1,349 research outputs found
A connection between the Camassa-Holm equations and turbulent flows in channels and pipes
In this paper we discuss recent progress in using the Camassa-Holm equations
to model turbulent flows. The Camassa-Holm equations, given their special
geometric and physical properties, appear particularly well suited for studying
turbulent flows. We identify the steady solution of the Camassa-Holm equation
with the mean flow of the Reynolds equation and compare the results with
empirical data for turbulent flows in channels and pipes. The data suggests
that the constant version of the Camassa-Holm equations, derived under
the assumptions that the fluctuation statistics are isotropic and homogeneous,
holds to order distance from the boundaries. Near a boundary, these
assumptions are no longer valid and the length scale is seen to depend
on the distance to the nearest wall. Thus, a turbulent flow is divided into two
regions: the constant region away from boundaries, and the near wall
region. In the near wall region, Reynolds number scaling conditions imply that
decreases as Reynolds number increases. Away from boundaries, these
scaling conditions imply is independent of Reynolds number. Given the
agreement with empirical and numerical data, our current work indicates that
the Camassa-Holm equations provide a promising theoretical framework from which
to understand some turbulent flows.Comment: tex file, 29 pages, 4 figures, Physics of Fluids (in press
Recurrence of Ganglion Cysts Following Re-excision
Previous studies have examined the recurrence of ganglion cysts after surgical excision at a rate of 4 to 40%. However, recurrence after revision surgical excision is unknown. The purpose of this study was to define the incidence of recurrent ganglion cysts in patients who underwent a 2nd excisional procedure.https://jdc.jefferson.edu/cwicposters/1032/thumbnail.jp
Psychosocial characteristics and social networks of suicidal prisoners: towards a model of suicidal behaviour in detention
Prisoners are at increased risk of suicide. Investigation of both individual and environmental risk factors may assist in developing suicide prevention policies for prisoners and other high-risk populations. We conducted a matched case-control interview study with 60 male prisoners who had made near-lethal suicide attempts in prison (cases) and 60 male prisoners who had not (controls). We compared levels of depression, hopelessness, self-esteem, impulsivity, aggression, hostility, childhood abuse, life events (including events occurring in prison), social support, and social networks in univariate and multivariate models. A range of psychosocial factors was associated with near-lethal self-harm in prisoners. Compared with controls, cases reported higher levels of depression, hopelessness, impulsivity, and aggression, and lower levels of self-esteem and social support (all p values <0.001). Adverse life events and criminal history factors were also associated with near-lethal self-harm, especially having a prior prison spell and having been bullied in prison, both of which remained significant in multivariate analyses. The findings support a model of suicidal behaviour in prisoners that incorporates imported vulnerability factors, clinical factors, and prison experiences, and underscores their interaction. Strategies to reduce self-harm and suicide in prisoners should include attention to such factors
Optimal Schedules for Parallelizing Anytime Algorithms: The Case of Shared Resources
The performance of anytime algorithms can be improved by simultaneously
solving several instances of algorithm-problem pairs. These pairs may include
different instances of a problem (such as starting from a different initial
state), different algorithms (if several alternatives exist), or several runs
of the same algorithm (for non-deterministic algorithms). In this paper we
present a methodology for designing an optimal scheduling policy based on the
statistical characteristics of the algorithms involved. We formally analyze the
case where the processes share resources (a single-processor model), and
provide an algorithm for optimal scheduling. We analyze, theoretically and
empirically, the behavior of our scheduling algorithm for various distribution
types. Finally, we present empirical results of applying our scheduling
algorithm to the Latin Square problem
Toughening and asymmetry in peeling of heterogeneous adhesives
The effective adhesive properties of heterogeneous thin films are
characterized through a combined experimental and theoretical investigation. By
bridging scales, we show how variations of elastic or adhesive properties at
the microscale can significantly affect the effective peeling behavior of the
adhesive at the macroscale. Our study reveals three elementary mechanisms in
heterogeneous systems involving front propagation: (i) patterning the elastic
bending stiffness of the film produces fluctuations of the driving force
resulting in dramatically enhanced resistance to peeling; (ii) optimized
arrangements of pinning sites with large adhesion energy are shown to control
the effective system resistance, allowing the design of highly anisotropic and
asymmetric adhesives; (iii) heterogeneities of both types result in front
motion instabilities producing sudden energy releases that increase the overall
adhesion energy. These findings open potentially new avenues for the design of
thin films with improved adhesion properties, and motivate new investigation of
other phenomena involving front propagation.Comment: Physical Review Letters (2012)
Prevention of suicidal behaviour in prisons: an overview of initiatives based on a systematic review of research on near-lethal suicide attempts
Background: Worldwide, prisoners are at high risk of suicide. Research on near-lethal suicide attempts can provide important insights into risk and protective factors, and inform suicide prevention initiatives in prison. Aims: To synthesize findings of research on near-lethal attempts in prisons, and consider their implications for suicide prevention policies and practice, in the context of other research in custody and other settings. Method: We searched two bibliographic indexes for studies in any language on near-lethal and severe self-harm in prisoners, supplemented by targeted searches over the period 2000–2014. We extracted information on risk factors descriptively. Data were not meta-analyzed owing to heterogeneity of samples and methods. Results: We identified eight studies reporting associations between prisoner near-lethal attempts and specific factors. The latter included historical, prison-related, and clinical factors, including psychiatric morbidity and comorbidity, trauma, social isolation, and bullying. These factors were also identified as important in prisoners' own accounts of what may have contributed to their attempts (presented in four studies). Conclusion: Factors associated with prisoners' severe suicide attempts include a range of potentially modifiable clinical, psychosocial, and environmental factors. We make recommendations to address these factors in order to improve detection, management, and prevention of suicide risk in prisoners
The influence of the symmetry of identical particles on flight times
In this work, our purpose is to show how the symmetry of identical particles
can influence the time evolution of free particles in the nonrelativistic and
relativistic domains. For this goal, we consider a system of either two
distinguishable or indistinguishable (bosons and fermions) particles. Two
classes of initial conditions have been studied: different initial locations
with the same momenta, and the same locations with different momenta. The
flight time distribution of particles arriving at a `screen' is calculated in
each case. Fermions display broader distributions as compared with either
distinguishable particles or bosons, leading to earlier and later arrivals for
all the cases analyzed here. The symmetry of the wave function seems to speed
up or slow down propagation of particles. Due to the cross terms, certain
initial conditions lead to bimodality in the fermionic case. Within the
nonrelativistic domain and when the short-time survival probability is
analyzed, if the cross term becomes important, one finds that the decay of the
overlap of fermions is faster than for distinguishable particles which in turn
is faster than for bosons. These results are of interest in the short time
limit since they imply that the well-known quantum Zeno effect would be
stronger for bosons than for fermions.Fermions also arrive earlier than bosons
when they are scattered by a delta barrier. Furthermore, the particle symmetry
does not affect the mean tunneling flight time and it is given by the phase
time for the distinguishable particle.Comment: 25 pages, 1 table, 5 figure
Learning Arbitrary Statistical Mixtures of Discrete Distributions
We study the problem of learning from unlabeled samples very general
statistical mixture models on large finite sets. Specifically, the model to be
learned, , is a probability distribution over probability
distributions , where each such is a probability distribution over . When we sample from , we do not observe
directly, but only indirectly and in very noisy fashion, by sampling from
repeatedly, independently times from the distribution . The problem is
to infer to high accuracy in transportation (earthmover) distance.
We give the first efficient algorithms for learning this mixture model
without making any restricting assumptions on the structure of the distribution
. We bound the quality of the solution as a function of the size of
the samples and the number of samples used. Our model and results have
applications to a variety of unsupervised learning scenarios, including
learning topic models and collaborative filtering.Comment: 23 pages. Preliminary version in the Proceeding of the 47th ACM
Symposium on the Theory of Computing (STOC15
Influence of Mo on the Fe:Mo:C nano-catalyst thermodynamics for single-walled carbon nanotube growth
We explore the role of Mo in Fe:Mo nanocatalyst thermodynamics for
low-temperature chemical vapor deposition growth of single walled carbon
nanotubes (SWCNTs). By using the size-pressure approximation and ab initio
modeling, we prove that for both Fe-rich (~80% Fe or more) and Mo-rich (~50% Mo
or more) Fe:Mo clusters, the presence of carbon in the cluster causes
nucleation of Mo2C. This enhances the activity of the particle since it
releases Fe, which is initially bound in a stable Fe:Mo phase, so that it can
catalyze SWCNT growth. Furthermore, the presence of small concentrations of Mo
reduce the lower size limit of low-temperature steady-state growth from ~0.58nm
for pure Fe particles to ~0.52nm. Our ab initio-thermodynamic modeling explains
experimental results and establishes a new direction to search for better
catalysts.Comment: 7 pages, 3 figures. submitte
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