45,319 research outputs found
Decay of weak turbulence
Weak turbulence fields generated by single and multiple stage grids covering Reynolds numbers between 7 and 70 showing decay of energy spectr
Prototype selection for parameter estimation in complex models
Parameter estimation in astrophysics often requires the use of complex
physical models. In this paper we study the problem of estimating the
parameters that describe star formation history (SFH) in galaxies. Here,
high-dimensional spectral data from galaxies are appropriately modeled as
linear combinations of physical components, called simple stellar populations
(SSPs), plus some nonlinear distortions. Theoretical data for each SSP is
produced for a fixed parameter vector via computer modeling. Though the
parameters that define each SSP are continuous, optimizing the signal model
over a large set of SSPs on a fine parameter grid is computationally infeasible
and inefficient. The goal of this study is to estimate the set of parameters
that describes the SFH of each galaxy. These target parameters, such as the
average ages and chemical compositions of the galaxy's stellar populations, are
derived from the SSP parameters and the component weights in the signal model.
Here, we introduce a principled approach of choosing a small basis of SSP
prototypes for SFH parameter estimation. The basic idea is to quantize the
vector space and effective support of the model components. In addition to
greater computational efficiency, we achieve better estimates of the SFH target
parameters. In simulations, our proposed quantization method obtains a
substantial improvement in estimating the target parameters over the common
method of employing a parameter grid. Sparse coding techniques are not
appropriate for this problem without proper constraints, while constrained
sparse coding methods perform poorly for parameter estimation because their
objective is signal reconstruction, not estimation of the target parameters.Comment: Published in at http://dx.doi.org/10.1214/11-AOAS500 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Combinatorial Gradient Fields for 2D Images with Empirically Convergent Separatrices
This paper proposes an efficient probabilistic method that computes
combinatorial gradient fields for two dimensional image data. In contrast to
existing algorithms, this approach yields a geometric Morse-Smale complex that
converges almost surely to its continuous counterpart when the image resolution
is increased. This approach is motivated using basic ideas from probability
theory and builds upon an algorithm from discrete Morse theory with a strong
mathematical foundation. While a formal proof is only hinted at, we do provide
a thorough numerical evaluation of our method and compare it to established
algorithms.Comment: 17 pages, 7 figure
Tangling clustering of inertial particles in stably stratified turbulence
We have predicted theoretically and detected in laboratory experiments a new
type of particle clustering (tangling clustering of inertial particles) in a
stably stratified turbulence with imposed mean vertical temperature gradient.
In this stratified turbulence a spatial distribution of the mean particle
number density is nonuniform due to the phenomenon of turbulent thermal
diffusion, that results in formation of a gradient of the mean particle number
density, \nabla N, and generation of fluctuations of the particle number
density by tangling of the gradient, \nabla N, by velocity fluctuations. The
mean temperature gradient, \nabla T, produces the temperature fluctuations by
tangling of the gradient, \nabla T, by velocity fluctuations. These
fluctuations increase the rate of formation of the particle clusters in small
scales. In the laboratory stratified turbulence this tangling clustering is
much more effective than a pure inertial clustering that has been observed in
isothermal turbulence. In particular, in our experiments in oscillating grid
isothermal turbulence in air without imposed mean temperature gradient, the
inertial clustering is very weak for solid particles with the diameter 10
microns and Reynolds numbers Re =250. Our theoretical predictions are in a good
agreement with the obtained experimental results.Comment: 16 pages, 4 figures, REVTEX4, revised versio
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