45 research outputs found
Shear dispersion along circular pipes is affected by bends, but the torsion of the pipe is negligible
The flow of a viscous fluid along a curving pipe of fixed radius is driven by
a pressure gradient. For a generally curving pipe it is the fluid flux which is
constant along the pipe and so I correct fluid flow solutions of Dean (1928)
and Topakoglu (1967) which assume constant pressure gradient. When the pipe is
straight, the fluid adopts the parabolic velocity profile of Poiseuille flow;
the spread of any contaminant along the pipe is then described by the shear
dispersion model of Taylor (1954) and its refinements by Mercer, Watt et al
(1994,1996). However, two conflicting effects occur in a generally curving
pipe: viscosity skews the velocity profile which enhances the shear dispersion;
whereas in faster flow centrifugal effects establish secondary flows that
reduce the shear dispersion. The two opposing effects cancel at a Reynolds
number of about 15. Interestingly, the torsion of the pipe seems to have very
little effect upon the flow or the dispersion, the curvature is by far the
dominant influence. Lastly, curvature and torsion in the fluid flow
significantly enhance the upstream tails of concentration profiles in
qualitative agreement with observations of dispersion in river flow
Correcting Experience Replay for Multi-Agent Communication
We consider the problem of learning to communicate using multi-agent
reinforcement learning (MARL). A common approach is to learn off-policy, using
data sampled from a replay buffer. However, messages received in the past may
not accurately reflect the current communication policy of each agent, and this
complicates learning. We therefore introduce a 'communication correction' which
accounts for the non-stationarity of observed communication induced by
multi-agent learning. It works by relabelling the received message to make it
likely under the communicator's current policy, and thus be a better reflection
of the receiver's current environment. To account for cases in which agents are
both senders and receivers, we introduce an ordered relabelling scheme. Our
correction is computationally efficient and can be integrated with a range of
off-policy algorithms. It substantially improves the ability of communicating
MARL systems to learn across a variety of cooperative and competitive tasks
Polynomial matrix reduction to linearised form
In many aspects of system analysis it is required to consider a set of
equations in order to infer the behaviour or, more simply, properties of the
system. In many cases these equations will be complex and consequently difficult
to analyse. It would be useful therefore from the analysis point of view if a
similar but equivalent set of equations describing the system's behaviour could
be found. [Continues.
Automatic phased mission system reliability model generation
There are many methods for modelling the reliability of systems based on component failure
data. This task becomes more complex as systems increase in size, or undertake missions
that comprise multiple discrete modes of operation, or phases. Existing techniques require
certain levels of expertise in the model generation and calculation processes, meaning that
risk and reliability assessments of systems can often be expensive and time-consuming.
This is exacerbated as system complexity increases.
This thesis presents a novel method which generates reliability models for phasedmission systems, based on Petri nets, from simple input files. The process has been
automated with a piece of software designed for engineers with little or no experience
in the field of risk and reliability. The software can generate models for both repairable
and non-repairable systems, allowing redundant components and maintenance cycles to be
included in the model.
Further, the software includes a simulator for the generated models. This allows a user
with simple input files to perform automatic model generation and simulation with a single
piece of software, yielding detailed failure data on components, phases, missions and the
overall system. A system can also be simulated across multiple consecutive missions. To
assess performance, the software is compared with an analytical approach and found to
match within 5% in both the repairable and non-repairable cases.
The software documented in this thesis could serve as an aid to engineers designing
new systems to validate the reliability of the system. This would not require specialist
consultants or additional software, ensuring that the analysis provides results in a timely
and cost-effective manner
Aspects classificatoires des systèmes à objets
Colloque avec actes et comité de lecture.Cet article cherche à mettre en évidence les divers aspects de la classification dans la variété des systèmes à objets. Pour cela, la classification est examinée en tant que processus de formation et d'organisation de classes, mais aussi en tant que processus de raisonnement, pour des besoins de reconnaissance par exemple. Il est montré que l'approche objet apporte un point de vue particulier et des interrogations nouvelles et complexes dans le domaine de la classification
Robust learning of acoustic representations from diverse speech data
Automatic speech recognition is increasingly applied to new domains. A key challenge is
to robustly learn, update and maintain representations to cope with transient acoustic
conditions. A typical example is broadcast media, for which speakers and environments
may change rapidly, and available supervision may be poor. The concern of this
thesis is to build and investigate methods for acoustic modelling that are robust to the
characteristics and transient conditions as embodied by such media.
The first contribution of the thesis is a technique to make use of inaccurate transcriptions as supervision for acoustic model training. There is an abundance of audio
with approximate labels, but training methods can be sensitive to label errors, and their
use is therefore not trivial. State-of-the-art semi-supervised training makes effective
use of a lattice of supervision, inherently encoding uncertainty in the labels to avoid
overfitting to poor supervision, but does not make use of the transcriptions. Existing
approaches that do aim to make use of the transcriptions typically employ an algorithm
to filter or combine the transcriptions with the recognition output from a seed model,
but the final result does not encode uncertainty. We propose a method to combine the
lattice output from a biased recognition pass with the transcripts, crucially preserving
uncertainty in the lattice where appropriate. This substantially reduces the word error
rate on a broadcast task.
The second contribution is a method to factorise representations for speakers and
environments so that they may be combined in novel combinations. In realistic scenarios,
the speaker or environment transform at test time might be unknown, or there may be
insufficient data to learn a joint transform. We show that in such cases, factorised, or
independent, representations are required to avoid deteriorating performance. Using
i-vectors, we factorise speaker or environment information using multi-condition training
with neural networks. Specifically, we extract bottleneck features from networks trained
to classify either speakers or environments. The resulting factorised representations
prove beneficial when one factor is missing at test time, or when all factors are seen,
but not in the desired combination.
The third contribution is an investigation of model adaptation in a longitudinal
setting. In this scenario, we repeatedly adapt a model to new data, with the constraint
that previous data becomes unavailable. We first demonstrate the effect of such a
constraint, and show that using a cyclical learning rate may help. We then observe
that these successive models lend themselves well to ensembling. Finally, we show
that the impact of this constraint in an active learning setting may be detrimental to
performance, and suggest to combine active learning with semi-supervised training to
avoid biasing the model.
The fourth contribution is a method to adapt low-level features in a parameter-efficient and interpretable manner. We propose to adapt the filters in a neural feature
extractor, known as SincNet. In contrast to traditional techniques that warp the
filterbank frequencies in standard feature extraction, adapting SincNet parameters is
more flexible and more readily optimised, whilst maintaining interpretability. On a task
adapting from adult to child speech, we show that this layer is well suited for adaptation
and is very effective with respect to the small number of adapted parameters
Langages et modèles à objets - état des recherches et perspectives
Langages et modèles à objets donne un aperçu de la diversité des travaux développés autour de la notion d'objet, à un moment où leur incidence est de plus en plus grande dans tous les domaines de l'informatique. l'ouvrage est divisé en quatre parties, centrées autour de thèmes choisis : génie logiciel, concepts avancés, représentation des connaissances et applications. Tous les chapitres (sauf un) ont été spécialement conçus pour l'occasion et leurs auteurs ont été choisis parmi les meilleurs spécialistes français. l'ouvrage peut être lu sans la connaissance préalable d'un langage particulier et ne donne pas une connaissance approfondie d'un langage ou de son histoire. l'accent est plutôt mis sur les grands principes des thèmes traités, qui sont présentés en détail et caractérisés les uns par rapport aux autres
Cadre conceptuel pour la composition des objets et la spécification du comportement
Thèse numérisée par la Direction des bibliothèques de l'Université de Montréal