7,224 research outputs found
Laboratory studies of the roughness and suspended load of alluvial streams
This report describes research work done under Contract No. DA-25-075-eng-3866 with the U. S. Army, Corps of Engineers, Missouri River Division, Omaha, during the period 1954-1957, on problems of suspended load transport in alluvial streams.
A total of 94 experimental runs were made in two laboratory flumes charged with fine sand of several size distributions. Special attention was given to the variation of the friction factor caused by the changing bed configuration and the damping effect of suspended sediment. The relationship between the sediment transportation rate and the hydraulic variables was also investigated. Most of the runs (General Studies, Chap. V) were made with the bed of the flume completely covered with loose sand but some special runs (Special Studies, Chap. VII) were made with the sand bed chemically solidified in place to prevent sediment transport while preserving the bed configuration previously generated by a natural flow of the same velocity with loose sand. The principal laboratory results are as follows:
1. The friction factor f for a stream with a movable sand bed may vary several fold, being highest at low or medium flow velocities and lowest at high velocity.
2. The principal cause of the variation in f is the appearance of dunes at low or medium velocities and disappearance at high velocities.
3. A secondary cause for the reduction in f for high sediment transport rates is the damping effect of the suspended sediment on the turbulence, and the concomitant reduction in the turbulent diffusion coefficients. The maximum observed reduction due directly to the sediment load was only about 28 percent.
4. The discharge and sediment transportation rate are not unique functions of depth and slope because of the variable roughness. Slope (or shear) must probably be considered a dependent variable for alluvial streams because several equilibrium flows can yield the same slope and shear stress.
The laboratory data are compared with similar data for natural streams, and the most promising existing analyses for roughness and sediment load are discussed in the light of the present findings. In addition, a critical review of early and recent literature on the resistance of sediment-laden streams is presented in Chapter II
On the occurrence and detectability of Bose-Einstein condensation in helium white dwarfs
It has been recently proposed that helium white dwarfs may provide promising
conditions for the occurrence of the Bose-Einstein condensation. The argument
supporting this expectation is that in some conditions attained in the core of
these objects, the typical De Broglie wavelength associated with helium nuclei
is of the order of the mean distance between neighboring nuclei. In these
conditions the system should depart from classical behavior showing quantum
effects. As helium nuclei are bosons, they are expected to condense.
In order to explore the possibility of detecting the Bose-Einstein
condensation in the evolution of helium white dwarfs we have computed a set of
models for a variety of stellar masses and values of the condensation
temperature. We do not perform a detailed treatment of the condensation process
but mimic it by suppressing the nuclei contribution to the equation of state by
applying an adequate function. As the cooling of white dwarfs depends on
average properties of the whole stellar interior, this procedure should be
suitable for exploring the departure of the cooling process from that predicted
by the standard treatment.
We find that the Bose-Einstein condensation has noticeable, but not dramatic
effects on the cooling process only for the most massive white dwarfs
compatible with a helium dominated interior (\approx 0.50 M_\odot) and very low
luminosities (say, Log(L/L_\odot) < -4.0). These facts lead us to conclude that
it seems extremely difficult to find observable signals of the Bose-Einstein
condensation.
Recently, it has been suggested that the population of helium white dwarfs
detected in the globular cluster NGC 6397 is a good candidate for detecting
signals of the Bose-Einstein condensation. We find that these stars have masses
too low and are too bright to have an already condensed interior.Comment: 13 pages, 3 figures, 2 tables, accepted for publication in the
Journal of Cosmology and Astroparticle Physics (JCAP
An evolutionary model for the gamma-ray system PSR J1311-3430 and its companion
The most recent member of the millisecond pulsar with very low-mass
companions and short orbital periods class, PSR J1311-3430 (Pletsch et al.
2012) is a remarkable object in various senses. Besides being the first
discovered in gamma-rays, its measured features include the very low or absent
hydrogen content. We show in this Letter that this important piece of
information leads to a very restricted range of initial periods for a given
donor mass. For that purpose, we calculate in detail the evolution of the
binary system self-consistently, including mass transfer and evaporation,
finding the features of the new evolutionary path leading to the observed
configuration. It is also important to remark that the detailed evolutionary
history of the system naturally leads to a high final pulsar mass, as it seems
to be demanded by observations.Comment: 5 pages, 5 figures, 1 table. Accepted for publication in MNRAS
Letter
Scale Invariant Interest Points with Shearlets
Shearlets are a relatively new directional multi-scale framework for signal
analysis, which have been shown effective to enhance signal discontinuities
such as edges and corners at multiple scales. In this work we address the
problem of detecting and describing blob-like features in the shearlets
framework. We derive a measure which is very effective for blob detection and
closely related to the Laplacian of Gaussian. We demonstrate the measure
satisfies the perfect scale invariance property in the continuous case. In the
discrete setting, we derive algorithms for blob detection and keypoint
description. Finally, we provide qualitative justifications of our findings as
well as a quantitative evaluation on benchmark data. We also report an
experimental evidence that our method is very suitable to deal with compressed
and noisy images, thanks to the sparsity property of shearlets
Positive operator valued measures covariant with respect to an irreducible representation
Given an irreducible representation of a group G, we show that all the
covariant positive operator valued measures based on G/Z, where Z is a central
subgroup, are described by trace class, trace one positive operators.Comment: 9 pages, Latex2
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