wherein a cantilevered thin flexible plate is aligned with a uniform flow with the upstream end of the plate attached to a spring-mass system. This allows the entire system to oscillate in a direction perpendicular to that of the flow as a result of the mounting’s dynamic interaction with the flow-induced oscillations, or flutter, of the flexible plate. While a fundamental problem in FSI, the study of this variation on classical plate flutter is also motivated by its potential as an energy-harvesting system in which the reciprocating motion of the support system would be tapped for energy production. Initial results show that compared to a fixed cantilever, the introduction of the dynamic support system yields lower flutter-onset flow speeds and a reduction of the order of the mode that yields the critical flow speed; these effects would be desirable for energy harvesting applications.

Non-linear C-F beam in inviscid flow

The focus of the research to be undertaken is to further the understanding of the mechanisms involved in the phenomenon of flutter, a very common Fluid-Structure Interaction (FSI) problem, with many applications including paper manufacturing, snoring simulation and energy harvesting. Previous work carried out in examining these mechanisms has yielded many positive results. However, there are still a number of discrepancies between published theoretical models developed and experimental results which have yet to be fully explained. These discrepancies are namely the hysteresis observed in experiments and the difference in predicted and actual values for critical velocity (the flow velocity at which the onset of flutter occurs). During the course of this research a method for modelling the geometrically non-linear defection of a cantilever-free beam in inviscid flow and its associated wake and boundary layer will be developed. This model will be used to examine the influence of boundary layer separation on flutter in an attempt to explain the discrepancies. Physical experiments will also be carried out and energy harvesting methods examined.

FSI of a compliant wall in boundary-layer and channel flow

A study of FSI with compliant panels in both a boundary-layer flow and a channel flow using numerical methods. The purpose of this is to maximise the transition delay from laminar to turbulent flow that is possible for well designed compliant panels. Both inviscid and viscous fluids will be considered.A better understanding of FSI with compliant walls could lead to drag reduction of marine vehicles, more accurate biomechanical modelling and increased insight into the swimming capabilities of marine animals.

2d and 3d modelling of a boundary layer over a flexible surface

We perform modal and non-modal global linear stability analysis to 2D and 3D disturbances of the blasius basic flow profile above a compliant wall. The final goal is a) to identify possibly new instabilities relative to 2D or 3D disturbances of the external non-parallel flow above a compliant wall, b) to find the most “dangerous” initial disturbance which can cause early transition to turbulence and investigate the effect of the compliant wall on it.

Filters, fibres, and droplets

Mist Filter Simulation

A custom solver has been developed that incorporates discrete particle tracking, droplet agglomeration and break-up, and liquid coalescence and transport. This has been applied to fibrous filters, as used in the treatment of oil-mists. As well as being further developed for this application, the solver is being developed for the simulations of knitted filters.

Droplet fibre systems

Related to mist filtration, but also relevant to a wide range of topics – from shampooing your hair through to fuel cells, is the study of the dynamics of droplet-fibre systems. The work is conducted using numerical models (MATLAB/Python simulation), OpenFOAM VOF simulation, and experimental work. Most of the experimental work uses Atomic Force Microscopy.