5,167 research outputs found
The 3D structure of the Lagrangian acceleration in turbulent flows
We report experimental results on the three dimensional Lagrangian
acceleration in highly turbulent flows. Tracer particles are tracked optically
using four silicon strip detectors from high energy physics that provide high
temporal and spatial resolution. The components of the acceleration are shown
to be statistically dependent. The probability density function (PDF) of the
acceleration magnitude is comparable to a log-normal distribution. Assuming
isotropy, a log-normal distribution of the magnitude can account for the
observed dependency of the components. The time dynamics of the acceleration
components is found to be typical of the dissipation scales whereas the
magnitude evolves over longer times, possibly close to the integral time scale.Comment: accepted for publication in Physical Review Letter
Measurement of Lagrangian velocity in fully developed turbulence
We have developed a new experimental technique to measure the Lagrangian
velocity of tracer particles in a turbulent flow, based on ultrasonic Doppler
tracking. This method yields a direct access to the velocity of a single
particule at a turbulent Reynolds number . Its dynamics is
analyzed with two decades of time resolution, below the Lagrangian correlation
time. We observe that the Lagrangian velocity spectrum has a Lorentzian form
, in agreement
with a Kolmogorov-like scaling in the inertial range. The probability density
function (PDF) of the velocity time increments displays a change of shape from
quasi-Gaussian a integral time scale to stretched exponential tails at the
smallest time increments. This intermittency, when measured from relative
scaling exponents of structure functions, is more pronounced than in the
Eulerian framework.Comment: 4 pages, 5 figures. to appear in PR
Fast Domain Growth through Density-Dependent Diffusion in a Driven Lattice Gas
We study electromigration in a driven diffusive lattice gas (DDLG) whose
continuous Monte Carlo dynamics generate higher particle mobility in areas with
lower particle density. At low vacancy concentrations and low temperatures,
vacancy domains tend to be faceted: the external driving force causes large
domains to move much more quickly than small ones, producing exponential domain
growth. At higher vacancy concentrations and temperatures, even small domains
have rough boundaries: velocity differences between domains are smaller, and
modest simulation times produce an average domain length scale which roughly
follows , where varies from near .55 at 50% filling
to near .75 at 70% filling. This growth is faster than the behavior
of a standard conserved order parameter Ising model. Some runs may be
approaching a scaling regime. At low fields and early times, fast growth is
delayed until the characteristic domain size reaches a crossover length which
follows . Rough numerical estimates give and simple theoretical arguments give . Our conclusion that
small driving forces can significantly enhance coarsening may be relevant to
the YBCuO electromigration experiments of Moeckly {\it et
al.}(Appl. Phys. Let., {\bf 64}, 1427 (1994)).Comment: 18 pages, RevTex3.
Improved Lagrangian mixing models for passive scalars in isotropic turbulence
Lagrangian data for velocity, scalars, and energy and scalar dissipation from direct numerical simulations are used to validate Lagrangian mixing models for inert passive scalars in stationary isotropic turbulence. The scalar fluctuations are nearly Gaussian, and, as a result of production by uniform mean gradients, statistically stationary. Comparisons are made for Taylor-scale Reynolds numbers in the range 38 to about 240 and Schmidt numbers in the range 1/8 to 1. Model predictions for one-point, one-time Eulerian statistics ~Eulerian correspondence! and one-particle, two-time Lagrangian statistics ~Lagrangian correspondence! are examined. Two scalar mixing models, namely the Lagrangian Fokker–Planck model and the Lagrangian colored-noise ~LCN! model, are proposed and written in terms of stochastic differential equations ~SDE! with specified drift and diffusion terms. Both of these models rely on statistics of the scalar field conditioned upon the energy dissipation, as provided by the Lagrangian spectral relaxation ~LSR! model. With the exception of the scalar dissipation, the models are shown to capture the Reynolds and Schmidt-number dependence of the Lagrangian integral time scales. However, the LCN model provides a more realistic description of the Lagrangian scalar fluctuations as differentiable time series having the correct form of the scalar autocorrelation function. Further extensions of the new mixing models to non-Gaussian scalars are conceptually straightforward, but require a closure for the scalar-conditioned scalar dissipation rate matrix. Likewise, accurate prediction of joint statistics for differential diffusion between different scalars with unequal molecular diffusivities will require the formulation of a multiscale SDE similar to the LSR model
Means and method of detection in chemical separation procedures
A means and method for indirect detection of constituent components of a mixture separated in a chemical separation process. Fluorescing ions are distributed across the area in which separation of the mixture will occur to provide a generally uniform background fluorescence intensity. For example, the mixture is comprised of one or more charged analytes which displace fluorescing ions where its constituent components separate to. Fluorescing ions of the same charge as the charged analyte components cause a displacement. The displacement results in the location of the separated components having a reduced fluorescence intensity to the remainder of the background. Detection of the lower fluorescence intensity areas can be visually, by photographic means and methods, or by automated laser scanning
Perceived stigmatisation of patients with mental illness and its psychosocial correlates: a prospective cohort study
Acceleration and vortex filaments in turbulence
We report recent results from a high resolution numerical study of fluid
particles transported by a fully developed turbulent flow. Single particle
trajectories were followed for a time range spanning more than three decades,
from less than a tenth of the Kolmogorov time-scale up to one large-eddy
turnover time. We present some results concerning acceleration statistics and
the statistics of trapping by vortex filaments.Comment: 10 pages, 5 figure
Phase Separation Kinetics in a Model with Order-Parameter Dependent Mobility
We present extensive results from 2-dimensional simulations of phase
separation kinetics in a model with order-parameter dependent mobility. We find
that the time-dependent structure factor exhibits dynamical scaling and the
scaling function is numerically indistinguishable from that for the
Cahn-Hilliard (CH) equation, even in the limit where surface diffusion is the
mechanism for domain growth. This supports the view that the scaling form of
the structure factor is "universal" and leads us to question the conventional
wisdom that an accurate representation of the scaled structure factor for the
CH equation can only be obtained from a theory which correctly models bulk
diffusion.Comment: To appear in PRE, figures available on reques
Domain Growth in a 1-D Driven Diffusive System
The low-temperature coarsening dynamics of a one-dimensional Ising model,
with conserved magnetisation and subject to a small external driving force, is
studied analytically in the limit where the volume fraction \mu of the minority
phase is small, and numerically for general \mu. The mean domain size L(t)
grows as t^{1/2} in all cases, and the domain-size distribution for domains of
one sign is very well described by the form P_l(l) \propto
(l/L^3)\exp[-\lambda(\mu)(l^2/L^2)], which is exact for small \mu (and possibly
for all \mu). The persistence exponent for the minority phase has the value 3/2
for \mu \to 0.Comment: 8 pages, REVTeX, 7 Postscript figures, uses multicol.sty and
epsf.sty. Submitted to Phys. Rev.
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