59 research outputs found
Structural design studies of a supersonic cruise arrow wing configuration
Structural member cross sections were sized with a system of integrated computer programs to satisfy strength and flutter design requirements for several variants of the arrow wing supersonic cruise vehicle. The resulting structural weights provide a measure of the structural efficiency of the planform geometry, structural layout, type of construction, and type of material including composites. The material distribution was determined for a baseline metallic structure and the results indicate that an approximate fatigue constraint has an important effect on the structural weight required for strength but, in all cases, additional material had to be added to satisfy flutter requirements with lighter mass engines with minimum fuel onboard. The use of composite materials on the baseline configuration was explored and indicated increased structural efficiency. In the strength sizing, the all-composite construction provided a lower weight design than the hybrid construction which contained composites only in the wing cover skins. Subsequent flutter analyses indicated a corresponding lower flutter speed
Noise and dynamical pattern selection
In pattern forming systems such as Rayleigh-Benard convection or directional
solidification, a large number of linearly stable, patterned steady states
exist when the basic, simple steady state is unstable. Which of these steady
states will be realized in a given experiment appears to depend on unobservable
details of the system's initial conditions. We show, however, that weak,
Gaussian white noise drives such a system toward a preferred wave number which
depends only on the system parameters and is independent of initial conditions.
We give a prescription for calculating this wave number, analytically near the
onset of instability and numerically otherwise.Comment: 12 pages, REVTEX, no figures. Submitted to Phys. Rev. Let
Identity of the universal repulsive-core singularity with Yang-Lee edge criticality
Lattice and continuum fluid models with repulsive-core interactions typically
display a dominant, critical-type singularity on the real, negative activity
axis. Lai and Fisher recently suggested, mainly on numerical grounds, that this
repulsive-core singularity is universal and in the same class as the Yang-Lee
edge singularities, which arise above criticality at complex activities with
positive real part. A general analytic demonstration of this identification is
presented here using a field-theory approach with separate representation of
the repulsive and attractive parts of the pair interactions.Comment: 6 pages, 3 figure
Transfer Matrices and Partition-Function Zeros for Antiferromagnetic Potts Models. IV. Chromatic polynomial with cyclic boundary conditions
We study the chromatic polynomial P_G(q) for m \times n square- and
triangular-lattice strips of widths 2\leq m \leq 8 with cyclic boundary
conditions. This polynomial gives the zero-temperature limit of the partition
function for the antiferromagnetic q-state Potts model defined on the lattice
G. We show how to construct the transfer matrix in the Fortuin--Kasteleyn
representation for such lattices and obtain the accumulation sets of chromatic
zeros in the complex q-plane in the limit n\to\infty. We find that the
different phases that appear in this model can be characterized by a
topological parameter. We also compute the bulk and surface free energies and
the central charge.Comment: 55 pages (LaTeX2e). Includes tex file, three sty files, and 22
Postscript figures. Also included are Mathematica files transfer4_sq.m and
transfer4_tri.m. Journal versio
Fisher zeros of the Q-state Potts model in the complex temperature plane for nonzero external magnetic field
The microcanonical transfer matrix is used to study the distribution of the
Fisher zeros of the Potts models in the complex temperature plane with
nonzero external magnetic field . Unlike the Ising model for
which has only a non-physical critical point (the Fisher edge singularity), the
Potts models have physical critical points for as well as the
Fisher edge singularities for . For the cross-over of the Fisher
zeros of the -state Potts model into those of the ()-state Potts model
is discussed, and the critical line of the three-state Potts ferromagnet is
determined. For we investigate the edge singularity for finite lattices
and compare our results with high-field, low-temperature series expansion of
Enting. For we find that the specific heat, magnetization,
susceptibility, and the density of zeros diverge at the Fisher edge singularity
with exponents , , and which satisfy the scaling
law .Comment: 24 pages, 7 figures, RevTeX, submitted to Physical Review
Traffic Equations and Granular Convection
We investigate both numerically and analytically the convective instability
of granular materials by two dimensional traffic equations. In the absence of
vibrations the traffic equations assume two distinctive classes of fixed bed
solutions with either a spatially uniform or nonuniform density profile. The
former one exists only when the function V(\rho) that monitors the relaxation
of grains assumes a cut off at the closed packed density, \rho_c, with
V(\rho_c)=0, while the latter one exists for any form of V. Since there is
little difference between the uniform and nonuniform solution deep inside the
bed, the convective instability of the bulk may be studied by focusing on the
stability of the uniform solution. In the presence of vibrations, we find that
the uniform solution bifurcates into a bouncing solution, which then undergoes
a supercritical bifurcation to the convective instability. We determine the
onset of convection as a function of control parameters and confirm this
picture by solving the traffic equations numerically, which reveals bouncing
solutions, two convective rolls, and four convective rolls. Further, convective
patterns change as the aspect ratio changes: in a vertically long container,
the rolls move toward the surface, and in a horizontally long container, the
rolls move toward the walls. We compare these results with those reported
previously with a different continuum model by Hayakawa, Yue and Hong[Phys.
Rev. Lett. 75,2328, 1995]. Finally, we also present a derivation of the traffic
equations from Enskoq equation.Comment: 34 pages, 10 figure
Effect of noise on coupled chaotic systems
Effect of noise in inducing order on various chaotically evolving systems is
reviewed, with special emphasis on systems consisting of coupled chaotic
elements. In many situations it is observed that the uncoupled elements when
driven by identical noise, show synchronization phenomena where chaotic
trajectories exponentially converge towards a single noisy trajectory,
independent of the initial conditions. In a random neural network, with
infinite range coupling, chaos is suppressed due to noise and the system
evolves towards a fixed point. Spatiotemporal stochastic resonance phenomenon
has been observed in a square array of coupled threshold devices where a
temporal characteristic of the system resonates at a given noise strength. In a
chaotically evolving coupled map lattice with logistic map as local dynamics
and driven by identical noise at each site, we report that the number of
structures (a structure is a group of neighbouring lattice sites for whom
values of the variable follow certain predefined pattern) follow a power-law
decay with the length of the structure. An interesting phenomenon, which we
call stochastic coherence, is also reported in which the abundance and
lifetimes of these structures show characteristic peaks at some intermediate
noise strength.Comment: 21 page LaTeX file for text, 5 Postscript files for figure
Using motivational techniques to reduce cardiometabolic risk factors in long term psychiatric inpatients: A naturalistic interventional study
Background
People with severe mental illness have markedly reduced life expectancy; cardiometabolic disease is a major cause. Psychiatric hospital inpatients have elevated levels of cardiometabolic risk factors and are to a high degree dependent of the routines and facilities of the institutions. Studies of lifestyle interventions to reduce cardiometabolic risk in psychiatric inpatients are few. The current study aimed at assessing the feasibility and effects of a lifestyle intervention including Motivational Interviewing (MI) on physical activity levels, cardiometabolic risk status and mental health status in psychotic disorder inpatients.
Methods
Prospective naturalistic intervention study of 83 patients at long term inpatient psychosis treatment wards in South-Eastern Norway. Patients were assessed 3–6 months prior to, at start and 6 months after a life-style intervention program including training of staff in MI, simple changes in routines and improvements of facilities for physical exercise. Assessments were done by clinical staff and included level of physical activity, motivation, life satisfaction, symptom levels (MADRS, AES-C, PANSS, and GAF) as well as anthropometric and biochemical markers of cardiometabolic risk. A mixed model was applied to analyze change over time.
Results
A total of 88% of patients received MI interventions, with a mean of 2.5 MI interventions per week per patient. The physical activity level was not increased, but activity level was positively associated with motivation and negatively associated with positive symptoms. Triglyceride levels and number of smokers were significantly reduced and a significant decrease in symptom levels was observed.
Conclusions
The current results suggest that a simple, low cost life-style intervention program focusing on motivational change is feasible and may reduce symptoms and improve lifestyle habits in psychosis patients in long term treatment facilities. Similar programs may easily be implemented in other psychiatric hospitals.submittedVersio
Physical activity in Hodgkin's lymphoma survivors with and without chronic fatigue compared with the general population – a cross-sectional study
<p>Abstract</p> <p>Background</p> <p>Hodgkin's lymphoma survivors (HLSs) commonly report chronic fatigue, defined as high levels of fatigue for 6 months or more. Underlying mechanisms are poorly understood. Based upon knowledge from other populations, lifestyle parameters may be related to this increased and persistent fatigue. The primary objective of the present study was to assess self-reported levels of physical activity, smoking habits and sleep patterns in HLSs with and without chronic fatigue. The secondary objective was to compare these results with data from age and gender adjusted data from the general population (Gen-Pop).</p> <p>Methods</p> <p>The Fatigue Questionnaire (FQ) and questions about daily smoking, sleep patterns and level of physical activity were completed by 476 HLSs treated at Rikshospitalet-Radiumhospitalet Trust (RR). The Gen-Pop data was derived from 56.999 inhabitants in a Norwegian county responding to a mail survey. Fischer's exact test, chi square test and t-tests were used to compare groups. P-values < .05 were considered statistically significant. A logistic regression analysis was performed in comparing the Gen-Pop with the HLSs.</p> <p>Results</p> <p>Level of physical activity, smoking habits and sleep patterns did not differ significantly between HLSs with and without chronic fatigue. The multivariate logistic regression analysis adjusting for different covariates, showed significantly more physically active men among HLSs compared with the Gen-Pop (OR = 1.50, CI 1.04 – 2.17), p = .031. No significant difference was found among females (OR = 1.20, CI = 0.83 – 1.74), p = .33.</p> <p>Conclusion</p> <p>Lifestyle parameters did not seem to be related to increased and persistent fatigue among HLSs. The results may indicate that the experience of Hodgkin's lymphoma increases the level of physical activity among male HLSs.</p
Students' Models of Newton's Second Law in Mechanics and Electromagnetism
We investigated students' use of Newton's second law in mechanics and
electromagnetism contexts by interviewing students in a two-semester
calculus-based physics course. We observed that students' responses are
consistent with three mental models. These models appeard in mechanics contexts
and were transferred to electromagnetism contexts. We developed an inventory to
help instructors identify these models and direct students towards the correct
one.Comment: 15 pages, 3 figues and 4 table
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