Heriot-Watt University
ROS: The Research Output Service. Heriot-Watt University EdinburghNot a member yet
4689 research outputs found
Sort by
Understanding the microbial community dynamics in response to biocides for the improvement of reservoir souring prediction and mitigation
Get PDFReservoir souring is the term used to depict the phenomenon that the level of hydrogen
sulfide (H2S) increases over time during the oil and gas production from a reservoir that
previously produced no H2S. Reservoir souring is usually caused by the activity of a group
of microorganisms capable of producing H2S in the reservoir, mainly sulfate-reducing
microorganisms (SRM). Souring control is critical for the safe and economical production
of oil and gas because H2S is both toxic and corrosive. Biocides have been increasingly
used for souring control, mainly to inhibit microbial growth. Biocides often fail, and the
poor understanding of the microbial community dynamics in response to biocides limits
the optimisation of souring prediction and mitigation. In this PhD project, the microbial
community dynamics in response to biocides was extensively explored using both static
microcosms and dynamic sand-packed flow-through bioreactors. Spectrophotometric
measurement and ion chromatography were used to assess the concentrations of the
products and substrates of microbial activity. DNA assays (i.e. qPCR and 16S rRNA gene
sequencing) were used to quantitatively assess a variety of microbial ecology metrics,
which represent the key microbial properties delineating the microbial community
responses (i.e. the abundance, alpha diversity and structure of the microbial community).
In two of the studies, PMA (propidium monoazide) technique was incorporated before
the DNA extraction so that dead cells were removed from the downstream DNA assays,
allowing for the assessment of the microbial community responses for the live-only
fraction of the microbial community. The studies revealed close and complex connections
between the development of souring under biocide treatments and the changes in the key
microbial ecological properties. In particular, the studies suggested that the change in
alpha diversity could be an early warning sign for the failure of souring control using the
biocides investigated in this project. The studies further revealed that biocides might
result in changes in the spatial pattern of the microbial abundance by introducing a high
abundance “shelter” zone, as well as leading to the long-term microbial community shift
towards the enrichment of spore-forming SRM (e.g. Desulfotomaculum and
Desulfurispora). Collectively, the studies demonstrated the importance and potential
practical benefits of understanding and monitoring the ecological properties of the biocide
treated microbial community. Finally, based on the findings of the studies, several
conceptual models were developed to describe the responses of the souring microbial
community to biocides. These conceptual models might provide novel theoretical
foundations for the development of next-generation souring modelling tools, which may
further contribute to the improvement of current souring control strategies
Revivals in time evolution problems
Get PDFSubject to periodic boundary conditions, it is known that the solution to a certain family of linear dispersive partial differential equations, such as the free linear
Schr¨odinger and Airy evolution, exhibits a dichotomy at rational and irrational
times. At rational times, the solution is decomposed into a finite number of translated copies of the initial condition. Consequently, when the initial function has a
jump discontinuity, then the solution also exhibits finitely many jump discontinuities. On the other hand, at irrational times the solution becomes a continuous, but
nowhere differentiable function. These two effects form the revival and fractalisation
phenomenon at rational and irrational times, respectively.
The main aim of the thesis is to further investigate the phenomenon of revivals
in time evolution problems posed under appropriate boundary conditions on a finite
interval. We consider both first-order and second-order in time problems. For
the former, we examine the influence of non-periodic boundary conditions on the
revival effect. For the latter, we study the revivals under periodic and non-periodic
boundary conditions.
In terms of first-order in time evolution problems, we show that the revival
phenomenon persists in the free linear Schr¨odinger equation under pseudo-periodic
and Robin-type boundary conditions. Moreover, we prove that under quasi-periodic
boundary conditions, the Airy equation does not in general exhibit revivals. With
respect to second-order in time equations, we first formulate an abstract setting
for the revival phenomenon, which we then apply to establish that the periodic,
even-order poly-harmonic wave equation exhibits revivals. Finally, following the
lack of revivals in Airy’s quasi-periodic problem, we characterise quasi-periodic and
periodic problems, either of first-order or second-order in time, for which the revival
effect breaks.
In general, our approach relies on identifying the canonical periodic components
of the generalised Fourier series representations of solutions, in order to utilise the
classical periodic theory of revivals
New devices and techniques for quantum information protocols
Get PDFQuantum technologies aim to make direct use of the physics of the very small to revolutionise
how we sense, compute, and communicate information. Quantum communications promises
theoretically unbreakable future security and fully operational quantum key distribution (QKD)
systems are already available commercially. As the information carrier of choice for quantum
communications is the photon, uptake will be facilitated by the increased reproducibility and
scalability offered by integrated photonic devices.
The first experimental section of the thesis makes direct use of fibre-coupled femtosecond laser
written (FLW) devices in two experimental studies. The first presents an implementation of the
phase-sensitive state comparison amplifier, or SCAMP, and represents the first example of probabilistic amplification making use of an integrated component. This demonstrates the ability
of fibre-coupled FLW devices to integrate seamlessly into modern telecommunications infrastructure. The second study proposes a novel technique using the photonic lantern as a low-loss temporal multiplexer for the output of a quantum communications receiver. The technique
trades optimal key generation rate for cost effectiveness by facilitating single-detector QKD.
Femtosecond laser writing is unique so far among integrated photonic platforms in its ability
to implement the photonic lantern; the technique is proved in principle via characterisation of a
polarisation Bennett-Brassard 1984 QKD receiver temporally multiplexed by an FLW photonic
lantern combined with staggered optical delays. This experiment represents the first use case
of the photonic lantern in a quantum communications protocol and is an important step towards
mass consumer uptake.
The second experimental section introduces a creative new technique for manipulation of single-photon level thermal light, displaced photon subtraction, which presents as an optical loss but
allows manipulation of the relative amplification and suppression of the conditioned output mean
photon number via the displacement amplitude. The operation is translated to a realistic experimental implementation and proved in principle using commercially available fibre optical components, showing its immediate compatibility with current infrastructure and potential compatibility with integrated platforms. This technique represents a new tool for protocols in quantum
information that make use of thermal light which have enjoyed a recent resurgence of interest
in the literature as an easy to produce source of two-mode classically correlated single-photon
level light
Fluctuation-induced phases and localisation in quasicrystalline systems
Get PDFIn physics, quasicrystals are a unique kind of condensed matter system that lie
in between the usual paradigms of periodic and disordered matter. While they lack
any form of short-range translational invariance, they will retain long-range order,
which can allow for the observation of distinct physical properties. Different kinds of
quasicrystals will manifest their quasiperiodic order to model parameters in distinct
ways, including the geometry of a discrete lattice or a separate energy potential.
In this thesis, we will show how different quasicrystals can lead to the formation of
fluctuation-induced phases and localisation across a lattice. The models we consider
will concern that of interacting systems composed of bosonic atoms, allowing for
close analogies to be made to the field of ultracold gases. Indeed, ultracold atom
experiments with optical lattices offer the possibility to realise highly controllable
lattice environments, including those that are quasicrystalline.
By first considering quasicrystalline lattices, we show that non-uniform coordination numbers can introduce off-site disorder. If the interactions between atoms
are then non-local, the system can exhibit fluctuation-induced insulators and structures which have no crystalline counterpart. On the other hand, when considering
quasicrystals that contain on-site disorder, such as the 2D Aubry-Andr´e model,
long-range order can also affect the percolation of observables. For this scenario,
the physics of a small region in a quasicrystal can play a vital role in the underlying
quantum phase transitions. Finally, if a magnetic field interacts with atoms confined
to a quasiperiodic lattice, the induced cyclical motion can also lead to localisation.
In particular, we will show how off-site disorder from a magnetic field can introduce
localised, incompressible phases, i.e. current-carrying states that are robust against
particle number fluctuations. The study of interacting, quasicrystalline systems is
at an early stage of its development, especially in regards to the numerical methods that can solve these models. By studying the different quasicrystals across this
thesis, we will be able to uncover the rich potential of these fascinating systems and
their possible realisation in experiments.Engineering and Physical Sciences Research Council (EPSRC) CM-CDT Grant No. EP/L015110/1
Learning-based communication system design – autoencoder for (differential) block coded modulation designs and path loss predictions
Get PDFShannon’s channel coding theorem states the existence of long random codes that can
make the error probability arbitrarily small. Recently, advanced error-correcting codes
such as turbo and low-density parity-check codes have almost reached the theoretical
Shannon limit for binary additive white Gaussian noise channels. However, designing
optimal high-rate short-block codes with automatic bit-labeling for various wireless networks is still an unsolved problem.
Deep-learning-based autoencoders (AE) have appeared as a potential near-optimal
solution for designing wireless communications systems. We take a holistic approach that
jointly optimizes all the components of the communication networks by performing data-driven end-to-end learning of the neural network-based transmitter and receiver together.
Specifically, to tackle the fading channels, we show that AE frameworks can perform
near-optimal block coded-modulation (BCM) and differential BCM (d-BCM) designs in
the presence and absence of the channel state information knowledge. Moreover, we
focus on AE-based designing of high-rate short block codes with automatic bit-labeling
that are capable of outperforming conventional networks with larger margins as the rate
R increases. We also investigate the BCM and d-BCM from an information-theoretic
perspective.
With the advent of internet-of-things (IoT) networks and the widespread use of small
devices, we face the challenge of limited available bandwidth. Therefore, novel techniques need to be utilized, such as full-duplex (FD) mode transmission reception at the
base station for the full utilization of the spectrum, and non-orthogonal multiple access
(NOMA) at the user-end for serving multiple IoT devices while fulfilling their quality-of-service requirement. Furthermore, the deployment of relay nodes will play a pivotal
role in improving network coverage, reliability, and spectral efficiency for the future 5G
networks. Thus, we design and develop novel end-to-end-learning-based AE frameworks
for BCM and d-BCM in various scenarios such as amplify-and-forward and decode-and-forward relaying networks, FD relaying networks, and multi-user downlink networks.
We focus on interpretability and understand the AE-based BCM and d-BCM from an
information-theoretic perspective, such as the AE’s estimated mutual information, convergence, loss optimization, and training principles. We also determine the distinct properties of AE-based (differential) coded-modulation designs in higher-dimensional space.
Moreover, we also studied the reproducibility of the trained AE framework.
In contrast, large bandwidth and worldwide spectrum availability at mm-wave bands
have also shown a great potential for 5G and beyond, but the high path loss (PL) and
significant scattering/absorption loss make the signal propagation challenging. Highly
accurate PL prediction is fundamental for mm-wave network planning and optimization,
whereas existing methods such as slope-intercept models and ray tracing fall short in
capturing the large street-by-street variation seen in urban cities. We also exploited the
potential benefits of AE framework-based compression capabilities in mm-wave PL prediction. Specifically, we employ extensive 28 GHz measurements from Manhattan Street
canyons and model the street clutters via a LiDAR point cloud dataset and 3D-buildings
by a mesh-grid building dataset. We aggressively compress 3D-building shape information using convolutional-AE frameworks to reduce overfitting and propose a machine
learning (ML)-based PL prediction model for mm-wave propagation.EPSRC-UKRI fundin
Comparing Gaussian and Bessel-Gauss beams for translating ultrafast laser ablation towards soft tissue surgery
Get PDFThe goal of this research was to further improve existing ultrafast laser surgery techniques. To do so,
different beam shapes (Bessel-Gauss and Gaussian) were compared for performing ultrashort picosecond
pulsed surgery on various soft biological tissues, with the goal of minimising collateral thermal damage.
Initially, theoretical modelling was performed using OpticStudio to test axicons of various conical angles.
A 20° axicon was selected, but unfortunately early tests on murine intestinal tissue indicated a lack of
sufficient intensity to achieve plasma-mediated ablation of the tissue with the 6ps input pulses of 85 µJ
energy. Subsequently, a reimaged setup was designed in OpticStudio to demagnify the beam by a factor of
1.4x. The ability of this demagnified Bessel-Gauss beam to perform plasma-mediated ablation of murine
intestinal tissue was confirmed through histological analysis. Another setup was also designed to produce
a Gaussian beam of equivalent spot size.
These beams were then tested on porcine intestinal tissue using lower pulse repetition rates of 1, 2 and 3
kHz, with optimal ablation and thermal damage margins of less than 20 µm (confirmed through histological
analysis) being achieved with the Bessel-Gauss beam for spatial pulse overlaps of 70%, while for the
Gaussian beam the prominence of cavitation bubble formation at both 2 and 3 kHz inhibited the respective
ablation processes at this same spatial pulse overlap. As the numbers of passes were increased, the Bessel-Gauss beam also showed a trend of increased ablation depths. This was attributed to its large depth of focus
of over 1 mm, compared to the theoretical 48 µm depth of focus for the Gaussian beam.
After characterisation of fixated, non-ablated porcine intestine sample surfaces to quantify the
inhomogeneity, another set of ablation trials was performed at higher pulse repetition rates (5, 10 and 20
kHz) to test more clinically viable processes. For the Bessel-Gauss beam, spatial pulse overlaps of up to
around 50% at 5, 10 and 20 kHz offered excellent thermal confinement (with damage margins of < 30 µm,
< 50 µm and < 25 µm respectively) and shape control, but at 70% and greater pulse overlaps the ablated
feature became hard to control despite good thermal confinement (< 40 µm).
The Gaussian beam, while having the advantage of achieving plasma formation at lower input pulse
energies, was again found to be more prone to undesirable cavitation effects. Cavitation bubbles were
observed in the histology images for spatial pulse overlaps as low as 15% for 5 kHz and 30% for both 10
and 20 kHz. From the histology images it is clear to see that these effects became more pronounced as the
pulse repetition rate was increased. Conversely, the more consistent spot size of the Bessel-Gauss beam
across its longer focal depth resulted in a higher tolerance to cavitation bubble formation. This was also
demonstrated by high-speed videos of the beams being scanned across porcine skin samples. This could be
significant as it may allow for higher ablation rates. In addition, it could ease the design constraint of the
maximum speed at which the beam can be scanned at the distal end of an endoscopic device.
Despite this, both beams were able to achieve distinct ablation with high thermal confinement for certain
parameters. This work further highlights fibre-delivered ultrashort laser pulses as a promising alternative
to existing endoscopic tumour resection techniques, which carry a higher risk of bowel perforation.James Watt Scholarshi
Contribution of tidal energy to an integrated island energy system
Get PDFTidal stream energy is an emerging sector of the energy generation industry. Compared to many other renewable energy resources it has high potential to provide base-load
power due to the predictability of the speed and direction of tidal currents. However, the
practical use of tidal stream energy requires extraction that is both efficient and appropriate; in engineering design, social impact; and economic viability. This study designs a
tidal array for Orkney waters, testing it against a wide range of constraints from engineering efficiency to market suitability. It explores various approaches to achieving efficient
energy extraction. Different energy generation patterns are examined to find the strategy
that best fits the pattern of energy demand of the islands, without conflicting with the
existing supply. The study demonstrates the potential of integrating tidal energy into an
island energy system without the need for expensive grid upgrades. It shows that arranging the turbines in a staggered sub-array (SSA) layout, and regulating the power output
of the tidal device, increases the capacity factor of the installed system. This strategy
improves the economic viability and commercial competitiveness of tidal energy.James Watt scholarshi
Investigating the self-monitoring potentials of an engineered cementitious composite
Get PDFCement-based materials are an important group of structural materials, and the ability of
such materials to respond to internal and external changes could provide an added
feature which could further enhance their range of application. One area of development
that has received increasing attention within the research community is making use of
the self-monitoring features of concrete with respect to deformation and damage.
Ordinary concrete is, however, a poor conductor of electricity, particularly after
cracking and under dry conditions, and attention is therefore directed towards a highly
damage-tolerant family of concrete types with superior tensile strain capacities and
controllable crack widths, generally known as the Engineered Cementitious Composite
(ECC).
This thesis explores the self-monitoring capabilities of the ECC under mechanical and
non-mechanical loading and presents the a.c. electrical properties of ECC over the
frequency range 1 Hz–10 MHz. The project was developed on three general fronts,
focusing on key factors affecting the electrical properties of ECC:
(i) investigation of the influence of cement hydration and temperature;
(ii) evaluation of the influence of tensile straining and cracking; and
(iii) investigation of the influence of wetting and drying.
Laboratory samples of different geometries were fabricated and tested under various
curing regimes and test conditions. Results are presented from each of the sub-themes
listed above, with measured data presented in a range of formats to provide insights into
features that could potentially be exploited for self-monitoring. This includes the
Nyquist format, which has been generally used in a.c. electrical property measurements,
and the permittivity and conductivity, which were de-embedded from the measured
impedance and presented in the frequency domain to elucidate the nature of the
conduction and polarization processes. Equivalent circuit models were also developed
to simulate the measured response and offer a phenomenological interpretation of the
origin of some of the features observed in the electrical response.
It was found that, over a curing period of 180 days, the ECC displayed a classic
impedance response comprising an electrode spur, a weak intermediate "plateau" region
and a single bulk arc. Both conductivity and relative permittivity were found to be frequency dependent due to bulk relaxation processes operating within the composite. It
was found that cement hydration has a negligible effect on the relative permittivity at
high frequencies (i.e., > 1 MHz), as evidenced by the merging of the relative
permittivity at different curing ages when presented in a logarithmic format. Moreover,
the knowledge regarding the temperature effects on the electrical properties (through the
activation energy approach) will have direct practical significance for removing the
effect of natural temperature fluctuations.
Tensile straining was shown to result in a detectable change in the impedance response
but retained a similar overall profile. When presented in the frequency domain, a
downward displacement in relative permittivity at high frequency (i.e., 1 MHz) was
evident with increasing tensile strain for ECC with average crack widths in the range
50 μm–65 μm. In the ECC with larger average crack widths (i.e., >100 μm), a
downward displacement in relative permittivity profiles together with an enhancement
of the relative permittivity within the frequency range (> 10 kHz to ~low MHz) was
observed. Overall, it is shown that the relative permittivity at the high-frequency end
could be exploited as a potentially useful indicator for strain/damage detection.
The electrical properties of ECC display significant increases in the impedance response
when the material is subjected to drying. When presented in the frequency domain, an
enhancement of the relative permittivity within the frequency range > 1 kHz to
~low MHz was observed. Within the low-frequencies range of ~1Hz to < 1 kHz, the
relative permittivity of the un-cracked ECC curves showed a slight decrease, while the
cracked-ECC was sensitive to drying. When subjected to wetting, a reduction of the
impedance response was observed, and the enhancement of the relative permittivity at
high frequencies disappeared, due to the presence of water in the micro-cracks.
This thesis demonstrates the use of multi-frequency measurements to characterise the
electrical properties of ECC under mechanical and non-mechanical loading.Engineering and Physical Sciences Research Council, U.K. (Grant EP/N028597/1)
Scaling of soliton dynamics in gas-filled hollow capillary fibres
Get PDFSoliton-effect dynamics during the propagation of an ultrashort pump pulses in gas-filled hollow-core fibres offer unique capabilities for applications in many different
fields of science and technology, such as strong-field physics and time-resolved spectroscopy. In recent years many advancements have been made by the use of gas-filled
hollow-core micro-structured fibres. However, because of their small core sizes these
systems have been limited with respect to the maximum energy that can be used to
observe these dynamics.
In this thesis a method of scaling the energy of soliton-effect dynamics thought
the use of simple fused silica hollow capillary fibres (HCFs) is studied. First, the
viability of HCFs as a platform for high-energy soliton dynamics is demonstrated
by the compression of mJ-level pump pulses to sub-cycle duration, combined with
the emission of tunable ultrafast pulses, called resonant dispersive-wave (RDW)
emission, over the ultraviolet spectral region with brightness comparable to that
of free-electron lasers for these wavelengths. This is achieved by pumping a 3 - long helium-filled HCF with inner diameter of 250 µm with 10 fs pulses centered at
800 nm and 1 kHz repetition rate. The soliton-effect self-compression was shown to
yield pulses with envelope duration of 1.2 f fs and 340 µJ energy when the HCF was
filled with 400 mbar of helium. The wavelength of the RDW emission can be varied
simply by changing the filling gas pressure, and this tunability was demonstrated
for RDW emission from 120 nm to 350 nm by using He-pressures in the range of
230 mbar-4 bar. The energy of the UV-RDW was measured to nearly 1 µJ around
120 nm and up to 16 µJ around 220 nm. These represent two orders of magnitude
increase both in the self-compressed pulse energy and the RDW energy in comparison
to previous demonstrations in gas-filled hollow-core micro-structured fibres.
Further, the variation of the observed dynamics as a function of the dispersion
regime, in which the pulses are propagating, is studied using the same optical setup,
but using argon as HCF-filling gas. The filling-gas pressure is varied from 7 mbar to
3.344 bar in order to continuously change the dispersion at the pump wavelength of
800 nm from anomalous to normal. This has shown that the wide variety of soliton
dynamics, which have so far been observed in gas-filled micro-structured fibres, can
also be observed in HCFs. In addition, a new regime of interest is identified when
the short, 10 fs pump pulses are propagating near the zero-dispersion wavelength
(ZDW) of the gas-filled HCF. In this regime, in addition to spectral broadening,
a RDW-like band of radiation is generated at wavelengths shorter than the pump.
This is attributed to self-phase-modulation-induced pulse splitting of the pump pulse
near the ZDW and the consequent collision and cross-phase modulation of the split
pulses. In this study it is also shown that in certain cases, for example when the
pulse is experiencing significant self-focusing or ionisation, modelling by simply assuming pure fundamental-mode initial coupling into the HCF is not appropriate.
Instead, a full-spatial propagation of the pulse to the input of the fibre is necessary to appropriately calculate the initial modal excitation before the propagation
through the HCF.
Lastly, the short-wavelength extent of the process of RDW emission in different
noble gases is studied. A previous result present in the literature, the existence
of a gas-dependent RDW emission wavelength cut-off, which is shifted to shorter wavelengths for lighter noble gases, is experimentally confirmed. Using the same
HCF system as the previous studies, it is shown that the shortest RDW than can
be generated is 115 nm in helium, 125 nm in neon, 160 nm in argon, and 180 nm in
krypton. A possible reason for this cut-off is presented, linking it with the ionisation-influenced dynamics of the pump pulse. Further studies will aim to find a way of
extending the RDW emission cut-off to shorter wavelengths
Youth homelessness and the transition to adulthood : how understanding life stage informs effective interventions
Get PDFYoung people, because of their life stage, are widely thought to have distinct routes into
homelessness and require different interventions than older adults. Youth-specific
interventions are championed by those working in this sector. However, the scholarly
case for what form such responses should take remains unclear. This thesis explores and
strengthens the underpinning rationale and assumptions of current responses to youth
homelessness. Specific research questions are as follows: What are the key factors
influencing young people’s routes into homelessness? How useful is age as a proxy for
understanding young people’s support and housing needs? In what circumstances, if any,
is congregate supported accommodation a legitimate housing option for young people?
What difference do distinct national and local policy approaches make to young people’s
experiences of homelessness?
Answers to these research questions draw on youth studies literature and a critical realist
understanding of homelessness causation and new empirical data collection. The
fieldwork for this study compares two local case studies, Newcastle upon Tyne and
Glasgow, within the UK jurisdictions of England and Scotland. Interviews with National
Key Informants (n = 16) involved in statutory and third sector policy and practice roles
provided nation-level context on recent trends in youth homelessness. Local-level data
from the two case studies included interviews with local practitioners (n = 15) and young
people with current or recent experiences of homelessness (n = 23).
Key findings are that young people’s routes into homelessness can be better explained by
adopting learning from youth studies on the transition to adulthood. A proper
understanding of youth studies literature, in combination with the findings of this study,
makes clear that age has limited usefulness as a proxy for support and housing needs. It
is argued that congregate supported accommodation is not an appropriate housing option
for young people because of the intrinsically problematic impact such models have on
developmental processes. The examples of local policies on prevention and housing-led
approaches show meaningfully positive differences in young people’s homelessness
experiences. However, lower welfare entitlements for young people create barriers to
resolving homelessness and delay the developmental processes of reaching adulthood.
Based on these findings, this thesis makes the case that a greater understanding of life
stages is essential in delivering effective interventions for youth homelessness
Access Repository Dashboard
Do you manage ROS: The Research Output Service. Heriot-Watt University Edinburgh ? Access insider analytics, issue reports and manage access to outputs from your repository in the CORE Repository Dashboard!