1,791 research outputs found
Turbulence model reduction by deep learning
A central problem of turbulence theory is to produce a predictive model for
turbulent fluxes. These have profound implications for virtually all aspects of
the turbulence dynamics. In magnetic confinement devices, drift-wave turbulence
produces anomalous fluxes via cross-correlations between fluctuations. In this
work, we introduce a new, data-driven method for parameterizing these fluxes.
The method uses deep supervised learning to infer a reduced mean-field model
from a set of numerical simulations. We apply the method to a simple drift-wave
turbulence system and find a significant new effect which couples the particle
flux to the local \emph{gradient} of vorticity. Notably, here, this effect is
much stronger than the oft-invoked shear suppression effect. We also recover
the result via a simple calculation. The vorticity gradient effect tends to
modulate the density profile. In addition, our method recovers a model for
spontaneous zonal flow generation by negative viscosity, stabilized by
nonlinear and hyperviscous terms. We highlight the important role of symmetry
to implementation of the new method.Comment: To be published in Phys. Rev. E Rap. Comm. 6 pages, 7 figure
Odd Decays from Even Anomalies: Gauge Mediation Signatures Without SUSY
We analyze the theory and phenomenology of anomalous global chiral symmetries
in the presence of an extra dimension. We propose a simple extension of the
Standard Model in 5D whose signatures closely resemble those of supersymmetry
with gauge mediation, and we suggest a novel scalar dark matter candidate.Comment: 26 pages, 1 figure; v2: references added; discussion of direct
collider constraints added; v3: corrected dark matter calculation in chapter
4.2 and replaced figure 1
An open source approach for regional cortical bone mineral density analysis
Objective: Cortical porosity, particularly at the endocortical region, is recognised to play a central role in the pathogenesis of bone fragility. Therefore, the purpose of this study was to: 1) demonstrate how cortical volumetric BMD (vBMD) distribution can be analysed from (p)QCT images and 2) highlight the clinical significance of assessing regional density distribution of cortical bone. Methods: We used pQCT to compare mid-tibial cortical volumetric BMD distribution of 20 young (age 24(SD2) years, mass 77(11) kg, height 178(6) cm) and 25 elderly (72(4) years, 75(9) kg, 172(5) cm) men. Radial and polar cortical vBMD distributions were analysed using a custom built open source analysis tool which allowed the cortex to be divided into three concentric cortical divisions and in 36 cortical sectors originating from the centroid of the bone. Results: Mean vBMD did not differ between the groups (1135(16) vs. 1130(28) mg/cm, P=0.696). In contrast, there was a significant age-group by radial division interaction for radial cortical vBMD (P<0.001). Conclusions: The proposed analysis method for analysing cortical bone density distribution of pQCT images was effective for detecting regional differences in cortical density between young and elderly men, which would have been missed by just looking at mean vBMD values.<br /
Polymerase Chain Reaction Assay With Urine Specimens in the Diagnosis of Acute Chlamydia trachomatis Infection in Women
Objective: The purpose of this study was to evaluate the benefits achievable by Amplicor polymerase
chain reaction (PCR) (F. Hoffmann-LaRoche Ltd., Basel, Switzerland) with urine specimens
in addition to PACE 2 (Gen-Probe, Inc., San Diego, California) assay with cervical swab specimens
in the diagnosis of Chlamydia trachomatis in women
A full body musculoskeletal model based on flexible multibody simulation approach utilised in bone strain analysis during human locomotion
Load-induced strains applied to bone can stimulate its development and adaptation. In order to quantify the incident strains within the skeleton, in vivo implementation of strain gauges on the surfaces of bone is typically used. However, in vivo strain measurements require invasive methodology that is challenging and limited to certain regions of superficial bones only such as the anterior surface of the tibia. Based on our previous study [Al Nazer et al. (2008) J Biomech. 41:1036–1043], an alternative numerical approach to analyse in vivo strains based on the flexible multibody simulation approach was proposed. The purpose of this study was to extend the idea of using the flexible multibody approach in the analysis of bone strains during physical activity through integrating the magnetic resonance imaging (MRI) technique within the framework. In order to investigate the reliability and validity of the proposed approach, a three-dimensional full body musculoskeletal model with a flexible tibia was used as a demonstration example. The model was used in a forward dynamics simulation in order to predict the tibial strains during walking on a level exercise. The flexible tibial model was developed using the actual geometry of human tibia, which was obtained from three-dimensional reconstruction of MRI. Motion capture data obtained from walking at constant velocity were used to drive the model during the inverse dynamics simulation in order to teach the muscles to reproduce the motion in the forward dynamics simulation. Based on the agreement between the literature-based in vivo strain measurements and the simulated strain results, it can be concluded that the flexible multibody approach enables reasonable predictions of bone strain in response to dynamic loading. The information obtained from the present approach can be useful in clinical applications including devising exercises to prevent bone fragility or to accelerate fracture healing.<br /
Universal Equilibrium Currents in the Quantum Hall Fluid
The equilibrium current distribution in a quantum Hall fluid that is
subjected to a slowly varying confining potential is shown to generally consist
of strips or channels of current, which alternate in direction, and which have
universal integrated strengths. A measurement of these currents would yield
direct independent measurements of the proper quasiparticle and quasihole
energies in the fractional quantum Hall states.Comment: 4 pages, Revte
Dynamics of magnetization coupled to a thermal bath of elastic modes
We study the dynamics of magnetization coupled to a thermal bath of elastic
modes using a system plus reservoir approach with realistic magnetoelastic
coupling. After integrating out the elastic modes we obtain a self-contained
equation for the dynamics of the magnetization.
We find explicit expressions for the memory friction kernel and hence, {\em
via} the Fluctuation-Dissipation
Theorem, for the spectral density of the magnetization thermal fluctuations.
For magnetic samples in which the single domain approximation is valid, we
derive an equation for the dynamics of the uniform mode.
Finally we apply this equation to study the dynamics of the uniform
magnetization mode in insulating ferromagnetic thin films.
As experimental consequences we find that the fluctuation correlation time is
of the order of the ratio between the film thickness, , and the speed of
sound in the magnet and that the line-width of the ferromagnetic resonance peak
should scale as where is the magnetoelastic coupling constant.Comment: Revised version as appeared in print. 12 pages 9 figure
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