699 research outputs found
Matrix exponential-based closures for the turbulent subgrid-scale stress tensor
Two approaches for closing the turbulence subgrid-scale stress tensor in terms of matrix exponentials are introduced and compared. The first approach is based on a formal solution of the stress transport equation in which the production terms can be integrated exactly in terms of matrix exponentials. This formal solution of the subgrid-scale stress transport equation is shown to be useful to explore special cases, such as the response to constant velocity gradient, but neglecting pressure-strain correlations and diffusion effects. The second approach is based on an Eulerian-Lagrangian change of variables, combined with the assumption of isotropy for the conditionally averaged Lagrangian velocity gradient tensor and with the recent fluid deformation approximation. It is shown that both approaches lead to the same basic closure in which the stress tensor is expressed as the matrix exponential of the resolved velocity gradient tensor multiplied by its transpose. Short-time expansions of the matrix exponentials are shown to provide an eddy-viscosity term and particular quadratic terms, and thus allow a reinterpretation of traditional eddy-viscosity and nonlinear stress closures. The basic feasibility of the matrix-exponential closure is illustrated by implementing it successfully in large eddy simulation of forced isotropic turbulence. The matrix-exponential closure employs the drastic approximation of entirely omitting the pressure-strain correlation and other nonlinear scrambling terms. But unlike eddy-viscosity closures, the matrix exponential approach provides a simple and local closure that can be derived directly from the stress transport equation with the production term, and using physically motivated assumptions about Lagrangian decorrelation and upstream isotropy
Predicting new major depression symptoms from long working hours, psychosocial safety climate and work engagement: A population-based cohort study
Objectives This study sought to assess the association between long working hours, psychosocial safety climate (PSC), work engagement (WE) and new major depression symptoms emerging over the next 12 months. PSC is the work climate supporting workplace psychological health. Setting Australian prospective cohort population data from the states of New South Wales, Western Australia and South Australia. Participants At Time 1, there were 3921 respondents in the sample. Self-employed, casual temporary, unclassified, those with working hours <35 (37% of 2850) and participants with major depression symptoms at Time 1 (6.7% of 1782) were removed. The final sample was a population-based cohort of 1084 full-time Australian employees. Primary and secondary outcome measures The planned and measured outcomes were new cases of major depression symptoms. Results Long working hours were not significantly related to new cases of major depression symptoms; however, when mild cases were removed, the 41â48 and â„55 long working hour categories were positively related to major depression symptoms. Low PSC was associated with a threefold increase in risk for new major depression symptoms. PSC was not related to long working hours, and long working hours did not mediate the relationship between PSC and new cases of major depression symptoms. The inverse relationship between PSC and major depression symptoms was stronger for males than females. Additional analyses identified that WE was positively related to long working hours. Long working hours (41â48 and â„55 hours) mediated a positive relationship between WE and major depression symptoms when mild cases of major depression were removed. Conclusion The results suggest that low workplace PSC and potentially long working hours (41â48; â„55 hours/ week) increase the risk of new major depression symptoms. Furthermore, high WE may increase long working hours and subsequent major depression symptoms.Amy Jane Zadow, Maureen F Dollard, Christian Dormann, Paul Landsbergi
Fermi-Walker gauge in 2+1 dimensional gravity.
It is shown that the Fermi-Walker gauge allows the general solution of
determining the metric given the sources, in terms of simple quadratures. We
treat the general stationary problem providing explicit solving formulas for
the metric and explicit support conditions for the energy momentum tensor. The
same type of solution is obtained for the time dependent problem with circular
symmetry. In both cases the solutions are classified in terms of the invariants
of the Wilson loops outside the sources. The Fermi-Walker gauge, due to its
physical nature, allows to exploit the weak energy condition and in this
connection it is proved that, both for open and closed universes with
rotational invariance, the energy condition imply the total absence of closed
time like curves. The extension of this theorem to the general stationary
problem, in absence of rotational symmetry is considered. At present such
extension is subject to some assumptions on the behavior of the determinant of
the dreibein in this gauge. PACS number: 0420Comment: 28 pages, RevTex, no figure
A powerful intervention: general practitioners' use of sickness certification in depression
<b>Background</b> Depression is frequently cited as the reason for sickness absence, and it is estimated that sickness certificates are issued in one third of consultations for depression. Previous research has considered GP views of sickness certification but not specifically in relation to depression. This study aimed to explore GPs views of sickness certification in relation to depression.<p></p>
<b>Methods</b> A purposive sample of GP practices across Scotland was selected to reflect variations in levels of incapacity claimants and antidepressant prescribing. Qualitative interviews were carried out between 2008 and 2009.<p></p>
<b>Results</b> A total of 30 GPs were interviewed. A number of common themes emerged including the perceived importance of GP advocacy on behalf of their patients, the tensions between stakeholders involved in the sickness certification system, the need to respond flexibly to patients who present with depression and the therapeutic nature of time away from work as well as the benefits of work. GPs reported that most patients with depression returned to work after a short period of absence and that it was often difficult to predict which patients would struggle to return to work.<p></p>
<b>Conclusions</b>
GPs reported that dealing with sickness certification and depression presents distinct challenges. Sickness certificates are often viewed as powerful interventions, the effectiveness of time away from work for those with depression should be subject to robust enquiry
Tunneling times with covariant measurements
We consider the time delay of massive, non-relativistic, one-dimensional
particles due to a tunneling potential. In this setting the well-known Hartman
effect asserts that often the sub-ensemble of particles going through the
tunnel seems to cross the tunnel region instantaneously. An obstacle to the
utilization of this effect for getting faster signals is the exponential
damping by the tunnel, so there seems to be a trade-off between speedup and
intensity. In this paper we prove that this trade-off is never in favor of
faster signals: the probability for a signal to reach its destination before
some deadline is always reduced by the tunnel, for arbitrary incoming states,
arbitrary positive and compactly supported tunnel potentials, and arbitrary
detectors. More specifically, we show this for several different ways to define
``the same incoming state'' and ''the same detector'' when comparing the
settings with and without tunnel potential. The arrival time measurements are
expressed in the time-covariant approach, but we also allow the detection to be
a localization measurement at a later time.Comment: 12 pages, 2 figure
Modeling the pressure Hessian and viscous Laplacian in Turbulence: comparisons with DNS and implications on velocity gradient dynamics
Modeling the velocity gradient tensor A along Lagrangian trajectories in
turbulent flow requires closures for the pressure Hessian and viscous Laplacian
of A. Based on an Eulerian-Lagrangian change of variables and the so-called
Recent Fluid Deformation closure, such models were proposed recently. The
resulting stochastic model was shown to reproduce many geometric and anomalous
scaling properties of turbulence. In this work, direct comparisons between
model predictions and Direct Numerical Simulation (DNS) data are presented.
First, statistical properties of A are described using conditional averages of
strain skewness, enstrophy production, energy transfer and vorticity
alignments, conditioned upon invariants of A. These conditionally averaged
quantities are found to be described accurately by the stochastic model. More
detailed comparisons that focus directly on the terms being modeled in the
closures are also presented. Specifically, conditional statistics associated
with the pressure Hessian and the viscous Laplacian are measured from the model
and are compared with DNS. Good agreement is found in strain-dominated regions.
However, some features of the pressure Hessian linked to rotation dominated
regions are not reproduced accurately by the model. Geometric properties such
as vorticity alignment with respect to principal axes of the pressure Hessian
are mostly predicted well. In particular, the model predicts that an
eigenvector of the rate-of-strain will be also an eigenvector of the pressure
Hessian, in accord to basic properties of the Euler equations. The analysis
identifies under what conditions the Eulerian-Lagrangian change of variables
with the Recent Fluid Deformation closure works well, and in which flow regimes
it requires further improvements.Comment: 16 pages, 10 figures, minor revisions, final version published in
Phys. Fluid
Adiabatic non-equilibrium steady states in the partition free approach
Consider a small sample coupled to a finite number of leads, and assume that
the total (continuous) system is at thermal equilibrium in the remote past. We
construct a non-equilibrium steady state (NESS) by adiabatically turning on an
electrical bias between the leads. The main mathematical challenge is to show
that certain adiabatic wave operators exist, and to identify their strong limit
when the adiabatic parameter tends to zero. Our NESS is different from, though
closely related with the NESS provided by the Jak{\v s}i{\'c}-Pillet-Ruelle
approach. Thus we partly settle a question asked by Caroli {\it et al} in 1971
regarding the (non)equivalence between the partitioned and partition-free
approaches
A quasi classical approach to electron impact ionization
A quasi classical approximation to quantum mechanical scattering in the
Moeller formalism is developed. While keeping the numerical advantage of a
standard Classical--Trajectory--Monte--Carlo calculation, our approach is no
longer restricted to use stationary initial distributions. This allows one to
improve the results by using better suited initial phase space distributions
than the microcanonical one and to gain insight into the collision mechanism by
studying the influence of different initial distributions on the cross section.
A comprehensive account of results for single, double and triple differential
cross sections for atomic hydrogen will be given, in comparison with experiment
and other theories.Comment: 21 pages, 10 figures, submitted to J Phys
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