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234 results on '"Cassidy, M. J."'

1. Models for Chronology Selection

Author

Cassidy, M. J. and Hawking, S. W.

Subjects
High Energy Physics - Theory
Abstract

In this paper, we derive an expression for the grand canonical partition function for a fluid of hot, rotating massless scalar field particles in the Einstein universe. We consider the number of states with a given energy as one increases the angular momentum so that the fluid rotates with an increasing angular velocity. We find that at the critical value when the velocity of the particles furthest from the origin reaches the speed of light, the number of states tends to zero. We illustrate how one can also interpret this partition function as the effective action for a boosted scalar field configuration in the product of three dimensional de Sitter space and $S^1$. In this case, we consider the number of states with a fixed linear momentum around the $S^1$ as the particles are given more and more boost momentum. At the critical point when the spacetime is about to develop closed timelike curves, the number of states again tends to zero. Thus it seems that quantum mechanics naturally enforces the chronology protection conjecture by superselecting the causality violating field configurations from the quantum mechanical phase space., Comment: 20 pages, Latex

Published
1997
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2. Divergences in the Effective Action for Acausal Spacetimes

Author

Cassidy, M. J.

Subjects
General Relativity and Quantum Cosmology
Abstract

The 1--loop effective Lagrangian for a massive scalar field on an arbitrary causality violating spacetime is calculated using the methods of Euclidean quantum field theory in curved spacetime. Fields of spin 1/2, spin 1 and twisted field configurations are also considered. In general, we find that the Lagrangian diverges to minus infinity at each of the nth polarised hypersurfaces of the spacetime with a structure governed by a DeWitt-Schwinger type expansion., Comment: 17 pages, Latex

Published
1997
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3. Nonlinearity in Quantum Theory and Closed Timelike Curves

Author

Cassidy, M. J.

Subjects
General Relativity and Quantum Cosmology, High Energy Physics - Theory
Abstract

We examine consequences of the density matrix approach to quantum theory in the context of a model spacetime containing closed timelike curves and find that in general, an initially pure state will evolve in a nonlinear way to a mixed quantum state. CPT invariance and the implications of this nonlinearity for the statistical interpretation of quantum theory are discussed., Comment: 17 pages plain TeX, DAMTP R/94/34

Published
1994
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4. Quantum field theory and acausal spacetimes

Author

Cassidy, M. J.

Subjects
520
Abstract

This thesis is concerned with various problems that arise when one attempts to quantise fields on spacetimes which contain regions of closed time-like curves (CTCs). Chapter 1 presents a review of some of the major classical and quantum difficulties encountered by various authors investigating acausal spacetimes until 1993. In chapter 2, a number of more recent proposals that have been put forward in an attempt to construct a consistent dynamics are considered. Particular consideration is given to the density matrix proposal for Deutsch and Politzer, which is ultimately shown to give a nonlinear evolution through the acausal region. This work has been published in Phys. Rev. D 52, 5676 (1995). The chapter concludes with a discussion of the Euclidean approach, which is adopted by this author for much of the remainder of the thesis. It is argued that the inevitable loss of quantum coherence is physically preferable to the nonlinear evolution encountered in the other proposals. Chapter 3 is concerned with the effective action for matter fields defined on acausal spacetimes, in which an expression is derived for the 1 loop effective action for fields for arbitrary mass and spin on Euclidean spaces which have acausal analytic continuations. I will show that in general, after one has analytically continued back to the acausal section, the effective action diverges to minus affinity at each of the nth polarised hypersurfaces of the spacetime with a structure governed by a de Witt-Schwinger type expansion. In particular, I will show that even if →Tμνπ remains finite at the Cauchy horizon, the effective action will still diverge there. This work has been published in Classical and Quantum Gravity 14, 3031 (1997). In chapter 4 I attempt to give a sensible interpretation to Euclidean path integrals in the presence of causality violations, which was work carried out in collaboration with S.W. Hawking. If the action always diverges to minus infinity, then at first sight it seems as if any Euclidean path integrals will be completely ill defined. However, I will argue that if one focuses on the density of states, one can obtain physically reasonable results. Ultimately, I will derive an expression for the effective action of a boosted scalar field configuration in the product of three dimensional de Sitter space and S1, and I consider the number of states with a fixed linear momentum around the S1 as the particles are given more and more boost momentum. I will show that at the critical point when the spacetime is about to develop closed timelike curves, the number of states tends to zero. This suggest that quantum mechanics naturally enforces the Chronology Protection Conjecture by superselecting the causality violating field configurations from the quantum mechanical phase space. This work has been accepted for publication in Phys. Rev. D. Finally, in chapter 5 I consider what happens when one includes the effects of back reaction in a spacetime containing CTCs. Misner space is considered as an axisymmetric Bianchi I universe and it is assumed that near the horizon, the periodic identification in the x direction provides the dominant contribution to the energy-momentum tensor. I will show that, as the horizon is approached, any small perturbation grows until a singularity forms in a finite proper time, which therefore lends further support to the Chronology Protection Conjecture. This work has been submitted for publication to Classical and Quantum Gravity and is the subject of DAMTP preprint R/98/01.

Published
1998

5. Causes And Effects Of Phase Transitions In Highway Traffic

Author

Daganzo, C. F., Cassidy, M. J., and Bertini, R. L.

Subjects
Traffic flow--Mathematical models, Traffic congestion--Mathematical models, Queuing theory
Abstract

It is shown that all the phase transitions in and out of freely flowing traffic reported earlier for a German site could be caused by bottlenecks, as are all the transitions observed at two other sites examined here. Furthermore, all the evidence indicates that bottlenecks cause these transitions in a predictable way, and no evidence is found that stoppages (jams) appear spontaneously in free flow traffic for no apparent reason. The most salient phenomena observed at all locations are explained in terms of a simple theory specific to traffic.

Published
1997

8. Optimization of perforation drilling for mitigating punch-through in multi-layered clays

Author

Hossain, M. S., Cassidy, M. J., Baker, R., and Randolph, M. F.

Subjects
Clay minerals -- Research, Oil well drilling rigs -- Analysis, Earth sciences
Abstract

'Spudcan' foundations for mobile drilling rigs continue to exhibit a high failure rate in the offshore oil and gas industry. The more frequent use of larger jack-ups in highly stratified regions, such as the Sunda Shelf in Southeast Asia, contributes to this concerning increase in 'punch-through' incidents, which can lead to buckling of a leg or even toppling of the rig. An industry practice known as 'perforation drilling' is sometimes used to mitigate the punch-through risk in layered clays, extracting soil from the upper strong layer before the jack-up is installed. This paper reports results from centrifuge model tests exploring the efficiency of perforation drilling. The soil conditions tested simulate offshore strength profiles that have reported punch-through failures. An experimental method for 'drilling' sites in an enhanced gravity centrifuge environment was developed and the installation responses of model spudcan foundations penetrating through multi-layered clays with interbedded stiff layers were recorded. The experimental results show that the removal of soil inside the spudcan perimeter, with an area of 9% perforated, eliminated rapid leg run and severe punch-through on the two- and four-layer seabed profiles tested. This confirms the effectiveness of perforation drilling and indicates how the offshore drilling plan may be optimized. Key words: spudcan foundations, stratified clays, punch-through mitigation, perforation drilling, centrifuge tests, bearing capacity. Les fondations sur caissons de support des plates-formes de forage mobiles presentent un haut taux de rupture dans l'industrie petroliere et gaziere en mer. L'utilisation plus frequente de plates-formes autoelevatrices dans les regions tres stratifiees, comme la croute Sunda en Asie du sud-est, contribue a cette augmentation inquietante d'incidents de type ≪ perforation ≫, qui peuvent entrainer une flexion d'une patte ou meme le renversement de la plate-forme. Une pratique industrielle nommee ≪ forage de perforation ≫ est parfois utilisee pour prevenir les risques de perforation dans l'argile stratifiee. Cette pratique consiste a extraire le sol de la couche superieure resistante avant que la plate-forme autoelevatrice soit installee. Cet article presente des resultats d'essais modelise par centrifuge permettant d'explorer l'efficacite du forage de perforation. Les conditions de sol testees simulent les profils de resistance en mer qui ont engendre des ruptures par perforation. Une methode experimentale pour forer des sites dans un environnement a forte gravite de la centrifuge a ete developpee, et les comportements lies a l' installation de modeles de fondations sur caisson de support penetrant un sol argileux multicouches avec des couches rigides intercalees ont ete enregistres. Les resultats experimentaux demontrent que le fait de retirer le sol a l'interieur du perimetre du caisson de support, avec une aire perforee de 9 %, elimine l'enfoncement rapide de la patte et la perforation severe sur les profils de fonds marins testes a deux et quatre couches. Ceci confirme l'efficacite du forage de perforation et demontre comment le plan de forage en mer peut etre optimise. Mots-cles : fondations sur caisson de support, argiles stratifiees, prevention de la perforation, forage de perforation, essais en centrifuge, capacite portante. [Traduit par la Redaction], Introduction The world's appetite for energy is driving offshore explorations into new development regions and deeper waters. The manufacturers of mobile 'jack-up' drilling platforms are building more robust and heavier [...]

Published
2011
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23. Non-linear analysis of jack-up structures subjected to random waves

Author

Cassidy, M, Cassidy, M. J., Houlsby, G, and Eatock Taylor, R

Subjects
Geotechnical engineering, Offshore foundations, Engineering & allied sciences, Structural dynamics, Civil engineering
Abstract

There is a steadily increasing demand for the use of jack-up units in deeper water and harsher environments. Confidence in their use in these environments requires jack-up analysis techniques to reflect accurately the physical processes occurring. This thesis is concerned with the models appropriate for the dynamic assessment of jack-ups, an important issue in long-term reliability considerations. The motivation is to achieve a balanced approach in considering the non-linearities in the structure, foundations and wave loading. A work hardening plasticity model is outlined for the combined vertical, moment and horizontal loading of spudcan footings on dense sand. Empirical expressions for the yield surface in combined load space and a flow rule for prediction of footing displacements during yield are given. Theoretical lower bound bearing capacity factors for conical footings in sand have been derived and are used in a strain-hardening law to define the variation in size of the yield surface with the plastic component of vertical penetration. The complete incremental numerical model has been implemented into a plane frame analysis program named JAKUP. The spectral content of wave loading is considered using NewWave theory, and the importance of random wave histories shown by constraining the deterministic NewWave into a completely random surface elevation. Using this technique, a method for determining short-term extreme response statistics for a sea-state is demonstrated. A numerical experiment on an example jack-up and central North Sea location is shown to emphasise the difference in long-term extreme response according to various footing assumptions. The role of sea-state severity in the variation of short-term extreme response statistics is also highlighted. Finally, probabilistic methods are used to develop further understanding of the response behaviour of jack-ups. A sensitivity study of influential variables (with probabilistic formulations as opposed to deterministic values) has been conducted using the response surface methodology.

Published
2016

29. Probabilistic identification of soil stratification

Author

Li, J., Cassidy, M. J., Huang, Jinsong, Zhang, Li Min, Kelly, Richard B., Li, J., Cassidy, M. J., Huang, Jinsong, Zhang, Li Min, and Kelly, Richard B.

Abstract

Identification of soil stratification is vital to geotechnical structural design and construction where the soil layer, soil type and properties are necessary inputs. Although methods are available for classifying the soil profiling using measured cone penetration test (CPT) data, the identification of soil stratification at unsampled locations is still difficult due to significant variability of natural soil. The identification is further complicated by the considerable uncertainties in the CPT measurements and soil classification methods. This study aims to develop a probabilistic method to predict soil stratification at unsampled locations by explicitly filtering the uncertainties in soil classification systems. An established Kriging interpolation technique is used to estimate the CPT parameters which are further interpreted to identify the soil stratification. Equations are derived to quantify the degree of uncertainties reduced by this method. The approaches are illustrated using a database of 26 CPT tests recently sourced from a dike near Ballina, Australia. Results show that the majority of the uncertainties in the soil parameters are screened by a soil classification index. The remaining uncertainties are further filtered by the soil classification systems. A clear stratification with a high degree of confidence is obtained in both horizontal plane and vertical unsampled locations, which shows excellent agreement with the existing CPT tests. This study provides a methodology to clearly identify the soil strata and reduce the uncertainties in prediction of design properties, paving the way for a more cost-effective geotechnical design.

Published
2016

40. Advancing Australia's facilities for physical modelling in geotechnics

Author

Cassidy, M J, Gaudin, C, Bates, L, Indraratna, B, Nimbalkar, S, O'Loughlin, C D, White, D J, Sloan, S W, Cassidy, M J, Gaudin, C, Bates, L, Indraratna, B, Nimbalkar, S, O'Loughlin, C D, White, D J, and Sloan, S W

Abstract

This paper presents details of the advancements of the Australian Research Council Centre of Excellence for Geotechnical Science and Engineering to the apparatus, facilities and methods for physical modelling in geotechnics. This advancement includes (i) the launch of a National Geotechnical Centrifuge Facility with a new 10 m diameter fixed beam centrifuge that will be capable of spinning 2.4 tonnes of soil at 100 gravities, (ii) a new mobile soft soil in situ testing laboratory, (iii) a new national facility for the cyclic testing of high-speed rail and (iv) three recirculating flumes, called O-tubes, which are presented in another paper of this special issue. This paper provides an overview of this new equipment and the aims of the research that it will underpin. The equipment will provide enhanced possibilities for Australia to conduct project specific testing for future energy and transportation infrastructure developments, nationally and internationally.

Published
2014

44. Proactive detection of high collision concentration locations on highways

Author

Cassidy, M J, Skabardonis, A, Chung, Koohong, Jang, Kitae, Madanat, Samer, Washington, Simon, Cassidy, M J, Skabardonis, A, Chung, Koohong, Jang, Kitae, Madanat, Samer, and Washington, Simon

Abstract

In previous research (Chung et al., 2009), the potential of the continuous risk profile (CRP) to proactively detect the systematic deterioration of freeway safety levels was presented. In this paper, this potential is investigated further, and an algorithm is proposed for proactively detecting sites where the collision rate is not sufficiently high to be classified as a high collision concentration location but where a systematic deterioration of safety level is observed. The approach proposed compares the weighted CRP across different years and uses the cumulative sum (CUSUM) algorithm to detect the sites where changes in collision rate are observed. The CRPs of the detected sites are then compared for reproducibility. When high reproducibility is observed, a growth factor is used for sequential hypothesis testing to determine if the collision profiles are increasing over time. Findings from applying the proposed method using empirical data are documented in the paper together with a detailed description of the method.

Published
2011

50. Reply

Author

Porter, C., primary, Stavroulopoulos, A., additional, Roe, S. D., additional, Pointon, K., additional, and Cassidy, M. J. D., additional

Published
2007
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