181 research outputs found
Pressure Hessian and viscous contributions to velocity gradient statistics based on Gaussian random fields
Understanding the non-local pressure contributions and viscous effects on the
small-scale statistics remains one of the central challenges in the study of
homogeneous isotropic turbulence. Here we address this issue by studying the
impact of the pressure Hessian as well as viscous diffusion on the statistics
of the velocity gradient tensor in the framework of an exact statistical
evolution equation. This evolution equation shares similarities with earlier
phenomenological models for the Lagrangian velocity gradient tensor evolution,
yet constitutes the starting point for a systematic study of the unclosed
pressure Hessian and viscous diffusion terms. Based on the assumption of
incompressible Gaussian velocity fields, closed expressions are obtained as the
results of an evaluation of the characteristic functionals. The benefits and
shortcomings of this Gaussian closure are discussed, and a generalization is
proposed based on results from direct numerical simulations. This enhanced
Gaussian closure yields, for example, insights on how the pressure Hessian
prevents the finite-time singularity induced by the local self-amplification
and how its interaction with viscous effects leads to the characteristic strain
skewness phenomenon
On the small-scale structure of turbulence and its impact on the pressure field
Understanding the small-scale structure of incompressible turbulence and its
implications for the non-local pressure field is one of the fundamental
challenges in fluid mechanics. Intense velocity gradient structures tend to
cluster on a range of scales which affects the pressure through a Poisson
equation. Here we present a quantitative investigation of the spatial
distribution of these structures conditional on their intensity for
Taylor-based Reynolds numbers in the range [160, 380]. We find that the
correlation length, the second invariant of the velocity gradient, is
proportional to the Kolmogorov scale. It also is a good indicator for the
spatial localization of intense enstrophy and strain-dominated regions, as well
as the separation between them. We describe and quantify the differences in the
two-point statistics of these regions and the impact they have on the
non-locality of the pressure field as a function of the intensity of the
regions. Specifically, across the examined range of Reynolds numbers, the
pressure in strong rotation-dominated regions is governed by a
dissipation-scale neighbourhood. In strong strain-dominated regions, on the
other hand, it is determined primarily by a larger neighbourhood reaching
inertial scales.Comment: Accepted for publication by the Journal of Fluid Mechanic
Optimal noise-canceling networks
Natural and artificial networks, from the cerebral cortex to large-scale
power grids, face the challenge of converting noisy inputs into robust signals.
The input fluctuations often exhibit complex yet statistically reproducible
correlations that reflect underlying internal or environmental processes such
as synaptic noise or atmospheric turbulence. This raises the practically and
biophysically relevant of question whether and how noise-filtering can be
hard-wired directly into a network's architecture. By considering generic phase
oscillator arrays under cost constraints, we explore here analytically and
numerically the design, efficiency and topology of noise-canceling networks.
Specifically, we find that when the input fluctuations become more correlated
in space or time, optimal network architectures become sparser and more
hierarchically organized, resembling the vasculature in plants or animals. More
broadly, our results provide concrete guiding principles for designing more
robust and efficient power grids and sensor networks.Comment: 6 pages, 3 figures, supplementary materia
Relic gravitational waves and extended inflation
In extended inflation, a new version of inflation where the transition from the false-vacuum phase to a radiation-dominated Universe is accomplished by bubble nucleation and percolation, bubble collisions supply a potent-and potentially detectable-source of gravitational waves. The present energy density in relic gravity waves from bubble collisions is expected to be about 10(exp -5) of closure density-many orders of magnitude greater than that of the gravity waves produced by quantum fluctuations. Their characteristic wavelength depends upon the reheating temperature T(sub RH): lambda is approximately 10(exp 4) cm (10(exp 14) GeV/T(sub RH)). If large numbers of black holes are produced, a not implausible outcome, they will evaporate producing comparable amounts of shorter wavelength waves, lambda is approximately 10(exp -6) cm (T(sub RH)/10(exp 14) GeV)
Positron line radiation from halo WIMP annihilations as a dark matter signature
We suggest a new signature for dark matter annihilation in the halo: high energy positron line radiation. Because the cosmic ray positron spectrum falls rapidly with energy, e+'s from halo WIMP annihilations can be a significant, clean signal for very massive WIMP's (approx. greater than 30 GeV). In the case that the e+e- annihilation channel has an appreciable branch, the e+ signal should be above background in a future detector, such as have been proposed for ASTROMAG, and of potential importance as a dark matter signature. A significant e+e- branching ratio can occur for neutralinos or Dirac neutrinos. High-energy, continuum positron radiation may also be an important signature for massive neutralino annihilations, especially near or above the threshold of the W+W- and ZoZo annihilation channels
Persistent accelerations disentangle Lagrangian turbulence
Particles in turbulence frequently encounter extreme accelerations between
extended periods of quiescence. The occurrence of extreme events is closely
related to the intermittent spatial distribution of intense flow structures
such as vorticity filaments. This mixed history of flow conditions leads to
very complex particle statistics with a pronounced scale dependence, which
presents one of the major challenges on the way to a non-equilibrium
statistical mechanics of turbulence. Here, we introduce the notion of
persistent Lagrangian acceleration, quantified by the squared particle
acceleration coarse-grained over a viscous time scale. Conditioning Lagrangian
particle data from simulations on this coarse-grained acceleration, we find
remarkably simple, close-to-Gaussian statistics for a range of Reynolds
numbers. This opens the possibility to decompose the complex particle
statistics into much simpler sub-ensembles. Based on this observation, we
develop a comprehensive theoretical framework for Lagrangian single-particle
statistics that captures the acceleration, velocity increments as well as
single-particle dispersion
Hands-on osteoporosis screening with digital x-ray radiogrammetry
According to epidemiological studies about one third of women and one fifth of men over
50 years will experience a fragility fracture. These fractures account for substantial
mortality, morbidity and health care cost. Osteoporosis is a silent disease and thus often
goes undiagnosed for a long time. Second to trauma it is considered to be the main cause of
fractures among elderly. Several methods to reduce fracture risk have been developed.
Identifying individuals with high fracture risk who would most benefit from such measures
is of utmost importance for cost-efficient fracture prevention. Dual-energy X-ray
absorptiometry (DXA) is widely considered to be the gold standard for assessing bone
mass, diagnosing osteoporosis and estimating fracture risk. However, access to DXA is
limited and not everyone in need of an examination is able to have one. Other fracture risk
prediction models have therefore been developed, e.g. questionnaire-based tools. Different
bone mass measuring devices have also been invented, e.g. qualitative ultrasound,
peripheral DXA and digital X-ray radiogrammetry (DXR). None of these methods has been
investigated nor validated as much as DXA. The aim of this study was to investigate the
clinical use of DXR, which uses hand radiographs to determine bone mass.
In paper I, we retrospectively analyzed already obtained radiographs from 8,257 patients
with DXR and found that DXR was highly predictive for hip fractures in both women and
men.
Later we recruited study participants from the Swedish mammography screening program
and sampled a prospective, population-based cohort, the so-called STOP cohort. In paper II,
the cohort was described, and it was shown that self-reported information about established
clinical risk factors for osteoporosis were significantly associated with DXR T-score.
A subset of the STOP cohort based on those with the lowest bone mass for their age (Zscore)
was studied in paper III. In this subset we found a high prevalence of DXA-verified
osteoporosis. Underlying causes for secondary osteoporosis and risk factors for primary
osteoporosis were also overrepresented.
In paper IV the STOP cohort was matched with fracture data from the Swedish National
Inpatient Register and fracture prediction with DXR-BMD with and without clinical risk
factors was examined. DXR T-score was significantly associated with hip, major
osteoporotic and any clinical fracture.
In summary DXR derived bone mass was associated with fracture risk and known clinical
risk factors for osteoporosis. Further research should focus on longer follow-up of the
STOP cohort and health economical assessments concerning potential clinical
implementation of the method
Nonuniversal power-law spectra in turbulent systems
Turbulence is generally associated with universal power-law spectra in scale
ranges without significant drive or damping. Although many examples of
turbulent systems do not exhibit such an inertial range, power-law spectra may
still be observed. As a simple model for such situations, a modified version of
the Kuramoto-Sivashinsky equation is studied. By means of semi-analytical and
numerical studies, one finds power laws with nonuniversal exponents in the
spectral range for which the ratio of nonlinear and linear time scales is
(roughly) scale-independent.Comment: 5 pages, 5 figure
Turbulence and turbulent pattern formation in a minimal model for active fluids
Active matter systems display a fascinating range of dynamical states,
including stationary patterns and turbulent phases. While the former can be
tackled with methods from the field of pattern formation, the spatio-temporal
disorder of the active turbulence phase calls for a statistical description.
Borrowing techniques from turbulence theory, we here establish a quantitative
description of correlation functions and spectra of a minimal continuum model
for active turbulence. Further exploring the parameter space, we also report on
a surprising type of turbulence-driven pattern formation far beyond linear
onset: the emergence of a dynamic hexagonal vortex lattice state after an
extended turbulent transient, which can only be explained taking into account
turbulent energy transfer across scales.Comment: Supplemental videos available at https://youtu.be/gbf6cRho03w
https://youtu.be/n0qUUhAUJFQ https://youtu.be/LGmamkM012
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