No approximate complex fermion coherent states

Whereas boson coherent states with complex parametrization provide an elegant, and intuitive representation, there is no counterpart for fermions using complex parametrization. However, a complex parametrization provides a valuable way to describe amplitude and phase of a coherent beam. Thus we pose the question of whether a fermionic beam can be described, even approximately, by a complex-parametrized coherent state and define, in a natural way, approximate complex-parametrized fermion coherent states. Then we identify four appealing properties of boson coherent states (eigenstate of annihilation operator, displaced vacuum state, preservation of product states under linear coupling, and factorization of correlators) and show that these approximate complex fermion coherent states fail all four criteria. The inapplicability of complex parametrization supports the use of Grassman algebras as an appropriate alternative.Comment: Argumentation made cleare

Fluid pressure penetration for advanced FEA of metal-to-metal seals

This numerical study investigates the behaviour of the contact faces in the metal-to-metal seal of a typical pressure relief valve in the commercial FE-package ANSYS. The valve geometry is simplified to an axisymmetric problem, which comprises a simple representative geometry consisting of only three components. A cylindrical nozzle, which has a valve seat on top, contacts with a disk, which is preloaded by a compressed linear spring. Analysis considerations include the effects of the Fluid Pressure Penetration (FPP) across the valve seat which exists at two different scales. In-service observations show that there is certain limited fluid leakage through the valve seat at operational pressures about 90% of the set pressure, which is caused by the fluid penetrating into surface asperities at the microscale. At the macroscale, non-linear FE-analysis using the FPP technique available in ANSYS revealed that there is also a limited amount of fluid penetrating into gap, which is caused primarily by the global plastic deformation of the valve seat. Accurate prediction of the fluid pressure profile over the valve seat is addressed in this study by considering the FPP interaction on both scales. The shape of this pressure profile introduces an additional component of the spring force, which needs to be considered to provide a reliable sealing

Discussion of “Learning from Failure of a Long Curved Veneer Wall: Structural Analysis and Repair” by Paulo B. Lourenço and Pedro Medeiros

The authors, in an interesting and valuable paper, describe the failure of the brick-masonry veneer façade of a multiuse public hall in Gondomar, Portugal. Damage occurred within 2 years of construction. The veneer was a single leaf of continuous brick masonry tied to a RC structural wall, forming a cavity of nominal width 0.07 m that was partly filled with foamed polyurethane. The veneer wall was 242 m in length and 15 m in height, without movement joints, and extended around most of the elliptical perimeter of the building with portions facing north, east, and south. From their site investigations and technical analysis, the authors attributed the failure primarily to effects of “the irreversible expansion of clay brick,” apparent both from cracking and from extensive out-of-plane deformation of the wall, which had widened the cavity to as much as 0.13 m. Effects were greater on parts of the wall facing south. The failure analysis made use of a power law proposed by Wilson et al. (2003) to describe how expansive strain develops in fired-clay ceramics with time. Here, the discussers comment on recent fundamental work on moisture expansion in brick, and in particular on its temperature dependence, matters of direct relevance to the paper under discussion. The discussers’ comments support and extend the conclusions of the authors, with which the discussers broadly agree. Irreversible moisture expansion occurs as a result of slow chemical reactions between components of the fired-clay ceramic and environmental moisture (Hamilton and Hall 2012). The magnitude of the expansion varies strongly with brick mineralogy and kiln firing history, but a predictive model for expansion based on these factors does not yet exist. However, in general, highly crystalline engineering ceramics produced at high kiln temperatures expand less than low-fired ceramics with a higher amorphous content. The penalty is that high-fired ceramics tend to be more brittle and prone to cracking. It is now established that the expansive reaction continues indefinitely, although at a diminishing rate over all timescales; therefore, there is no well-defined time at which it ceases. Recent reanalysis of published data (Hall et al. 2011; Hall and Hoff 2012) shows that the equation e=at1/4 accurately describes expansion strain e over periods of time t as long as 65 years. It follows from this equation that expansive strain at 16 years is double the value at 1 year and three times the 1-year value at 81 years. The persistence of the expansion reaction, albeit at a diminishing rate, emphasizes the need to incorporate appropriate movement joints in masonry design. The authors mention the possibility of using a “poor mortar” to accommodate some of the expansive strain. The use of weak mortars undoubtedly explains the absence of expansion damage in some much-older buildings with thick brick walls. However, the discussers consider that in thin brick veneers, such as those used in Gondomar, a weak mortar is potentially dangerous. It is unfortunate both for design and for failure analysis that the test procedures generally used to characterize clay brick do not provide values of the expansivity a that are needed to apply the equation e=at1/4 . Accelerated steam tests, such as EN772-19 cited by the authors, are at best semiquantitative. In our view, it is essential to determine the expansivity from measurements of expansion strain made over an appropriate period of time under controlled conditions (Hall and Hoff 2012). The discussers also draw attention to the important practical matter of the temperature dependence of the moisture expansivity (Hall et al. 2013). The fact that moisture expansion is the direct consequence of a chemical rehydroxylation reaction (Hamilton and Hall 2012) ensures that the expansivity increases notably with temperature. Available data indicate that the activation energy (which controls the temperature dependence) is about 70 kJ/mol. This means, for example, that the expansivity a of any brick material is about 60% greater at a temperature of 30°C than it is at 10°C. Thus, if a limit expansion strain (say, 1×10−3) is reached in a particular material in 50 years at 10°C, the same strain is attained in the same material in only 7 years at 30°C. It seems likely that its strong temperature dependence explains why moisture expansion is perceived differently in different geographical regions [e.g., McNeilly (1985)] and generally receives more attention in regions with warmer climates, such as Australia, southern Asia, and Brazil. However, in any particular region, the magnitude of expansion and the associated damage within individual buildings are influenced by local temperature variations, in particular variations due to solar heat gain. In the Gondomar structure, deformation and cavity expansion were greatest in parts of the structure with a southern aspect, where the summer temperatures of the veneer are highest. The influence of aspect here is presumably exacerbated by the open situation of the building and by the insulation of the cavity where large temperature gradients might be expected. The discussers believe this large gradient acting over a thin veneer may partly explain such dramatic damage over a short period of time. A thicker brick cladding would probably fare better. Undoubtedly, there are also seasonal modulations of the expansion. The discussers have shown elsewhere how related thermal effects in the rehydroxylation of archaeological ceramics may be calculated (Hall et al. 2013)

Pattern formation in annular convection

This study of spatio-temporal pattern formation in an annulus is motivated by two physical problems on vastly different scales. The first is atmospheric convection in the equatorial plane between the warm surface of the Earth and the cold tropopause, modeled by the two dimensional Boussinesq equations. The second is annular electroconvection in a thin semetic film, where experiments reveal the birth of convection-like vortices in the plane as the electric field intensity is increased. This is modeled by two dimensional Navier-Stokes equations coupled with a simplified version of Maxwell's equations. The two models share fundamental mathematical properties and satisfy the prerequisites for application of O(2)-equivariant bifurcation theory. We show this can give predictions of interesting dynamics, including stationary and spatio-temporal patterns

Study of mechanical aspects of leak tightness in a pressure relief valve using advanced FE-analysis

This paper presents a numerical study involving the deformation of contact faces in the metal-to-metal seal in a typical pressure relief valve. The valve geometry is simplified to an axisymmetric problem, which comprises a simple geometry consisting of only 3 components. A cylindrical nozzle, which has a valve seat on top, contacts with a disk, which is preloaded by a compressed linear spring. All the components are made of AISI type 316N(L) steel defined using the multilinear kinematic hardening model based on monotonic and cyclic tests at 20°C. In-service observations show that there is a limited fluid leakage through the valve seat at operational pressures about 90% of the set pressure, which is caused by the fluid penetrating into surface asperities at the microscale. Nonlinear FEA in ANSYS using the fluid pressure penetration (FPP) technique revealed that there is a limited amount of fluid penetrating into gap, which is caused by the plastic deformation of the valve seat at the macroscale. Prediction of the fluid pressure distribution over the valve seat just before the valve lift is addressed in this study considering the FPP interaction on multiscale. This is the principal scope, since it allows adjustment of the valve spring force in order to improve the leak tightness

The Boston equalens: A clinical evaluation for wear

Recent advances in gas permeable contact lens materials have accounted for their successful use in extended wear. In this study, 18 subjects were fitted with the Boston Equalens, a new silicon/acrylate lens which is combined with a flourinated monomer. The subjects wore one lens as a daily wear contact lens and the other lens as an extended wear contact lens for a period of 90 days. Four patients successfully completed the 90 days of extended wear and five patients completed between 30 and 90 days of extended wear. No significant differences of corneal curvature, refractive error, corrected visual acuity, or subjective responses were noted between the daily wear eye and the extended wear eye. Also there were no reports of lens adhesion on any of the subjects\u27 eyes

Clarifying Obfuscation: Improving the Security of White-Box Encoding

To ensure the security of software executing on malicious hosts, as in digital rights management (DRM) applications, it is desirable to encrypt or decrypt content using white-box encoded cryptographic algorithms in the manner of Chow et al. Such encoded algorithms must run on an adversary’s machine without revealing the private key information used, despite the adversary’s ability to observe and manipulate the running algorithm. We have implemented obfuscated (white-box) DES and 3DES algorithms along the lines of Chow et al., with alterations that improve the security of the key, eliminating attacks that extract the key from Chow et al.’s obfuscated DES. Our system is secure against two previously published attacks on Chow et al.’s system, as well as a new adaptation of a statistical bucketing attack on their system. During implementation of white-box DES we found that a number of optimizations were needed for practical generation and execution. On a typical laptop we can generate obfuscated DES functions in a Lisp environment in under a minute allocating 11 MB, including the space required for the resulting function. The resulting function occupies 4.5 MB and encrypts or decrypts each block in approximately 30 ms on an 800 MHz G4 processor; slight run-time performance of the obfuscated DES could be traded to further reduce our algorithm’s representation to 2.3 MB. Although it is over an order of magnitude slower than typical DES systems, we believe it is fast enough for application to some DRM problems

Low nutritive quality as defense against herbivores

Contrary to a widespread assumption in the literature on plant-herbivore interactions, individual plants do not necessarily benefit by possessing traits which lower herbivore fitness. In particular, genes conferring lowered nutritive quality could even increase herbivore damage under certain circumstances. Three special sets of conditions are outlined in which low nutritive quality would lower herbivore-induced damage to an individual plant. These sets are far from exhaustive. It is concluded that the adaptiveness of lowered nutritive quality in herbivore defense is widely possible but in no case demonstrated.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/23144/1/0000068.pd