1,261 research outputs found
Combined 3D thinning and greedy algorithm to approximate realistic particles with corrected mechanical properties
The shape of irregular particles has significant influence on micro- and
macro-scopic behavior of granular systems. This paper presents a combined 3D
thinning and greedy set-covering algorithm to approximate realistic particles
with a clump of overlapping spheres for discrete element method (DEM)
simulations. First, the particle medial surface (or surface skeleton), from
which all candidate (maximal inscribed) spheres can be generated, is computed
by the topological 3D thinning. Then, the clump generation procedure is
converted into a greedy set-covering (SCP) problem.
To correct the mass distribution due to highly overlapped spheres inside the
clump, linear programming (LP) is used to adjust the density of each component
sphere, such that the aggregate properties mass, center of mass and inertia
tensor are identical or close enough to the prototypical particle. In order to
find the optimal approximation accuracy (volume coverage: ratio of clump's
volume to the original particle's volume), particle flow of 3 different shapes
in a rotating drum are conducted. It was observed that the dynamic angle of
repose starts to converge for all particle shapes at 85% volume coverage
(spheres per clump < 30), which implies the possible optimal resolution to
capture the mechanical behavior of the system.Comment: 34 pages, 13 figure
Episodic jet power extracted from a spinning black hole surrounded by a neutrino-dominated accretion flow in gamma-ray bursts
It was suggested that the relativistic jets in gamma-ray bursts (GRBs) are
powered via the Blandford-Znajek (BZ) mechanism or the annihilation of
neutrinos and anti-neutrinos from a neutrino cooling-dominated accretion flow
(NDAF). The advection and diffusion of the large-scale magnetic field of a NDAF
is calculated, and the external magnetic field is found to be dragged inward
efficiently by the accretion flow for a typical magnetic Prandtl number P_m=1.
The maximal BZ jet power can be ~10^53-10^54 erg/sec for an extreme Kerr black
hole, if an external magnetic field with 10^14 Gauss is advected by the NDAF.
This is roughly consistent with the field strength of the disk formed after a
tidal disrupted magnetar. The accretion flow near the black hole horizon is
arrested by the magnetic field if the accretion rate is below than a critical
value for a given external field. The arrested accretion flow fails to drag the
field inward and the field strength decays, and then the accretion re-starts,
which leads to oscillating accretion. The typical timescale of such episodic
accretion is in an order of one second. This can qualitatively explain the
observed oscillation in the soft extend emission of short-type GRBs.Comment: 8 pages, to appear in ApJ, references update
Strong enhancement of chlorophyll a concentration by a weak typhoon
Recent studies demonstrate that chlorophyll a (chl a) concentrations in the
surface ocean can be significantly enhanced due to typhoons. The present study
investigated chl a concentrations in the middle of the South China Sea (SCS)
from 1997-2007. Only the Category1 (minimal) Typhoon Hagibis (2007) had a
notable effect on the chl a concentrations. Typhoon Hagibis had a strong
upwelling potential due to its location near the equator, and the forcing time
of the typhoon (>82 h) was much longer than the geostrophic adjustment time
(~63 h). The higher upwelling velocity and the longer forcing time increased
the depth of the mixed-layer, which consequently induced a strong phytoplankton
bloom that accounted for about 30% of the total annual chl a concentration in
the middle of the SCS. The implication is that the forcing time of a typhoon
should be long enough to establish a strong upwelling and consequently for the
induction of significant upper ocean responses.Comment: Typhoon-Ocean Environment interaction
Simpson's Bias in NLP Training
In most machine learning tasks, we evaluate a model on a given data
population by measuring a population-level metric . Examples of
such evaluation metric include precision/recall for (binary) recognition,
the F1 score for multi-class classification, and the BLEU metric for language
generation. On the other hand, the model is trained by optimizing a
sample-level loss at each learning step , where is a subset
of (a.k.a. the mini-batch). Popular choices of include cross-entropy
loss, the Dice loss, and sentence-level BLEU scores. A fundamental assumption
behind this paradigm is that the mean value of the sample-level loss , if
averaged over all possible samples, should effectively represent the
population-level metric of the task, such as, that .
In this paper, we systematically investigate the above assumption in several
NLP tasks. We show, both theoretically and experimentally, that some popular
designs of the sample-level loss may be inconsistent with the true
population-level metric of the task, so that models trained to optimize the
former can be substantially sub-optimal to the latter, a phenomenon we call it,
Simpson's bias, due to its deep connections with the classic paradox known as
Simpson's reversal paradox in statistics and social sciences.Comment: AAAI 202
Connected and Disconnected Sea Partons from CT18 Parametrization of PDFs
The separation of the connected and disconnected sea partons, which were
uncovered in the Euclidean path-integral formulation of the hadronic tensor, is
accommodated with an alternative parametrization of the non-perturbative parton
distribution functions in the CT18 global analysis. This is achieved with the
help of the distinct small behaviours of these two sea partons and the
constraint from the lattice calculation of the ratio of the strange momentum
fraction to that of the or in the disconnected insertion. The
whole dataset of CT18 is used in this CT18CS fit. The impact of the recent
SeaQuest data on the distribution of CT18CS is also
discussed. The separate momentum fractions for the valence, the connected sea
and disconnected sea of and , the strange and the gluon partons are
presented at GeV for the first time. They can be compared
term-by-term with systematic error controlled lattice calculations.Comment: Revised manuscript accepted for publication in PR
Guidezilla extension catheter for percutaneous interventional therapy of complex lesions via a transradial approach: Case series from a single-center experience
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 Background: Guide catheter extension systems have become one of the most powerful tools for addressÂing complex lesions during percutaneous coronary intervention (PCI), but data on a new-generation rapid exchange extension catheter — the Guidezilla catheter — are limited. Summarized herein reports on experience using the Guidezilla catheter for complex coronary lesions via a transradial approach at the documented institution an evaluation of its safety and efficacy.
Methods: A total of 25 patients (19 males and 6 females) who underwent PCI via the radial approach with the Guidezilla catheter for adequate back-up support and to facilitate equipment delivery were enrolled. The clinical, angiographic and procedural data of all 26 procedures in 25 patients (1 patient underwent two PCI procedures on different lesions) were collected to evaluate the safety and efficacy of this novel equipment.
Results: The mean age of the enrolled patients was 67.7 ± 8.41 years old. The mean depth of intubaÂtion was 27.90 ± 12.23 mm. Stent implantation was successful in 23 out of 26 procedures (88.5%) and failed in 3 cases: 1 case of tortuosity and severe angulation in a chronic total occlusion lesion; 1 case of an existing type B dissection (NHLBI classification system for coronary artery dissection types); and 1 case in which a stent was stripped off its balloon. None of the patients experienced coronary dissection, perforation, air embolism, pressure dampening or other major complications during the procedure.
Conclusions: The Guidezilla extension catheter is an effective and safe tool that provides improved back-up support and increases the success rate of PCI for complex coronary lesion by radial access
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