5,858 research outputs found
Research on the drag reduction performance induced by the counterflowing jet for waverider with variable blunt radii
Waverider will endure the huge aero-heating in the hypersonic flow, thus, it need be blunt for the leading edge. However, the aerodynamic performance will decrease for the blunt waverider because of the drag hoik. How to improve the aerodynamic performance and reduce the drag and aero-heating is very important. The variable blunt radii method will improve the aerodynamic performance, however, the huge aero-heating and bow shock wave at the head is still serious. In the current study, opposing jet is used in the waverider with variable blunt radii to improve its performance. The three-dimensional coupled implicit Reynolds-averaged Navier-Stokes(RANS) equation and the two equation SST k–ω turbulence model have been utilized to obtain the flow field properties. The numerical method has been validated against the available experimental data in the open literature. The obtained results show that the L/D will drop 7–8% when R changes from 2 to 8. The lift coefficient will increase, and the drag coefficient almost keeps the same when the variable blunt radii method is adopted, and the L/D will increase. The variable blunt radii method is very useful to improve the whole characteristics of blunt waverider and the L/D can improve 3%. The combination of the variable blunt radii method and opposing jet is a novel way to improve the whole performance of blunt waverider, and L/D can improve 4–5%. The aperture as a novel way of opposing jet is suitable for blunt waverider and also useful to improve the aerodynamic and aerothermodynamic characteristics of waverider in the hypersonic flow. There is the optimal P0in/P0 that can make the detached shock wave reattach the lower surface again so that the blunt waverider can get the better aerodynamic performance
How Massless Neutrinos Affect the Cosmic Microwave Background Damping Tail
We explore the physical origin and robustness of constraints on the energy
density in relativistic species prior to and during recombination, often
expressed as constraints on an effective number of neutrino species, Neff.
Constraints from current data combination of Wilkinson Microwave Anisotropy
Probe (WMAP) and South Pole Telescope (SPT) are almost entirely due to the
impact of the neutrinos on the expansion rate, and how those changes to the
expansion rate alter the ratio of the photon diffusion scale to the sound
horizon scale at recombination. We demonstrate that very little of the
constraining power comes from the early Integrated Sachs-Wolfe (ISW) effect,
and also provide a first determination of the amplitude of the early ISW
effect. Varying the fraction of baryonic mass in Helium, Yp, also changes the
ratio of damping to sound-horizon scales. We discuss the physical effects that
prevent the resulting near-degeneracy between Neff and Yp from being a complete
one. Examining light element abundance measurements, we see no significant
evidence for evolution of Neff and the baryon-to-photon ratio from the epoch of
big bang nucleosynthesis to decoupling. Finally, we consider measurements of
the distance-redshift relation at low to intermediate redshifts and their
implications for the value of Neff.Comment: 11 pages. Replaced version extends our discussion of origin of
constraints and updates for current data, submitted to PR
and the Higgs mass from high scale supersymmetry
In the framework in which supersymmetry is used for understanding fermion
masses rather than stabilizing the electroweak scale, we elaborate the
phenomenological analysis for the neutrino physics. A relatively large
is the natural result. The model further predicts
vanishingly small CP violation in neutrino oscillations. And is
not necessarily maximal. While the high scale supersymmetry generically results
in a Higgs mass of about 141 GeV, our model reduces this mass via introducing
SU(2) triplet fields which also contribute to neutrino masses.Comment: 13 pages, no figure, revtex4, revised versio
\u3ci\u3eAb initio\u3c/i\u3e insight into the electrolysis of water on basal and edge (fullerene C\u3csub\u3e20\u3c/sub\u3e) surfaces of 4 Å single-walled carbon nanotubes
The extreme surface reactivity of 4 Å single-walled carbon nanotubes (SWCNTs) makes for a very promising catalytic material, however, controlling it experimentally has been found to be challenging. Here, we employ ab initio calculations to investigate the extent of surface reactivity and functionalization of 4 Å SWCNTs. We study the kinetics of water dissociation and adsorption on the surface of 4 Å SWCNTs with three different configurations: armchair (3,3), chiral (4,2) and zigzag (5,0). We reveal that out of three different configurations of 4 Å SWCNTs, the surface of tube (5,0) is the most reactive due to its small HOMO–LUMO gap. The dissociation of 1 H2O molecule into an OH/H pair on the surface of tube (5,0) has an adsorption energy of −0.43 eV and an activation energy barrier of 0.66 eV at 298.15 K in pure aqueous solution, which is less than 10% of the activation energy barrier of the same reaction without the catalyst present. The four steps of H+/e− transfer in the oxygen evolution reaction have also been studied on the surface of tube (5,0). The low overpotential of 0.38 V indicates that tube (5,0) has the highest potential efficiency among all studied carbon-based catalysts. We also reveal that the armchair edge of tube (5,0) is reconstructed into fullerene C20. The dangling bonds on the surface of fullerene C20 result in a more reactive surface than the basal surface of tube (5,0), however the catalytic ability was also inhibited in the later oxygen reduction processes
Time-Dependent Spin-Polarized Transport Through a Resonant Tunneling Structure with Multi-Terminal
The spin-dependent transport of the electrons tunneling through a resonant
tunneling structure with ferromagnetic multi-terminal under dc and ac fields is
explored by means of the nonequilibrium Green function technique. A general
formulation for the time-dependent current and the time-averaged current is
established. As its application the systems with two and three terminals in
noncollinear configurations of the magnetizations under dc and ac biases are
investigated, respectively. The asymmetric factor of the relaxation times for
the electrons with different spin in the central region is uncovered to bring
about various behaviours of the TMR. The present three-terminal device is
different from that discussed in literature, which is coined as a spin
transistor with source. The current-amplification effect is found. In addition,
the time-dependent spin transport for the two-terminal device is studied. It is
found that the photonic sidebands provide new channels for the electrons
tunneling through the barriers, and give rise to new resonances of the TMR,
which is called as the photon-asisted spin-dependent tunneling. The asymmetric
factor of the relaxation times is observed to lead to additional resonant peaks
besides the photon-asisted resonances.Comment: 32 pages,14 figure
5 GHz TMRT observations of 71 pulsars
We present integrated pulse profiles at 5~GHz for 71 pulsars, including eight
millisecond pulsars (MSPs), obtained using the Shanghai Tian Ma Radio Telescope
(TMRT). Mean flux densities and pulse widths are measured. For 19 normal
pulsars and one MSP, these are the first detections at 5~GHz and for a further
19, including five MPSs, the profiles have a better signal-to-noise ratio than
previous observations. Mean flux density spectra between 400~MHz and 9~GHz are
presented for 27 pulsars and correlations of power-law spectral index are found
with characteristic age, radio pseudo-luminosity and spin-down luminosity. Mode
changing was detected in five pulsars. The separation between the main pulse
and interpulse is shown to be frequency independent for six pulsars but a
frequency dependence of the relative intensity of the main pulse and interpulse
is found. The frequency dependence of component separations is investigated for
20 pulsars and three groups are found: in seven cases the separation between
the outmost leading and trailing components decreases with frequency, roughly
in agreement with radius-to-frequency mapping; in eleven cases the separation
is nearly constant; in the remain two cases the separation between the outmost
components increases with frequency. We obtain the correlations of pulse widths
with pulsar period and estimate the core widths of 23 multi-component profiles
and conal widths of 17 multi-component profiles at 5.0~GHz using Gaussian
fitting and discuss the width-period relationship at 5~GHz compared with the
results at at 1.0~GHz and 8.6~GHz.Comment: 46 pages, 14 figures, 8 Tables, accepted by Ap
Spatiotemporal Fluctuation Induced Transition in a Tumor Model with Immune Surveillance
We report on a simple model of spatial extend anti-tumor system with a
fluctuation in growth rate, which can undergo a nonequilibrium phase
transition. Three states as excited, sub-excited and non-excited states of a
tumor are defined to describe its growth. The multiplicative noise is found to
be double-face: The positive effect on a non-excited tumor and the negative
effect on an excited tumor.Comment: 8pages,5figure
Meta-Learning with Variational Semantic Memory for Word Sense Disambiguation
A critical challenge faced by supervised word sense disambiguation (WSD) is
the lack of large annotated datasets with sufficient coverage of words in their
diversity of senses. This inspired recent research on few-shot WSD using
meta-learning. While such work has successfully applied meta-learning to learn
new word senses from very few examples, its performance still lags behind its
fully supervised counterpart. Aiming to further close this gap, we propose a
model of semantic memory for WSD in a meta-learning setting. Semantic memory
encapsulates prior experiences seen throughout the lifetime of the model, which
aids better generalization in limited data settings. Our model is based on
hierarchical variational inference and incorporates an adaptive memory update
rule via a hypernetwork. We show our model advances the state of the art in
few-shot WSD, supports effective learning in extremely data scarce (e.g.
one-shot) scenarios and produces meaning prototypes that capture similar senses
of distinct words.Comment: 15 pages, 5 figure
The single t-quark productions via the flavor-changing processes in the topcolor-assisted technicolor model at the hadron colliders
In the framework of topcolor-assisted technicolor(TC2) model, there exist
tree-level flavor-changing (FC) couplings which can result in the loop-level FC
coupling . Such coupling can contribute significant clues at the
forthcoming Large Hadron Collider (LHC) experiments. In this paper, based on
the TC2 model, we study some single t-quark production processes involving
coupling at the Tevatron and LHC: .
We calculate the cross sections of these processes. The results show that the
cross sections at the Tevatron are too small to observe the signal, but at the
LHC it can reach a few pb. With the high luminosity, the LHC has considerable
capability to find the single t-quark signal produced via some FC processes
involving coupling . On the other hand, these processes can also provide
some valuable information of the coupling with detailed study of the
processes and furthermore provide the reliable evidence to test the TC2 model.Comment: 15 pages, 10 figure
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