49,346 research outputs found
The effect of lift on the wave-making resistance of multi-hull craft
A potential based panel method is presented to estimate the wave-making characteristics of multi-hull craft. In order to simulate the lifting potential flow around the sub-hulls, the method adopts mixed source/doublet distributions on the sub-hulls and their wake surface, while sources are distributed on the main hull and the free surface. In this way, the asymmetric flow characteristics of the sub-hull are properly simulated, i.e., a Kutta condition is satisfied at the trailing edge of the sub-hull. Comparison is made between the numerical and model experimental measurements, and a good correlation has been found. The wave-making characteristics and pressure distributions on the sub-hull predicted by the present method can differ from those based on a distribution of sources alone, especially the pressure distributions at the stern of the sub-hulls
Testing the Empirical Shock Arrival Model using Quadrature Observations
The empirical shock arrival (ESA) model was developed based on quadrature
data from Helios (in-situ) and P-78 (remote-sensing) to predict the Sun-Earth
travel time of coronal mass ejections (CMEs) [Gopalswamy et al. 2005a]. The ESA
model requires earthward CME speed as input, which is not directly measurable
from coronagraphs along the Sun-Earth line. The Solar Terrestrial Relations
Observatory (STEREO) and the Solar and Heliospheric Observatory (SOHO) were in
quadrature during 2010 - 2012, so the speeds of Earth-directed CMEs were
observed with minimal projection effects. We identified a set of 20 full halo
CMEs in the field of view of SOHO that were also observed in quadrature by
STEREO. We used the earthward speed from STEREO measurements as input to the
ESA model and compared the resulting travel times with the observed ones from
L1 monitors. We find that the model predicts the CME travel time within about
7.3 hours, which is similar to the predictions by the ENLIL model. We also find
that CME-CME and CME-coronal hole interaction can lead to large deviations from
model predictions.Comment: 17 pages, 4 figures, 3 table
A seakeeping analysis method for an air-lifted vessel
A seakeeping analysis in the frequency domain is presented to predict the motion response of an airlifted vessel (ALV) in waves. The ALV is supported by pressurised air in two separate cushion chambers; the pressure variation in the cushions has a significant effect on the motions of the vessel. The adiabatic gas law is used to couple cushion pressure and the free-surface elevation of water inside the chamber. Attention is focused on the waves generated by the pressure, and a method is presented to compute the corresponding free-surface elevation. New numerical schemes are proposed for calculating the threedimensional free-surface elevation for the four wave numbers. Numerical results of the free-surface elevation, escape area, escape volume and motion responses of the ALV are provided. & 2008 Elsevier Ltd. All rights reserved
A Hierarchical Relationship between the Fluence Spectra and CME Kinematics in Large Solar Energetic Particle Events: A Radio Perspective
We report on further evidence that solar energetic particles are organized by
the kinematic properties of coronal mass ejections (CMEs)[1]. In particular, we
focus on the starting frequency of type II bursts, which is related to the
distance from the Sun where the radio emission starts. We find that the three
groups of solar energetic particle (SEP) events known to have distinct values
of CME initial acceleration, also have distinct average starting frequencies of
the associated type II bursts. SEP events with ground level enhancement (GLE)
have the highest starting frequency (107 MHz), while those associated with
filament eruption (FE) in quiescent regions have the lowest starting frequency
(22 MHz); regular SEP events have intermediate starting frequency (81 MHz).
Taking the onset time of type II bursts as the time of shock formation, we
determine the shock formation heights measured from the Sun center. We find
that the shocks form on average closest to the Sun (1.51 Rs) in GLE events,
farthest from the Sun in FE SEP events (5.38 Rs), and at intermediate distances
in regular SEP events (1.72 Rs). Finally, we present the results of a case
study of a CME with high initial acceleration (~3 km s^-2) and a type II radio
burst with high starting frequency (~200 MHz) but associated with a minor SEP
event. We find that the relation between the fluence spectral index and CME
initial acceleration continues to hold even for this minor SEP event.Comment: 11 pages, 7 figures, 1 table, to appear in Journal of Physics:
Conference Series (JPCS), Proceedings of the 16th Annual International
Astrophysics Conference held in Santa Fe, NM, 201
Ground Level Enhancement in the 2014 January 6 Solar Energetic Particle Event
We present a study of the 2014 January 6 solar energetic particle (SEP)
event, which produced a small ground level enhancement (GLE), making it the
second GLE of this unusual solar cycle 24. This event was primarily observed by
the South Pole neutron monitors (increase of ~2.5%) whereas a few other neutron
monitors recorded smaller increases. The associated coronal mass ejection (CME)
originated behind the western limb and had the speed of 1960 km/s. The height
of the CME at the start of the associated metric type II radio burst, which
indicates the formation of a strong shock, was measured to be 1.61 Rs using a
direct image from STEREO-A/EUVI. The CME height at the time of GLE particle
release (determined using the South Pole neutron monitor data) was directly
measured as 2.96 Rs, from the STEREO-A/COR1 white-light observations. These CME
heights are consistent with those obtained for the GLE71, the only other GLE of
the current cycle as well as cycle-23 GLEs derived using back-extrapolation.
GLE72 is of special interest because it is one of the only two GLEs of cycle
24, one of the two behind-the-limb GLEs and one of the two smallest GLEs of
cycles 23 and 24
Spin current through an ESR quantum dot: A real-time study
The spin transport in a strongly interacting spin-pump nano-device is studied
using the time-dependent variational-matrix-product-state (VMPS) approach. The
precession magnetic field generates a dissipationless spin current through the
quantum dot. We compute the real time spin current away from the equilibrium
condition. Both transient and stationary states are reached in the simulation.
The essentially exact results are compared with those from the Hartree-Fock
approximation (HFA). It is found that correlation effect on the physical
quantities at quasi-steady state are captured well by the HFA for small
interaction strength. However the HFA misses many features in the real time
dynamics. Results reported here may shed light on the understanding of the
ultra-fast processes as well as the interplay of the non-equilibrium and
strongly correlated effect in the transport properties.Comment: 5 pages, 5 figure
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