18,307 research outputs found
Direct Numerical Simulation of a separated channel flow with a smooth profile
A direct numerical simulation (DNS) of a channel flow with one curved surface
was performed at moderate Reynolds number (Re_tau = 395 at the inlet). The
adverse pressure gradient was obtained by a wall curvature through a
mathematical mapping from physical coordinates to Cartesian ones. The code,
using spectral spanwise and normal discretization, combines the advantage of a
good accuracy with a fast integration procedure compared to standard numerical
procedures for complex geometries. The turbulent flow slightly separates on the
profile at the lower curved wall and is at the onset of separation at the
opposite flat wall. The thin separation bubble is characterized with a reversal
flow fraction. Intense vortices are generated near the separation line on the
lower wall but also at the upper wall. Turbulent normal stresses and kinetic
energy budget are investigated along the channel.Comment: 23 pages, submitted to Journal of Turbulenc
Bound States in the Continuum Realized in the One-Dimensional Two-Particle Hubbard Model with an Impurity
We report a bound state of the one-dimensional two-particle (bosonic or
fermionic) Hubbard model with an impurity potential. This state has the
Bethe-ansatz form, although the model is nonintegrable. Moreover, for a wide
region in parameter space, its energy is located in the continuum band. A
remarkable advantage of this state with respect to similar states in other
systems is the simple analytical form of the wave function and eigenvalue. This
state can be tuned in and out of the continuum continuously.Comment: A semi-exactly solvable model (half of the eigenstates are in the
Bethe form
Evaluation of Bacillus thuringiensis Berliner as an alternative control of small hive beetles, Aethina tumida Murray (Coleoptera: Nitidulidae)
Small hive beetles, Aethina tumida Murray, are parasites and scavengers of honeybee colonies, Apis mellifera L., and have become an invasive species that can cause considerable damage in its new distribution areas. An effective subspecies of Bacillus thuringiensis Berliner (=Bt) would provide an alternative to chemical control of this pest. Therefore, we tested three different Bt strains [B. thuringiensis, var. aizawai (B401®), B. thuringiensis var. kurstaki (Novodor®) and B. thuringiensis var. San Diego tenebrionis (Jackpot®)] and Perizin® (3.2% coumaphos), each applied on combs with a pollen diet fed to pairs of adult beetles. This evaluates the products for the suppression of successful small hive beetle reproduction. While none of the tested Bt strains showed a significant effect on the number of produced wandering larvae, we could confirm the efficacy of coumaphos for the control of small hive beetles. We further show that it is also efficient when applied with a lower concentration as a liquid on the combs. We suggest the continued search for efficient Bt strains naturally infesting small hive beetles in its endemic and new ranges, which may become a part of the integrated management of this pest
An exact Riemann solver based solution for regular shock refraction
We study the classical problem of planar shock refraction at an oblique
density discontinuity, separating two gases at rest. When the shock impinges on
the density discontinuity, it refracts and in the hydrodynamical case 3 signals
arise. Regular refraction means that these signals meet at a single point,
called the triple point.
After reflection from the top wall, the contact discontinuity becomes
unstable due to local Kelvin-Helmholtz instability, causing the contact surface
to roll up and develop the Richtmyer-Meshkov instability. We present an exact
Riemann solver based solution strategy to describe the initial self similar
refraction phase, by which we can quantify the vorticity deposited on the
contact interface. We investigate the effect of a perpendicular magnetic field
and quantify how addition of a perpendicular magnetic field increases the
deposition of vorticity on the contact interface slightly under constant Atwood
number. We predict wave pattern transitions, in agreement with experiments, von
Neumann shock refraction theory, and numerical simulations performed with the
grid-adaptive code AMRVAC. These simulations also describe the later phase of
the Richtmyer-Meshkov instability.Comment: 21 pages, 17 figures in 41 ps-files, accepted by J. Fluid Mec
Performance of a centrifugal pump running in inverse mode
This paper presents the functional characterization of a centrifugal pump used as a turbine. It shows the characteristics of the machine involved at several rotational speeds, comparing the respective flows and heads. In this way, it is possible to observe the influence of the rotational speed on efficiency, as well as obtaining the characteristics at constant head and runaway speed. Also, the forces actuating on the impeller were studied. An uncertainty analysis was made to assess the accuracy of the results. The research results indicate that the turbine characteristics can be predicted to some extent from the pump characteristics, that water flows out of the runner free of swirl flow at the best efficiency point, and that radial stresses are lower than in pump mode
Mass-Temperature Relation of Galaxy Clusters: A Theoretical Study
Combining conservation of energy throughout nearly-spherical collapse of
galaxy clusters with the virial theorem, we derive the mass-temperature
relation for X-ray clusters of galaxies . The normalization factor
and the scatter of the relation are determined from first principles with
the additional assumption of initial Gaussian random field. We are also able to
reproduce the recently observed break in the M-T relation at T \sim 3 \keV,
based on the scatter in the underlying density field for a low density
CDM cosmology. Finally, by combining observational data of high
redshift clusters with our theoretical formalism, we find a semi-empirical
temperature-mass relation which is expected to hold at redshifts up to unity
with less than 20% error.Comment: 43 pages, 13 figures, One figure is added and minor changes are made.
Accepted for Publication in Ap
Continuous measurements in a composite quantum system and possible exchange of information between its parts
We study an influence of the continuous measurement in a composite quantum
system C on the evolution of the states of its parts. It is shown that the
character of the evolution (decoherence or recoherence) depends on the type of
the measured quantity and on the initial state of the system. A number of
conditions under which the states of the subsystems of C decohere during the
measuring process are established. We propose a model of the composite system
and specify the observable the measurement of which may result in the
recoherence of the state of one of the subsystems of C. In the framework of
this model we find the optimal regime for the exchange of information between
the parts of C during the measurement. The main characteristics of such a
process are computed. We propose a scheme of detection of the recoherence under
the measurement in a concrete physical experiment.Comment: 6 page
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