DSpace Collection:
http://hdl.handle.net/2381/4819
2019-07-23T09:09:55Z
2019-07-23T09:09:55Z
Comparison of CdZnTe neutron detector models using MCNP6 and Geant4
Wilson, Emma
Anderson, Mike
Prendergasty, David
Cheneler, David
http://hdl.handle.net/2381/44398
2019-06-15T02:09:08Z
2019-06-14T11:11:20Z
Title: Comparison of CdZnTe neutron detector models using MCNP6 and Geant4
Authors: Wilson, Emma; Anderson, Mike; Prendergasty, David; Cheneler, David
Abstract: The production of accurate detector models is of high importance in the development and use of detectors. Initially, MCNP and Geant were developed to specialise in neutral particle models and accelerator models, respectively; there is now a greater overlap of the capabilities of both, and it is therefore useful to produce comparative models to evaluate detector characteristics. In a collaboration between Lancaster University, UK, and Innovative Physics Ltd., UK, models have been developed in both MCNP6 and Geant4 of Cadmium Zinc Telluride (CdZnTe) detectors developed by Innovative Physics Ltd. Herein, a comparison is made of the relative strengths of MCNP6 and Geant4 for modelling neutron flux and secondary γ-ray emission. Given the increasing overlap of the modelling capabilities of MCNP6 and Geant4, it is worthwhile to comment on differences in results for simulations which have similarities in terms of geometries and source configurations.
2019-06-14T11:11:20Z
Convergence of multilevel stationary Gaussian convolution
Hubbert, Simon
Levesley, Jeremy
http://hdl.handle.net/2381/44369
2019-06-13T02:11:58Z
2019-06-12T09:32:27Z
Title: Convergence of multilevel stationary Gaussian convolution
Authors: Hubbert, Simon; Levesley, Jeremy
Abstract: In this paper we give a short note showing convergence rates for periodic approximation of smooth functions by multilevel Gaussian convolution. We will use the Gaussian scaling in the convolution at the finest level as a proxy for degrees of freedom d in the model. We will show that, for functions in the native space of the Gaussian, convergence is of the order (Formula Presented.). This paper provides a baseline for what should be expected in discrete convolution, which will be the subject of a follow up paper.
Description: The file associated with this record is under embargo until 12 months after publication, in accordance with the publisher's self-archiving policy. The full text may be available through the publisher links provided above.
2019-06-12T09:32:27Z
The five factor model of personality and evaluation of drug consumption risk
Fehrman, Elaine
Muhammad, Awaz K.
Mirkes, Evgeny M.
Egan, Vincent
Gorban, Alexander N.
http://hdl.handle.net/2381/42588
2019-01-11T12:28:51Z
2018-07-26T11:17:48Z
Title: The five factor model of personality and evaluation of drug consumption risk
Authors: Fehrman, Elaine; Muhammad, Awaz K.; Mirkes, Evgeny M.; Egan, Vincent; Gorban, Alexander N.
Abstract: The problem of evaluating an individual’s risk of drug consumption and misuse is highly important and novel. An online survey methodology was employed to collect data including personality traits (NEO-FFI-R), impulsivity (BIS-11), sensation seeking (ImpSS), and demographic information. The data set contained information on the consumption of 18 central nervous system psychoactive drugs. Correlation analysis using a relative information gain model demonstrates the existence of a group of drugs (amphetamines, cannabis, cocaine, ecstasy, legal highs, LSD, and magic mushrooms) with strongly correlated consumption. An exhaustive search was performed to select the most effective subset of input features and data mining methods to classify users and non-users for each drug. A number of classification methods were employed (decision tree, random forest, k-nearest neighbours, linear discriminant analysis, Gaussian mixture, probability density function estimation, logistic regression, and naïve Bayes) and the most effective method selected for each drug. The quality of classification was surprisingly high. The best results with sensitivity and specificity being greater than 75% were achieved for cannabis, crack, ecstasy, legal highs, LSD, and volatile substance abuse. Sensitivity and specificity greater than 70% were achieved for amphetamines, amyl nitrite, benzodiazepines, chocolate, caffeine, heroin, ketamine, methadone, and nicotine. The poorest result was obtained for prediction of alcohol consumption.
Description: The file associated with this record is under embargo while permission to archive is sought from the publisher. The full text may be available through the publisher links provided above.
2018-07-26T11:17:48Z
Generation of explicit knowledge from empirical data through pruning of trainable neural networks
Gorban, Alexander N.
Mirkes, Eugeniy M.
Tsaregorodtsev, Victor G.
http://hdl.handle.net/2381/42416
2018-06-07T02:27:59Z
2018-06-06T09:08:35Z
Title: Generation of explicit knowledge from empirical data through pruning of trainable neural networks
Authors: Gorban, Alexander N.; Mirkes, Eugeniy M.; Tsaregorodtsev, Victor G.
Abstract: This paper presents a generalized technology of extraction of explicit knowledge from data. The main ideas are 1) maximal reduction of network complexity (not only removal of neurons or synapses, but removal all the unnecessary elements and signals and reduction of the complexity of elements), 2) using of adjustable and flexible pruning process (the pruning sequence shouldn't be predetermined - the user should have a possibility to prune network on his own way in order to achieve a desired network structure for the purpose of extraction of rules of desired type and form), and 3) extraction of rules not in predetermined but any desired form. Some considerations and notes about network architecture and training process and applicability of currently developed pruning techniques and rule extraction algorithms are discussed. This technology, being developed by us for more than 10 years, allowed us to create dozens of knowledge-based expert systems.
2018-06-06T09:08:35Z
Robust principal graphs for data approximation
Gorban, Alexander N.
Mirkes, Evgeny M.
Zinovyev, Andrei
http://hdl.handle.net/2381/42288
2018-05-26T02:39:18Z
2018-05-25T10:46:32Z
Title: Robust principal graphs for data approximation
Authors: Gorban, Alexander N.; Mirkes, Evgeny M.; Zinovyev, Andrei
Abstract: Revealing hidden geometry and topology in noisy data sets is a challenging task. Elastic principal graph is a computationally efficient and flexible data approximator based on embedding a graph into the data space and minimizing the energy functional penalizing the deviation of graph nodes both from data points and from pluri-harmonic configuration (generalization of linearity). The structure of principal graph is learned from data by application of a topological grammar which in the simplest case leads to the construction of principal curves or trees. In order to more efficiently cope with noise and outliers, here we suggest using a trimmed data approximation term to increase the robustness of the method. The modification of the method that we suggest does not affect either computational efficiency or general convergence properties of the original elastic graph method. The trimmed elastic energy functional remains a Lyapunov function for the optimization algorithm. On several examples of complex data distributions we demonstrate how the robust principal graphs learn the global data structure and show the advantage of using the trimmed data approximation term for the construction of principal graphs and other popular data approximators.
Description: The Parameters of the methods used are
provided together with the code from the corresponding GitHub https://github.com/auranic/Elastic-principal-graphs/wiki/Robust-principalgraphs
2018-05-25T10:46:32Z
Performance of Analog Nonlinear Filtering for Impulsive Noise Mitigation in OFDM-based PLC Systems
Barazideh, Reza
Natarajan, Balasubramaniam
Nikitin, Alexei V.
Davidchack, Ruslan L.
http://hdl.handle.net/2381/42280
2018-05-26T02:39:16Z
2018-05-25T09:11:41Z
Title: Performance of Analog Nonlinear Filtering for Impulsive Noise Mitigation in OFDM-based PLC Systems
Authors: Barazideh, Reza; Natarajan, Balasubramaniam; Nikitin, Alexei V.; Davidchack, Ruslan L.
Abstract: Asynchronous and cyclostationary impulsive noise can severely impact the bit-error-rate (BER) of OFDM-based powerline communication systems. In this paper, we analyze an adaptive nonlinear analog front end filter that mitigates various types of impulsive noise without detrimental effects such as self-interference and out-of-band power leakage caused by other nonlinear approaches like clipping and blanking. Our proposed Adaptive Nonlinear Differential Limiter (ANDL) is constructed from a linear analog filter by applying a feedback-based nonlinearity, controlled by a single resolution parameter. We present a simple practical method to find the value of this resolution parameter that ensures the mitigation of impulsive without impacting the desired OFDM signal. Unlike many prior approaches for impulsive noise mitigation that assume a statistical noise model, ANDL is blind to the exact nature of the noise distribution, and is designed to be fully compatible with existing linear front end filters. We demonstrate the potency of ANDL by simulating the OFDM-based narrowband PLC compliant with the IEEE standards. We show that the proposed ANDL outperforms other approaches in reducing the BER in impulsive noise environments.
2018-05-25T09:11:41Z
On Boiling of Crude Oil under Elevated Pressure
Pimenova, Anastasiya V.
Goldobin, Denis S.
http://hdl.handle.net/2381/42267
2018-05-25T02:26:56Z
2018-05-24T10:15:44Z
Title: On Boiling of Crude Oil under Elevated Pressure
Authors: Pimenova, Anastasiya V.; Goldobin, Denis S.
Abstract: We construct a thermodynamic model for theoretical calculation of the boiling process of multicomponent mixtures of hydrocarbons (e.g., crude oil). The model governs kinetics of the mixture composition in the course of the distillation process along with the boiling temperature increase. The model heavily relies on the theory of dilute solutions of gases in liquids. Importantly, our results are applicable for modelling the process under elevated pressure (while the empiric models for oil cracking are not scalable to the case of extreme pressure), such as in an oil field heated by lava intrusions.
2018-05-24T10:15:44Z
Coherence of Noisy Oscillators with Delayed Feedback Inducing Multistability
Pimenova, Anastasiya V.
Goldobin, Denis S.
http://hdl.handle.net/2381/42266
2018-05-25T02:26:55Z
2018-05-24T09:29:06Z
Title: Coherence of Noisy Oscillators with Delayed Feedback Inducing Multistability
Authors: Pimenova, Anastasiya V.; Goldobin, Denis S.
Abstract: For self-sustained oscillators subject to noise the coherence, understood as a constancy of the instantaneous oscillation frequency, is one of the primary characteristics. The delayed feedback has been previously revealed to be an efficient tool for controlling coherence of noise-driven self-sustained oscillator. The effect of the delayed feedback control on coherence is stronger for a longer delay time. Meanwhile, the instantaneous frequency of a noise-free oscillator can exhibit multistability for long delay time. The impact of the delay-feedback--induced multistability on the oscillation coherence, measured by the phase diffusion constant, of a noisy oscillator is studied in this work both numerically and analytically.
2018-05-24T09:29:06Z
Conway’s groupoid and its relatives
Gill, Nick
Gillespie, Neil I.
Praeger, Cheryl E.
Semeraro, Jason
http://hdl.handle.net/2381/42241
2018-05-22T02:25:14Z
2018-05-21T14:48:26Z
Title: Conway’s groupoid and its relatives
Authors: Gill, Nick; Gillespie, Neil I.; Praeger, Cheryl E.; Semeraro, Jason
Abstract: In 1997, John Conway constructed a 6-fold transitive subset M13
of permutations on a set of size 13 for which the subset fixing any given
point was isomorphic to the Mathieu group M12. The construction was via a
“moving-counter puzzle” on the projective plane PG(2, 3). We discuss consequences
and generalisations of Conway’s construction. In particular we explore
how various designs and hypergraphs can be used instead of PG(2, 3) to obtain
interesting analogues of M13; we refer to these analogues as Conway groupoids.
A number of open questions are presented.
Description: The file associated with this record is under embargo while permission to archive is sought from the publisher. The full text may be available through the publisher links provided above.
2018-05-21T14:48:26Z
The Five Factor Model of personality and evaluation of drug consumption risk
Ferhman, Elaine
Muhammad, Awaz K.
Mirkes, Evgeny M.
Egan, Vincent
Gorban, Alexander N.
http://hdl.handle.net/2381/42240
2018-05-22T02:25:58Z
2018-05-21T14:39:53Z
Title: The Five Factor Model of personality and evaluation of drug consumption risk
Authors: Ferhman, Elaine; Muhammad, Awaz K.; Mirkes, Evgeny M.; Egan, Vincent; Gorban, Alexander N.
Abstract: The problem of evaluating an individual’s risk of drug consumption and misuse is highly important and novel. An online survey methodology was employed to collect data including personality traits (NEO-FFI-R), impulsivity (BIS-11), sensation seeking (ImpSS), and demographic information. The data set contained information on the consumption of 18 central nervous system psychoactive drugs. Correlation analysis using a relative information gain model demonstrates the existence of a group of drugs (amphetamines, cannabis, cocaine, ecstasy, legal highs, LSD, and magic mushrooms) with strongly correlated consumption. An exhaustive search was performed to select the most effective subset of input features and data mining methods to classify users and non-users for each drug. A number of classification methods were employed (decision tree, random forest, k-nearest neighbours, linear discriminant analysis, Gaussian mixture, probability density function estimation, logistic regression, and naïve Bayes) and the most effective method selected for each drug. The quality of classification was surprisingly high. The best results with sensitivity and specificity being greater than 75% were achieved for cannabis, crack, ecstasy, legal highs, LSD, and volatile substance abuse. Sensitivity and specificity greater than 70% were achieved for amphetamines, amyl nitrite, benzodiazepines, chocolate, caffeine, heroin, ketamine, methadone, and nicotine. The poorest result was obtained for prediction of alcohol consumption.
Description: The file associated with this record is under embargo while permission to archive is sought from the publisher. The full text may be available through the publisher links provided above.
2018-05-21T14:39:53Z
The Lusternik-Schnirelmann Category for a Differentiable Stack
Alsulami, Samirah
Colman, Hellen
Neumann, Frank
http://hdl.handle.net/2381/40216
2018-02-07T02:45:09Z
2017-08-22T12:17:53Z
Title: The Lusternik-Schnirelmann Category for a Differentiable Stack
Authors: Alsulami, Samirah; Colman, Hellen; Neumann, Frank
Abstract: We introduce the notion of Lusternik-Schnirelmann category for differentiable stacks and establish its relation with the groupoid Lusternik-Schnirelmann category for Lie groupoids. This extends the notion of Lusternik-Schnirelmann category for smooth manifolds and orbifolds.
2017-08-22T12:17:53Z
Towards roughness-based drag reduction in cross-flow dominated flows
Garrett, Stephen J.
Cooper, A. J.
Ozkan, M.
Thomas, P. J.
http://hdl.handle.net/2381/37570
2019-07-03T13:58:38Z
2016-05-17T09:01:51Z
Title: Towards roughness-based drag reduction in cross-flow dominated flows
Authors: Garrett, Stephen J.; Cooper, A. J.; Ozkan, M.; Thomas, P. J.
Abstract: Recent theoretical results are presented from our ongoing study investigating the distinct convective instability properties of the boundary-layer flow over rough rotating disks. In this study, radial anisotropic surface roughness (concentric grooves) is modelled using the partial-slip approach of Miklavčič & Wang (2004) and the surface-geometry approach of Yoon et. Al (2007). An energy analysis reveals that for both instability modes, the main contributors to the energy balance are the energy production by the Reynolds stresses and conventional viscous dissipation. For the Type I mode, energy dissipation increases and the Reynolds-stress energy production decreases with roughness under both models. This suggests a clear stabilising effect of the anisotropic roughness on the Type I mode. For the Type II mode, the Reynolds-stress energy production increases with roughness under both models. However, the energy dissipation of the Type II mode decreases with the roughness under the surface-geometry model and increases under the partial-slip model. This sensitivity to the precise form of the anisotropic roughness suggests that maximising dissipation by an appropriately designed roughness can theoretically lead to an overall beneficial stabilisation of both the Type I and Type II modes. This is a potential route to overall boundary-layer-transition delay and drag reduction in cross-flow dominated flows.
2016-05-17T09:01:51Z
An energy analysis of convective instabilities of the Bödewadt and Ekman boundary layers over rough surfaces
Alveroglu, B.
Segalini, A.
Garrett, Stephen J.
http://hdl.handle.net/2381/37569
2017-10-03T01:45:08Z
2016-05-17T08:48:49Z
Title: An energy analysis of convective instabilities of the Bödewadt and Ekman boundary layers over rough surfaces
Authors: Alveroglu, B.; Segalini, A.; Garrett, Stephen J.
Abstract: An energy balance equation for the three-dimensional Bödewadt and Ekman layers of the so called “BEK family" of rotating boundary-layer flows is derived. A Chebyshev discretisation method is used to solve the equations and investigate the effect of surface roughness on the physical mechanisms of transition. All roughness types lead to a stabilization of the Type I (cross-flow) instability mode for both flows, with the exception of azimuthally-anisotropic roughness (radial grooves) within the Bödewadt layer which is destabilising. In the case of the viscous Type II instability mode, the results predict a destabilisation effect of radially-anisotropic roughness (concentric grooves) on both flows, whereas both azimuthally-anisotropic roughness and isotropic roughness have a stabilisation effect. The results presented here confirm the results of our prior linear stability analyses.
Description: The file associated with this record is under embargo until 12 months after publication, in accordance with the publisher's self-archiving policy. The full text may be available through the publisher links provided above.
2016-05-17T08:48:49Z
Stability modes in vortex structure formation: Canonical examples for rotating components
Gostelow, J. Paul
Garrett, Stephen J.
Rona, Aldo
Adebayo, David S.
http://hdl.handle.net/2381/37476
2016-05-05T02:09:18Z
2016-05-04T14:50:44Z
Title: Stability modes in vortex structure formation: Canonical examples for rotating components
Authors: Gostelow, J. Paul; Garrett, Stephen J.; Rona, Aldo; Adebayo, David S.
Abstract: Three rather different physical cases have been studied. All represent very practical geometries for which the modal behavior of vortex structures is not completely understood. The work on these problems is ongoing with the objective of obtaining physical confirmation, enhanced understanding and predictive capability for the vortex structures encountered in rotating machines.
2016-05-04T14:50:44Z
Bayesian Learning of Material Density Function by Multiple Sequential Inversions of 2-D Images in Electron Microscopy
Chakrabarty, Dalia
Paul, S.
http://hdl.handle.net/2381/31574
2015-10-30T14:59:37Z
2015-02-04T09:51:38Z
Title: Bayesian Learning of Material Density Function by Multiple Sequential Inversions of 2-D Images in Electron Microscopy
Authors: Chakrabarty, Dalia; Paul, S.
Editors: Polpo de Campos, A; Neto,; Ramos Rifo,; Stern,; Lauretto,
Abstract: We discuss a novel inverse problem in which the data is generated by the sequential contractive projections
of the convolution of two unknown functions, both of which we aim to learn. The method is illustrated
using an application that relates to the multiple inversions of image data recorded with a Scanning Electron
Microscope, with the aim of learning the density of a given material sample and the microscopy correction
function. Given the severe logistical difficulties in this application of taking multiple images at different
viewing angles, a novel imaging experiment is undertaken, resulting in expansion of information. In lieu of
training data, it is noted that the highly discontinuous material density function cannot be modelled using a
Gaussian Process (GP) as the parametrisation of the required non-stationary covariance function of such a
GP cannot be achieved without training data. Consequently, we resort to estimating values of the unknown
functions at chosen locations in their domain–locations at which an image data are available. Image data
across a range of resolutions leads to multiscale models which we use to estimate material densities from the
micro-metre to nano-metre length scales. We discuss applications of the method in non-destructive learning
of material density using simulated metallurgical image data, as well as perform inhomogeneity detection in
multi-component composite on nano metre scales, by inverting real image data of a brick of nano-particles.
2015-02-04T09:51:38Z
Simple Locally Finite Lie Algebras of Diagonal Type
Baranov, Alexander
http://hdl.handle.net/2381/31502
2015-01-27T14:40:49Z
2015-01-27T14:38:49Z
Title: Simple Locally Finite Lie Algebras of Diagonal Type
Authors: Baranov, Alexander
Abstract: We discuss various characterizations of simple locally finite Lie algebras of diagonal type over an algebraically closed field of characteristic zero.
2015-01-27T14:38:49Z
Polynomial programming approach to weak approximation of Lévy-driven stochastic differential equations with application to option pricing
Kashima, Kenji
Kawai, Reiichiro
http://hdl.handle.net/2381/10216
2012-03-20T02:01:38Z
2012-03-19T16:46:22Z
Title: Polynomial programming approach to weak approximation of Lévy-driven stochastic differential equations with application to option pricing
Authors: Kashima, Kenji; Kawai, Reiichiro
Abstract: We propose an optimization approach to weak approximation of Levy-driven stochastic differential equations. We employ a mathematical programming framework to obtain numerically upper and lower bound estimates of the target expectation, where the optimization procedure ends up with a polynomial programming problem. An advantage of our approach is that all we need is a closed form of the Levy measure, not the exact simulation knowledge of the increments or of a shot noise representation for the time discretization approximation. We present numerical examples of the computation of the moments, as well as the European call option premium, of the Doleacuteans-Dade exponential model.
2012-03-19T16:46:22Z
An optimization approach to weak approximation of Lévy-driven stochastic differential equations with application to option pricing
Kashima, Kenji
Kawai, Reiichiro
http://hdl.handle.net/2381/10189
2012-03-15T02:01:46Z
2012-03-14T11:34:46Z
Title: An optimization approach to weak approximation of Lévy-driven stochastic differential equations with application to option pricing
Authors: Kashima, Kenji; Kawai, Reiichiro
Abstract: We propose an optimization approach to weak
approximation of Lévy-driven stochastic differential equations.
We employ a mathematical programming framework to obtain
numerically upper and lower bound estimates of the target
expectation, where the optimization procedure ends up with
a polynomial programming problem. An advantage of our
approach is that all we need is a closed form of the Lévy
measure, not the exact simulation knowledge of the increments
or of a shot noise representation for the time discretization
approximation. We also investigate methods for approximation
at some different intermediate time points simultaneously.
Description: Personal use of this material is permitted. Permission from IEEE must be obtained for all other users, including reprinting/ republishing this material for advertising or promotional purposes, creating new collective works for resale or redistribution to servers or lists, or reuse of any copyrighted components of this work in other works.
2012-03-14T11:34:46Z
Transition mechanisms within the boundary-layer flow over slender vs. broad rotating cones
Hussain, Z.
Garrett, Stephen J.
Stephen, S.O.
http://hdl.handle.net/2381/8867
2014-02-28T13:26:28Z
2010-12-13T15:20:13Z
Title: Transition mechanisms within the boundary-layer flow over slender vs. broad rotating cones
Authors: Hussain, Z.; Garrett, Stephen J.; Stephen, S.O.
Abstract: We describe progress in classifying the convective instability characteristics of the boundarylayer
flow over the family of rotating cones. Existing experimental and theoretical studies
are discussed which lead to the clear hypothesis of a centrifugal instability mode that dominates
over slender cones and manifests as Görtler-type spiral vortices. Although a formulation
consistent with the classic rotating-disk problem has been successful in predicting the
stability characteristics over broad cones, it is unable to identify such a centrifugal mode
as the half-angle is reduced. An alternative formulation is introduced and an asymptotic
analysis summarized which identifies such a centrifugal mode.
Description: This paper was published as Proceedings of the 40th Fluid Dynamics Conference and Exhibit, 28 June - 1 July 2010, Chicago, Illinois, Paper AIAA-2010-4285. It is also available from http://www.aiaa.org/content.cfm?pageid=2. This paper appears here with the permission of the American Institute of Aeronautics and Astronautics, Inc.
2010-12-13T15:20:13Z
The convective instability of boundary-layer flows over rotating spheroids
Samad, A.
Garrett, S.J.
http://hdl.handle.net/2381/8849
2011-01-12T14:14:44Z
2010-12-08T15:48:56Z
Title: The convective instability of boundary-layer flows over rotating spheroids
Authors: Samad, A.; Garrett, S.J.
Abstract: The continuous development of spinning projectiles and
other industrial applications has led to the need to understand the
laminar boundary-layer flow and subsequent onset of transition
over the general family of rotating spheroids.
We begin by finding the laminar boundary-layer flow over a
general spheroid. In particular, we distinguish between prolate
and oblate spheroids and use an appropriate spheroidal coordinate
system in each case. The laminar-flow equations are established
for each family of spheroid rotating in otherwise still
fluid. An eccentricity parameter e is used to distinguish particular
bodies within the oblate or prolate families. In each case,
setting e = 0 reduces the equations to those already established
by Howarth [2] and Banks [4] for the rotating sphere.
In this preliminary study the laminar-flow equations at each
latitude are solved by extending the original series solutions due
to Howarth and Banks for the rotating sphere. The laminar flows
obtained are consistent with established results for the rotating
sphere as e tends to zero, and tend to the von Kármán [5] solution
for the rotating disk as the latitude is reduced close to the nose.
Analyses of the convective instability are performed on the
rotating prolate family. These extend the linear analyses previously
published by Malik, Lingwood and Garrett & Peake [6–10]
on related geometries. An investigation into the relative importance
of type I (crossflow) and type II (streamline curvature)
modes is also presented. At low latitudes increasing eccentricity
has negligible effects on the stability characteristics of the flow. However as the latitude increases, eccentricity is seen to
lower the upper (type I) branch of the neutral curve, reducing the
region of instability.
Description: This paper was published in Proceedings of FEDSM2009, ASME 2009 Fluids Engineering Summer Meeting, August 2-5, 2009, Vail, Colorado USA. It is available from http://www.asme.org/Publications/ConfProceedings/. Doi: 10.1115/FEDSM2009-78484; Metadata only entry
2010-12-08T15:48:56Z
On the global linear stability of the boundary layer on rotating bodies
Garrett, S.J.
Peake, N.
http://hdl.handle.net/2381/8847
2011-01-12T14:03:11Z
2010-12-08T15:23:09Z
Title: On the global linear stability of the boundary layer on rotating bodies
Authors: Garrett, S.J.; Peake, N.
Abstract: By taking the local approach of working at a fixed Reynolds number (equivalently
at fixed distance from the axis of rotation) and assuming that the
steady flow is spatially uniform, [1] shows that the boundary layer on a rotating
disk is locally absolutely unstable at Reynolds numbers in excess of a
critical value. The value of the critical Reynolds number agrees exceedingly
well with experimentally measured values of the transition Reynolds number,
leading to a clear hypothesis that absolute instability plays a role in turbulent
transition on the disk.
In contrast to this local analysis, [2] solve the linearised Navier–Stokes
equations directly for the rotating disk. When they make the same homogenous
flow approximation as in [1], they recover those results in full. However,
when the spatial inhomogeneity of the boundary layer is included there is no
evidence of an unstable global oscillator in the long-term response.
In order to address this discrepancy between the local results and the
numerical simulations of the full inhomogeneous flow, we consider the linear
global modes of the rotating disk/cone boundary layer.
Description: This paper was published as Advances in Turbulence XI: Proceedings of the 11th EUROMECH European Turbulence Conference, June 25-28, 2007, Porto, Portugal; Palma, J. M. L. M.; Silva Lopes, A. (Eds.), pp. 550-552. It is available from http://www.springer.com/materials/mechanics/book/978-3-540-72603-6; Metadata only entry
2010-12-08T15:23:09Z
Boundary-layer transition on broad cones rotating in an imposed axial flow
Garrett, Stephen J.
Hussain, Z.
Stephen, S. O.
http://hdl.handle.net/2381/8838
2016-04-19T13:44:58Z
2010-12-07T14:22:52Z
Title: Boundary-layer transition on broad cones rotating in an imposed axial flow
Authors: Garrett, Stephen J.; Hussain, Z.; Stephen, S. O.
Abstract: We present stability analyses for the boundary-layer flow over broad cones (half-angle
> 40◦) rotating in imposed axial flows. Preliminary convective instability analyses are
presented that are based on the Orr–Sommerfeld equation for a variety of axial-flow speeds.
The results are discussed in terms of the limited existing experimental data and previous
stability analyses on related bodies. The results of an absolute instability analysis are also
presented which are intended to further those by Garrett & Peake through the use of
a more rigorous steady-flow formulation. Axial flow is seen to delay the onset of both
convective and absolute instabilities.
2010-12-07T14:22:52Z
A Selective Review of Transition Modelling for CFD.
Di Pasquale, Davide
Rona, Aldo
Garrett, Stephen J.
http://hdl.handle.net/2381/7645
2011-01-12T14:10:49Z
2010-03-08T14:21:38Z
Title: A Selective Review of Transition Modelling for CFD.
Authors: Di Pasquale, Davide; Rona, Aldo; Garrett, Stephen J.
Abstract: This paper aims to give an overview of the more widely used approaches to model transition
in Computational Fluid Dynamics (CFD). Eight different methods are reviewed: the
stability theory approach, the low Reynolds number turbulent closure approach, the intermittency
transport method with integral correlations, the intermittency and the vorticity
Reynolds number approach, the laminar fluctuation energy method, the ν/-2- f model, and
the Large Eddy Simulations (LES) and Direct Numerical Simulations (DNS) for transition.
The approaches are compared to one another, highlighting their respective advantages and
drawbacks. From this analysis, a list of desirable features for CFD transition models is
drawn up, against which the eight approaches are scored.
Description: This paper was presented at the 39th AIAA Fluid Dynamics Conference, San Antonio, Texas, June 22-25, 2009. Copyright © 2009 by the American Institute of Aeronautics and Astronautics. It is available from http://www.aiaa.org/content.cfm?pageid=406. It appears here with the permission of the American Institute of Aeronautics and Astronautics Inc.
2010-03-08T14:21:38Z
Moduli stacks of vector bundles and Frobenius morphisms
Neumann, Frank
Stuhler, Ulrich
http://hdl.handle.net/2381/3451
2012-08-06T14:06:05Z
2009-12-08T16:13:12Z
Title: Moduli stacks of vector bundles and Frobenius morphisms
Authors: Neumann, Frank; Stuhler, Ulrich
2009-12-08T16:13:12Z
Non-uniform small-gain theorems for systems with unstable invariant sets
Tyukin, Ivan Yu.
Steur, Erik
Nijmeijer, Henk
van Leeuwen, Cees
http://hdl.handle.net/2381/4820
2016-03-21T09:36:49Z
2009-12-01T14:44:31Z
Title: Non-uniform small-gain theorems for systems with unstable invariant sets
Authors: Tyukin, Ivan Yu.; Steur, Erik; Nijmeijer, Henk; van Leeuwen, Cees
Abstract: We consider the problem of small-gain analysis of asymptotic behavior in interconnected nonlinear dynamic systems. Mathematical models of these systems are allowed to be uncertain and time-varying. In contrast to standard small-gain theorems that require global asymptotic stability of each interacting component in the absence of inputs, we consider interconnections of systems that can be critically stable and have infinite input-output Linfin gains. For this class of systems we derive small-gain conditions specifying state boundedness of the interconnection. The estimates of the domain in which the systempsilas state remains are also provided. Conditions that follow from the main results of our paper are non-uniform in space. That is they hold generally only for a set of initial conditions in the systempsilas state space. We show that under some mild continuity restrictions this set has a non-zero volume, hence such bounded yet potentially globally unstable motions are realizable with a non-zero probability. Proposed results can be used for the design and analysis of intermittent, itinerant and meta-stable dynamics which is the case in the domains of control of chemical kinetics, biological and complex physical systems, and non-linear optimization.
2009-12-01T14:44:31Z