3,380 research outputs found
Arbitrary slip length for fluid-solid interface of arbitrary geometry in smoothed particle dynamics
We model a slip boundary condition at fluid-solid interface of an arbitrary
geometry in smoothed particle hydrodynamics and smoothed dissipative particle
dynamics simulations. Under an assumption of linear profile of the tangential
velocity at quasi-steady state near the interface, an arbitrary slip length
can be specified and correspondingly, an artificial velocity for every boundary
particle can be calculated. Therefore, as an input parameter affects the
calculation of dissipative and random forces near the interface. For ,
the no-slip is recovered while for , the free-slip is achieved.
Technically, we devise two different approaches to calculate the artificial
velocity of any boundary particle. The first has a succinct principle and is
competent for simple geometries, while the second is subtle and affordable for
complex geometries. Slip lengths in simulations for both steady and transient
flows coincide with the expected ones. As demonstration, we apply the two
approaches extensively to simulate curvy channel flows, dynamics of an
ellipsoid in pipe flow and flows within complex microvessels, where desired
slip lengths at fluid-solid interfaces are prescribed. The proposed methodology
may apply equally well to other particle methods such as dissipative particle
dynamics and moving particle semi-implicit methods
Discovery of Eight z ~ 6 Quasars in the Sloan Digital Sky Survey Overlap Regions
We present the discovery of eight quasars at z~6 identified in the Sloan
Digital Sky Survey (SDSS) overlap regions. Individual SDSS imaging runs have
some overlap with each other, leading to repeat observations over an area
spanning >4000 deg^2 (more than 1/4 of the total footprint). These overlap
regions provide a unique dataset that allows us to select high-redshift quasars
more than 0.5 mag fainter in the z band than those found with the SDSS
single-epoch data. Our quasar candidates were first selected as i-band dropout
objects in the SDSS imaging database. We then carried out a series of follow-up
observations in the optical and near-IR to improve photometry, remove
contaminants, and identify quasars. The eight quasars reported here were
discovered in a pilot study utilizing the overlap regions at high galactic
latitude (|b|>30 deg). These quasars span a redshift range of 5.86<z<6.06 and a
flux range of 19.3<z_AB<20.6 mag. Five of them are fainter than z_AB=20 mag,
the typical magnitude limit of z~6 quasars used for the SDSS single-epoch
images. In addition, we recover eight previously known quasars at z~6 that are
located in the overlap regions. These results validate our procedure for
selecting quasar candidates from the overlap regions and confirming them with
follow-up observations, and provide guidance to a future systematic survey over
all SDSS imaging regions with repeat observations.Comment: AJ in press (8 pages
Probing the electroweak symmetry breaking history with Gravitational waves
We perform a three dimensional lattice simulation of the electroweak symmetry
breaking process through a two-step phase transition, where one of the two
steps is a first order phase transition. Our results show that: 1) when the
electroweak symmetry breaking is driven by the beyond Standard Model sector
around GeV, the gravitational wave spectra
produced from the phase transitions are of broken power-law double-peak shapes;
2) when the electroweak symmetry breaking is induced by a first-order phase
transition of a high-scale global U(1) theory, cosmic strings can form and then
disappear through particle radiation, and the yielded gravitational wave
spectra are of plateau shapes. The two scenarios can be distinguished through
probing gravitational wave spectra. Our study suggests that the stochastic
gravitational waves provide an alternative way to probe the beyond Standard
Model sector relevant to the electroweak symmetry breaking pattern in the early
Universe.Comment: 9 pages, 8 figures, comments welcome
Constraining C iii] Emission in a Sample of Five Luminous z = 5.7 Galaxies
Recent observations have suggested that the CIII] emission
lines could be alternative diagnostic lines for galaxies in the reionization
epoch. We use the F128N narrowband filter on the Hubble Space Telescope's
() Wide Field Camera 3 (WFC3) to search for CIII] emission in a
sample of five galaxies at z = 5.7 in the Subaru Deep Field and the
Subaru/XMM-Newton Deep Field. Using the F128N narrowband imaging, together with
the broadband imaging, we do not detect CIII] emission for the five galaxies
with ranging from 24.10 -- 27.00 in our sample. For the brightest
galaxy J132416.13+274411.6 in our sample (z = 5.70, ),
which has a significantly higher signal to noise, we report a CIII] flux of
, which places a
stringent 3- upper limit of $\mathrm{erg\
s^{-1}\ cm^{-2}}\rm\sigma\mathrm{2.55\times10^{-18}\ erg\ s^{-1}\ cm^{-2}}\rm\sigma>$ 5.70 exhibit a wide range
of distribution. Our strong limits on CIII] emission could be used as a guide
for future observations in the reionization epoch
Cerebral hemodynamic characteristics of acute mountain sickness upon acute high-altitude exposure at 3,700 m in young Chinese men.
PURPOSE: We aimed at identifying the cerebral hemodynamic characteristics of acute mountain sickness (AMS). METHODS: Transcranial Doppler (TCD) sonography examinations were performed between 18 and 24 h after arrival at 3,700 m via plane from 500 m (n = 454). A subgroup of 151 subjects received TCD examinations at both altitudes. RESULTS: The velocities of the middle cerebral artery, vertebral artery (VA) and basilar artery (BA) increased while the pulsatility indexes (PIs) and resistance indexes (RIs) decreased significantly (all p < 0.05). Velocities of BA were higher in AMS (AMS+) individuals when compared with non-AMS (AMS-) subjects (systolic velocity: 66 ± 12 vs. 69 ± 15 cm/s, diastolic velocity: 29 ± 7 vs. 31 ± 8 cm/s and mean velocity, 42 ± 9 vs. 44 ± 10 cm/s). AMS was characterized by higher diastolic velocity [V d_VA (26 ± 4 vs. 25 ± 4, p = 0.013)] with lower PI and RI (both p = 0.004) in VA. Furthermore, the asymmetry index (AI) of VAs was significantly lower in the AMS + group [-5.7 % (21.0 %) vs. -2.5 % (17.8 %), p = 0.016]. The AMS score was closely correlated with the hemodynamic parameters of BA and the V d_VA, PI, RI and AI of VA. CONCLUSION: AMS is associated with alterations in cerebral hemodynamics in the posterior circulation rather than the anterior one, and is characterized by higher blood velocity with lower resistance. In addition, the asymmetry of VAs may be involved in AMS
MApping the Most Massive Overdensities Through Hydrogen (MAMMOTH) I: Methodology
Modern cosmology predicts that a galaxy overdensity is associated to a large
reservoir of the intergalactic gas, which can be traced by the Ly
forest absorption. We have undertaken a systematic study of the relation
between Coherently Strong intergalactic Ly Absorption systems (CoSLAs),
which have highest optical depth () in distribution, and mass
overdensities on the scales of 10 - 20 comoving Mpc. On such
large scales, our cosmological simulations show a strong correlation between
the effective optical depth () of the CoSLAs and the 3-D mass
overdensities. In moderate signal-to-noise spectra, however, the profiles of
CoSLAs can be confused with high column density absorbers. For , where
the corresponding Ly is redshifted to the optical, we have developed the
technique to differentiate between these two alternatives. We have applied this
technique to SDSS-III quasar survey at - 3.3, and we present a sample
of five CoSLA candidates with on 15 Mpc greater than
the mean optical depth. At lower redshifts of , where the
background quasar density is higher, the overdensity can be traced by
intergalactic absorption groups using multiple sight lines. Our overdensity
searches fully utilize the current and next generation of Ly forest
surveys which cover a survey volume of Gpc). In addition,
systems traced by CoSLAs will build a uniform sample of the most massive
overdensities at to constrain the models of structure formation, and
offer a unique laboratory to study the interactions between galaxy
overdensities and the intergalactic medium.Comment: 24 pages, 30 figures, 8 tables, submitted to the Astrophysical
Journa
Self-Domain Adaptation for Face Anti-Spoofing
Although current face anti-spoofing methods achieve promising results under
intra-dataset testing, they suffer from poor generalization to unseen attacks.
Most existing works adopt domain adaptation (DA) or domain generalization (DG)
techniques to address this problem. However, the target domain is often unknown
during training which limits the utilization of DA methods. DG methods can
conquer this by learning domain invariant features without seeing any target
data. However, they fail in utilizing the information of target data. In this
paper, we propose a self-domain adaptation framework to leverage the unlabeled
test domain data at inference. Specifically, a domain adaptor is designed to
adapt the model for test domain. In order to learn a better adaptor, a
meta-learning based adaptor learning algorithm is proposed using the data of
multiple source domains at the training step. At test time, the adaptor is
updated using only the test domain data according to the proposed unsupervised
adaptor loss to further improve the performance. Extensive experiments on four
public datasets validate the effectiveness of the proposed method.Comment: Camera Ready, AAAI 202
Retinexformer: One-stage Retinex-based Transformer for Low-light Image Enhancement
When enhancing low-light images, many deep learning algorithms are based on
the Retinex theory. However, the Retinex model does not consider the
corruptions hidden in the dark or introduced by the light-up process. Besides,
these methods usually require a tedious multi-stage training pipeline and rely
on convolutional neural networks, showing limitations in capturing long-range
dependencies. In this paper, we formulate a simple yet principled One-stage
Retinex-based Framework (ORF). ORF first estimates the illumination information
to light up the low-light image and then restores the corruption to produce the
enhanced image. We design an Illumination-Guided Transformer (IGT) that
utilizes illumination representations to direct the modeling of non-local
interactions of regions with different lighting conditions. By plugging IGT
into ORF, we obtain our algorithm, Retinexformer. Comprehensive quantitative
and qualitative experiments demonstrate that our Retinexformer significantly
outperforms state-of-the-art methods on thirteen benchmarks. The user study and
application on low-light object detection also reveal the latent practical
values of our method. Code, models, and results are available at
https://github.com/caiyuanhao1998/RetinexformerComment: ICCV 2023; The first Transformer-based method for low-light image
enhancemen
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