DSpace Collection:
http://hdl.handle.net/2381/4819
2018-02-23T06:13:35Z
2018-02-23T06:13:35Z
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, Sephen J.
Cooper, A. J.
Ozkan, M.
Thomas, P. J.
http://hdl.handle.net/2381/37570
2016-05-18T02:08:20Z
2016-05-17T09:01:51Z
Title: Towards roughness-based drag reduction in cross-flow dominated flows
Authors: Garrett, Sephen 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.
Description: This paper is under embargo as it has been submitted for publication to European Journal of Mechanics - B/Fluids. If accepted the file associated with this record is embargoed until 24 months after the date of publication.
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