298 research outputs found
Modelling of turbulent jets and wall layers: extensions of Lighthill's acoustic analogy with application to computational aeroacoustics
Two extensions to Lighthill’s aeroacoustic analogy are presented. First, equivalent sources due to initial conditions are derived that supplement those due to boundary conditions, as given by Ffowcs Williams & Hawkings. The resulting exact inhomogeneous wave equation is then reformulated with pressure rather than density as the wave variable, and the right-hand side is rearranged using the energy equation with no additional assumptions. Applications to computational aeroacoustics are discussed, and illustrated with examples based on 2D and 3D simulations
Analysis of data on the relation between eddies and streaky structures in turbulent flows using the placebo method
An artificially synthesized velocity field with known properties is used as a test data set in analyzing and interpreting the turbulent flow velocity fields. The objective nature of this approach is utilized for studying the relation between streaky and eddy structures. An analysis shows that this relation may be less significant than is customarily supposed
Influence of casting temperature on the thermal stability of Cu- and Zr-based metallic glasses: theoretical analysis and experiments
Influence of casting temperature on the thermal stability of Cu- and Zr-based metallic glasses (MGs) was analyzed based on the monomer-cluster structural model using the Johnson-Mehl-Avrami (JMA) equation. The result indicates that increasing the casting temperature can enhance the thermal stability of MGs. It is suggested that it be attributed to the decrease in the amount of the local ordering clusters induced by the elevating casting temperature. The prediction is confirmed by continuous heating transformation diagrams constructed for the Cu- and Zr-amorphous samples obtained under different casting temperatures
Spin polarized tunneling in ferromagnet/unconventional superconductor junctions
We study tunneling in ferromagnet/unconventional superconductor (F/S)
junctions. We include the effects of spin polarization, interfacial resistance,
and Fermi wavevector mismatch (FWM) between the F and S regions. Andreev
reflection (AR) at the F/S interface, governing tunneling at low bias voltage,
is strongly modified by these parameters. The conductance exhibits a very wide
variety of features as a function of applied voltage.Comment: Revision includes new figures with angular averages and correction of
minor error
Spin-Polarized Transport Across an LaSrMnO/YBaCuO Interface: Role of Andreev Bound States
Transport across an
LaSr_{3}/YBa_{3}_{7}_{3}$/YBCO and Ag/YBCO. In all cases, YBCO is used as bottom layer to
eliminate the channel resistance and to minimize thermal effects. The observed
differential conductance re ects the role of Andreev bound states in a-b
planes, and brings out for the first time the suppression of such states by the
spin-polarized transport across the interface. The theoretical analysis of the
measured data reveals decay of the spin polarization near the LSMO surface with
temperature, consistent with the reported photoemission data.Comment: 5 pages LaTeX, 3 eps figures included, accepted by Physical Review
The Vacuum System of HIRFL
AbstractThe vacuum system of Heavy Ion Research Facility in Lanzhou (HIRFL) is a large and complex system. HIRFL consists of two ECR ion sources, a sector focus cyclotron (SFC), a separate sector cyclotron (SSC) and a multi-purpose cooling storage ring system which has a main ring (CSRm) and an experiment ring (CSRe). Several beam lines connect these accelerators together and transfer various heavy ion beams to more than 10 experiment terminals. According to the requirements of the ion acceleration and ion lifetime, the working pressure in each accelerator is different. SFC is nearly 50 years old. After upgrade, the working pressure in SFC is improved from 10-6mbar to 10-8mbar. The pressure in SSC which was built in the 1980s reaches the same level. The cooling storage ring system with a length of 500m came into operation in 2007. The average pressures in CSRm and CSRe are 5×10-12mbar and 8×10-12mbar respectively. Different designs were adopt for vacuum system of a dozen beam lines to meet specific requirement of each experiment terminal. Along with the extensive development of the heavy ion researches and applications, new accelerators of HIRFL are under construction. The vacuum system of the new machines will be designed and constructed followed the overall schedule
An improved version of Inverse Distance Weighting metamodel assisted Harmony Search algorithm for truss design optimization
Contributions to the cross shock electric field at supercritical perpendicular shocks: Impact of the pickup ions
A particle-in-cell code is used to examine contributions of the pickup ions
(PIs) and the solar wind ions (SWs) to the cross shock electric field at the
supercritical, perpendicular shocks. The code treats the pickup ions
self-consistently as a third component. Herein, two different runs with
relative pickup ion density of 25% and 55% are presented in this paper. Present
preliminary results show that: (1) in the low percentage (25%) pickup ion case,
the shock front is nonstationary. During the evolution of this perpendicular
shock, a nonstationary foot resulting from the reflected solar wind ions is
formed in front of the old ramp, and its amplitude becomes larger and larger.
At last, the nonstationary foot grows up into a new ramp and exceeds the old
one. Such a nonstationary process can be formed periodically. hen the new ramp
begins to be formed in front of the old ramp, the Hall term mainly contributed
by the solar wind ions becomes more and more important. The electric field Ex
is dominated by the Hall term when the new ramp exceeds the old one.
Furthermore, an extended and stationary foot in pickup ion gyro-scale is
located upstream of the nonstationary/self-reforming region within the shock
front, and is always dominated by the Lorentz term contributed by the pickup
ions; (2) in the high percentage (55%) pickup ion case, the amplitude of the
stationary foot is increased as expected. One striking point is that the
nonstationary region of the shock front evidenced by the self-reformation
disappears. Instead, a stationary extended foot dominated by Lorentz term
contributed by the pickup ions, and a tationary ramp dominated by Hall term
contributed by the solar wind ions are clearly evidenced. The significance of
the cross electric field on ion dynamics is also discussed.Comment: 11 pages, 6 figs and 1 table. This paper will be published in the
journal: Astrophysics and Space Scienc
Mapping of periodically poled crystals via spontaneous parametric down-conversion
A new method for characterization of periodically poled crystals is developed
based on spontaneous parametric down-conversion. The method is demonstrated on
crystals of Y:LiNbO3, Mg:Y:LiNbO3 with non-uniform periodically poled
structures, obtained directly under Czochralski growth procedure and designed
for application of OPO in the mid infrared range. Infrared dispersion of
refractive index, effective working periods and wavelengths of OPO were
determined by special treatment of frequency-angular spectra of spontaneous
parametric down-conversion in the visible range. Two-dimensional mapping via
spontaneous parametric down-conversion is proposed for characterizing spatial
distribution of bulk quasi-phase matching efficiency across the input window of
a periodically poled sample.Comment: 19 pages, 6 figure
k=0Magnetic Structure and Absence of Ferroelectricity in SmFeO3
SmFeO3 has attracted considerable attention very recently due to the reported
multiferroic properties above room-temperature. We have performed powder and
single crystal neutron diffraction as well as complementary polarization
dependent soft X-ray absorption spectroscopy measurements on floating-zone
grown SmFeO3 single crystals in order to determine its magnetic structure. We
found a k=0 G-type collinear antiferromagnetic structure that is not compatible
with inverse Dzyaloshinskii-Moriya interaction driven ferroelectricity. While
the structural data reveals a clear sign for magneto-elastic coupling at the
N\'eel-temperature of ~675 K, the dielectric measurements remain silent as far
as ferroelectricity is concerned
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