93,990 research outputs found
An experimental study of a self-confined flow with ring-vorticity distribution
A new form of self-confined flow was investigated in which a recirculation zone forms away from any solid boundary. An inviscid flow analysis indicated that in a purely meridional axisymmetric flow a stationary, spherical, self-confined region should occur in the center of a streamlined divergent-convergent enlargement zone. The spherical confinement region would be at rest and at constant pressure. Experimental investigations were carried out in a specially built test apparatus to establish the desired confined flow. The streamlined divergent-convergent interior shape of the test section was fabricated according to the theoretical calculation for a particular streamline. The required inlet vorticity distribution was generated by producing a velocity profile with a shaped gauze screen in the straight pipe upstream of the test section. Fluid speed and turbulence intensity were measured with a constant-temperature hot-wire anemometer system. The measured results indicated a very orderly and stable flow field
Towards Long-endurance Flight: Design and Implementation of a Variable-pitch Gasoline-engine Quadrotor
Majority of today's fixed-pitch, electric-power quadrotors have short flight
endurance ( 1 hour) which greatly limits their applications. This paper
presents a design methodology for the construction of a long-endurance
quadrotor using variable-pitch rotors and a gasoline-engine. The methodology
consists of three aspects. Firstly, the rotor blades and gasoline engine are
selected as a pair, so that sufficient lift can be comfortably provided by the
engine. Secondly, drivetrain and airframe are designed. Major challenges
include airframe vibration minimization and power transmission from one engine
to four rotors while keeping alternate rotors contra-rotating. Lastly, a PD
controller is tuned to facilitate preliminary flight tests. The methodology has
been verified by the construction and successful flight of our gasoline
quadrotor prototype, which is designed to have a flight time of 2 to 3 hours
and a maximum take-off weight of 10 kg.Comment: 6 page
Single-particle Excitation Spectra of C Molecules and Monolayers
In this paper we present calculations of single-particle excitation spectra
of neutral and three-electron-doped Hubbard C molecules and monolayers
from large-scale quantum Monte Carlo simulations and cluster perturbation
theory. By a comparison to experimental photoemission, inverse photoemission,
and angle-resolved photoemission data, we estimate the intermolecular hopping
integrals and the C molecular orientation angle, finding agreement with
recent X-ray photoelectron diffraction (XPD) experiments. Our results
demonstrate that a simple effective Hubbard model, with intermediate coupling,
, provides a reasonable basis for modeling the properties of C
compounds.Comment: 6 page
An MHD Model For Magnetar Giant Flares
Giant flares on soft gamma-ray repeaters that are thought to take place on
magnetars release enormous energy in a short time interval. Their power can be
explained by catastrophic instabilities occurring in the magnetic field
configuration and the subsequent magnetic reconnection. By analogy with the
coronal mass ejection (CME) events on the Sun, we develop a theoretical model
via an analytic approach for magnetar giant flares. In this model, the rotation
and/or displacement of the crust causes the field to twist and deform, leading
to flux rope formation in the magnetosphere and energy accumulation in the
related configuration. When the energy and helicity stored in the configuration
reach a threshold, the system loses its equilibrium, the flux rope is ejected
outward in a catastrophic way, and magnetic reconnection helps the catastrophe
develop to a plausible eruption. By taking SGR 1806 - 20 as an example, we
calculate the free magnetic energy released in such an eruptive process and
find that it is more than ergs, which is enough to power a giant
flare. The released free magnetic energy is converted into radiative energy,
kinetic energy and gravitational energy of the flux rope. We calculated the
light curves of the eruptive processes for the giant flares of SGR 1806 - 20,
SGR 0526-66 and SGR 1900+14, and compared them with the observational data. The
calculated light curves are in good agreement with the observed light curves of
giant flares.Comment: Accepted to Ap
Calibration and Irradiation Study of the BGO Background Monitor for the BEAST II Experiment
Beam commissioning of the SuperKEKB collider began in 2016. The Beam Exorcism
for A STable experiment II (BEAST II) project is particularly designed to
measure the beam backgrounds around the interaction point of the SuperKEKB
collider for the Belle II experiment. We develop a system using bismuth
germanium oxide (BGO) crystals with optical fibers connecting to a multianode
photomultiplier tube (MAPMT) and a field-programmable gate array (FPGA)
embedded readout board for monitoring the real-time beam backgrounds in BEAST
II. The overall radiation sensitivity of this system is estimated to be
Gy/ADU (analog-to-digital unit) with the standard
10 m fibers for transmission and the MAPMT operating at 700 V. Our -ray
irradiation study of the BGO system shows that the exposure of BGO crystals to
Co -ray doses of 1 krad has led to immediate light output
reductions of 25--40%, and the light outputs further drop by 30--45% after the
crystals receive doses of 2--4 krad. Our findings agree with those of the
previous studies on the radiation hard (RH) BGO crystals grown by the low
thermal gradient Czochralski (LTG Cz) technology. The absolute dose from the
BGO system is also consistent with the simulation, and is estimated to be about
1.18 times the equivalent dose. These results prove that the BGO system is able
to monitor the background dose rate in real time under extreme high radiation
conditions. This study concludes that the BGO system is reliable for the beam
background study in BEAST II
-Particle Spectrum in the Reaction p+B
Using a simple phenomenological parametrization of the reaction amplitude we
calculated -particle spectrum in the reaction p+B at the resonance proton energy 675 KeV. The parametrization
includes Breit-Wigner factor with an energy dependent width for intermediate
state and the Coulomb and the centrifugal factors in -particle
emission vertexes. The shape of the spectrum consists of a well defined peak
corresponding to emission of the primary and a flat shoulder going
down to very low energy. We found that below 1.5 MeV there are 17.5% of
's and below 1 MeV there are 11% of them.Comment: 6 pages, 3 figure
Phase transitions in exactly solvable decorated model of localized Ising spins and itinerant electrons
A hybrid lattice-statistical model of doubly decorated two-dimensional
lattices, which have localized Ising spins at its nodal sites and itinerant
electrons delocalized over decorating sites, is exactly solved with the help of
a generalized decoration-iteration transformation. Under the assumption of a
quarter filling of each couple of the decorating sites, the ground state
constitutes either spontaneously long-range ordered ferromagnetic or
ferrimagnetic phase in dependence on whether the ferromagnetic or
antiferromagnetic interaction between the localized Ising spins and itinerant
electrons is considered. The critical temperature of the spontaneously
long-range ordered phases monotonically increases upon strengthening the ratio
between the kinetic term and the Ising-type exchange interaction.Comment: 4 pages, 3 figures, presented at International Conference on
Magnetism 2009 to be held on July 26-31 in Karlsruhe, Germany. submitted to
J. Phys.: Conf. Se
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