4,178 research outputs found
Stability of a two-sublattice spin-glass model
We study the stability of the replica-symmetric solution of a two-sublattice
infinite-range spin-glass model, which can describe the transition from
antiferromagnetic to spin glass state. The eigenvalues associated with
replica-symmetric perturbations are in general complex. The natural
generalization of the usual stability condition is to require the real part of
these eigenvalues to be positive. The necessary and sufficient conditions for
all the roots of the secular equation to have positive real parts is given by
the Hurwitz criterion. The generalized stability condition allows a consistent
analysis of the phase diagram within the replica-symmetric approximation.Comment: 21 pages, 5 figure
A Double Myers-Perry Black Hole in Five Dimensions
Using the inverse scattering method we construct a six-parameter family of
exact, stationary, asymptotically flat solutions of the 4+1 dimensional vacuum
Einstein equations, with U(1)^2 rotational symmetry. It describes the
superposition of two Myers-Perry black holes, each with a single angular
momentum parameter, both in the same plane. The black holes live in a
background geometry which is the Euclidean C-metric with an extra flat time
direction. This background possesses conical singularities in two adjacent
compact regions, each corresponding to a set of fixed points of one of the U(1)
actions in the Cartan sub-algebra of SO(4). We discuss several aspects of the
black holes geometry, including the conical singularities arising from force
imbalance, and the torsion singularity arising from torque imbalance. The
double Myers-Perry solution presented herein is considerably simpler than the
four dimensional double Kerr solution and might be of interest in studying
spin-spin interactions in five dimensional general relativity.Comment: 23 pages, 7 figures, LaTeX. v2: minor changes, references added;
version published in JHE
Temperature, flow and eccentricity measurements in a journal bearing with a single axial groove at 90º to the load line
Parametric experiments have been conducted to analyse the influence of some supply conditions on the performance of a steadily loaded journal bearing. Temperature distribution on the internal surface, flowrate and bearing eccentricity were measured for different sets of operating conditions, at variable supply conditions. Quantitative information is provided which shows the effect of both shaft speed and applied load on maximum bush temperature and flowrate. It has been observed that flowrate was modestly affected by load and significantly affected by rotational speed, oil supply temperature and supply pressure. Maximum bush temperature was modestly affected by supply pressure, moderately affected by load and significantly affected by shaft speed. For low applied loads, the attitude angle was markedly affected by supply pressure. The experimental results also showed that for a small groove length there is a variation of bush temperature in axial direction in the groove region.Fundação para a Ciência e a Tecnologia (FCT) - PRAXIS XXI Programme/BD/13922/9
Measurement errors and uncertainty estimation of an experimental set up using a 2D PIV technique
The study of the flow interaction and the heat transfer between
air jets and a surface is of paramount importance in industrial
processes that apply multiple air jet impingement. To ensure a
good performance of the process, high heat transfer rates and
uniformization of the flow over the target plate are required. To
perform this analysis, a PIV technique was implemented for the
measurement of the velocity fields of the flow. However, as any
real experiment, the values recorded by the PIV method are
subjected to several errors that compromise the reliability and
accuracy of the measurements. These errors can have different
sources, from the installation and alignment to the particles
seeding and calibration procedure. To determine an interval that
contains the measurement error, the uncertainty quantification is
crucial. In that sense, this paper focus on the identification of
measurement errors and uncertainty quantification of an
experimental set up specially built for the analysis of the
interaction between a non-isothermal jets and non-flat surfaces
moving perpendicularly to the jet axis. To ensure the reliability
of the results, preliminary tests were performed to guarantee a
uniform and stable flow and to determine the range and
conditions of operation. In addition, this work presents an
analysis of the system, and the source of errors are identified,
quantified and, when possible, corrected. The particle seeding,
which consists of olive oil droplets, is characterized and its
efficiency for the flow tracking is analysed. The test facility was
tested to fully characterize the flow field in terms of mean velocity profile and turbulence intensity over a wide range of
Reynolds numbers and temperature. Several velocity fields are
then measured until convergence of the flow quantities is
reached. The combination of these measurements with high
spatial resolution and low measurement errors allow to obtain
accurate and precise measurement values.Portuguese Foundation for Science and
Technology (FCT) through the Research Grant
PD/BD/128216/2016. This work has been supported by FCT
within the Project Scope UID/CEC/00319/2019 (ALGORITMI)
and Project Scope UID/EMS/04077/2019 (METRICS)
Thermohydrodynamic modelling of journal bearings under varying load angle and negative groove flow rate
The performance of hydrodynamic journal bearings is affected by the conditions under which the lubricant is fed to the bearing gap. Axial grooves are often used and, depending on their location relatively to the load line, they might substantially interfere with the hydrodynamic pressure generation and the thermal behaviour of the bearing. However, many of the existing tools for predicting bearing performance are not able to suitably predict bearing behaviour under varying load angle given the oversimplified way under which they treat lubricant feeding conditions.
The present work proposes a detailed Thermohydrodynamic approach which realistically incorporates these conditions into the bearing analysis. Special care is put on the mass and energy-conserving models of the ruptured film region and on a detailed treatment of lubricant mixing within the vicinity of grooves. This includes the first full modelling of the effect of negative flow rate in a groove, a phenomenon originally described experimentally in detail by the authors in previous publications, and which happens for a broad range of load/groove angles.
An extensive investigation on the influence of loading direction on the performance of twin groove journal bearings has been performed. This parameter is found to affect deeply all major performance parameters due to the interference of groove regions in the hydrodynamic pressure generation and in the flow rates at each groove.Fundação para a Ciência e a Tecnologia (FCT
Dual-band implantable antennas for medical telemetry: a fast design methodology and validation for intra-cranial pressure monitoring
WOS:000323538100010 (Nº de Acesso Web of Science)In this study, we suggest and experimentally validate a methodology for fast and optimized design of dual-band implantable antennas for medical telemetry (MICS, 402-405 MHz, and ISM, 2400-2480 MHz). The methodology aims to adjust the design of a parametric dual-band antenna model towards optimally satisfying the requirements imposed by the antenna-fabrication procedure and medical application in hand. Design is performed in a systematic, fast, and accurate way. To demonstrate its effectiveness, the proposed methodology is applied to optimize the parametric antenna model for intra-cranial pressure (ICP) monitoring given a specific antenna-fabrication procedure. For validation purposes, a prototype of the optimized antenna is fabricated and experimentally tested. The proposed antenna is further evaluated within a 13-tissue anatomical head model in terms of resonance, radiation, and safety performance for ICP monitoring. Extensive parametric studies of the optimized antenna are, finally, performed. Feasibility of the proposed parametric antenna model to be optimally re-adjusted for various scenarios is demonstrated, and generic guidelines are provided for implantable antenna design. Dual-band operation is targeted to ensure energy autonomy for the implant. Finite Element (FE) and Finite Difference Time Domain (FDTD) simulations are carried out in homogeneous rectangular and anatomical head tissue models, respectively
Influence des conditions d’alimentation sur l’amélioration des performances et la réduction du frottement dans les paliers
The performance of hydrodynamic journal bearings is significantly affected by the conditions under which the lubricant is fed to the bearing. However, many conventional analyses are not prepared to suitably incorporate these parameters and their effect on bearing performance, due to the over-simplified way they treat them.
A thermohydrodynamic analysis suitable for conveniently deal with lubricant feeding conditions is now presented. It couples the numerical solution of the generalized Reynolds equation, the energy equations within the lubricant film and the heat transfer within the bush body. Special attention has been given to the treatment of the phenomena taking place within the grooves and in their vicinity, as well as to the ruptured film region.
The effect that lubricant feed pressure and temperature, groove length ratio, groove width ratio and groove number (single / twin) have on bearing performance has been analyzed for a broad range of conditions. The results were found to be in good agreement with experimental published results and the robustness of the model to suitably treat these phenomena has been confirmed. It was found that a careful tuning of the feeding conditions may indeed improve bearing performance.Les performances des paliers lisses hydrodynamiques sont significativement affectées
par les conditions dans lesquelles le lubrifiant est introduit dans le palier. Cependant, de
nombreuses analyses classiques ne sont pas prêtes à intégrer convenablement ces
paramètres et leurs effets sur les performances des paliers, en raison de la façon trop
simplifiée avec laquelle ils les traitent.
Une analyse thermohydrodynamique appropriée pour traiter convenablement les
conditions d'alimentation de lubrifiant est maintenant présenté. Elle intègre la solution
numérique simultanée des équations de Reynolds généralisée, de l'énergie dans le film
lubrifiant et de la de chaleur dans le coussinet. Une attention particulière a été portée
pour le traitement des phénomènes se produisant dans les rainures et dans leur
voisinage, ainsi que dans la région de rupture du film.
L'influence de la pression et de la température d'alimentation en lubrifiant, de la longueur
et largeur de la rainure et du nombre de rainures (simple ou double) sur les performances
du palier a été analysée pour un large éventail de conditions.
Les résultats numériques obtenus sont en bon accord avec ceux issus de la littérature
expérimentale et la robustesse du modèle à traiter convenablement ces phénomènes a
pu être confirmée. Il a été constaté qu'un réglage minutieux des conditions d'alimentation
peut effectivement améliorer les performances.FCT - POCTI/EME/39202/2001 & Postdoctoral grant SFRH/BPD/89553/201
On the occurrence of negative groove flow rate in twin groove hydrodynamic journal bearings
This paper describes the phenomenon of negative flow rate (hot oil reflux) occurring in one of the grooves of twin axial groove hydrodynamic journal bearings. This rarely described phenomenon, which can occur under severe operating conditions, increases the risk of bearing seizure. Existing codes for predicting bearing behaviour do not seem to be suitably incorporating this phenomenon into the analysis. The present work reports some experimental observations of the phenomenon and the way it can be taken into account in theoretical modelling.FCT - POCTI/39202/EME/2001 & PhD grant SFRH/BD/22278/200
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