2,509 research outputs found
Bending models of lipid bilayer membranes: spontaneous curvature and area-difference elasticity
We preset a computational study of bending models for the curvature
elasticity of lipid bilayer membranes that are relevant for simulations of
vesicles and red blood cells. We compute bending energy and forces on
triangulated meshes and evaluate and extend four well established schemes for
their approximation: Kantor and Nelson 1987, Phys. Rev. A 36, 4020, J\"ulicher
1996, J. Phys. II France 6, 1797, Gompper and Kroll 1996, J. Phys. I France 6,
1305, and Meyer et. al. 2003 in Visualization and Mathematics III, Springer,
p35, termed A, B, C, D. We present a comparative study of these four schemes on
the minimal bending model and propose extensions for schemes B, C and D. These
extensions incorporate the reference state and non-local energy to account for
the spontaneous curvature, bilayer coupling, and area-difference elasticity
models. Our results indicate that the proposed extensions enhance the models to
account for shape transformation including budding/vesiculation as well as for
non-axisymmetric shapes. We find that the extended scheme B is superior to the
rest in terms of accuracy, and robustness as well as simplicity of
implementation. We demonstrate the capabilities of this scheme on several
benchmark problems including the budding-vesiculating process and the
reproduction of the phase diagram of vesicles
Analysis of Noisy Evolutionary Optimization When Sampling Fails
In noisy evolutionary optimization, sampling is a common strategy to deal
with noise. By the sampling strategy, the fitness of a solution is evaluated
multiple times (called \emph{sample size}) independently, and its true fitness
is then approximated by the average of these evaluations. Previous studies on
sampling are mainly empirical. In this paper, we first investigate the effect
of sample size from a theoretical perspective. By analyzing the (1+1)-EA on the
noisy LeadingOnes problem, we show that as the sample size increases, the
running time can reduce from exponential to polynomial, but then return to
exponential. This suggests that a proper sample size is crucial in practice.
Then, we investigate what strategies can work when sampling with any fixed
sample size fails. By two illustrative examples, we prove that using parent or
offspring populations can be better. Finally, we construct an artificial noisy
example to show that when using neither sampling nor populations is effective,
adaptive sampling (i.e., sampling with an adaptive sample size) can work. This,
for the first time, provides a theoretical support for the use of adaptive
sampling
Constraining Sub-Parsec Binary Supermassive Black Holes in Quasars with Multi-Epoch Spectroscopy. II. The Population with Kinematically Offset Broad Balmer Emission Lines
A small fraction of quasars have long been known to show bulk velocity
offsets in the broad Balmer lines with respect to the systemic redshift of the
host galaxy. Models to explain these offsets usually invoke broad-line region
gas kinematics/asymmetry around single black holes (BHs), orbital motion of
massive (~sub-pc) binary black holes (BBHs), or recoil BHs, but single-epoch
spectra are unable to distinguish between these scenarios. The line-of-sight
(LOS) radial velocity (RV) shifts from long-term spectroscopic monitoring can
be used to test the BBH hypothesis. We have selected a sample of 399 quasars
with offset broad H-beta lines from the SDSS DR7 quasar catalog, and have
conducted second-epoch optical spectroscopy for 50 of them. Combined with the
existing SDSS spectra, the new observations enable us to constrain the LOS RV
shifts of broad H-beta lines with a rest-frame baseline of a few years to
nearly a decade. Using cross-correlation analysis, we detect significant (99%
confidence) radial accelerations in the broad H-beta lines in 24 of the 50
objects. We suggest that 9 of the 24 detections are sub-pc BBH candidates,
which show consistent velocity shifts independently measured from a second
broad line (either H-alpha or Mg II) without significant changes in the
broad-line profiles. Combining the results on the general quasar population
studied in Paper I, we find a tentative anti-correlation between the velocity
offset in the first-epoch spectrum and the average acceleration between two
epochs, which could be explained by orbital phase modulation when the time
separation between two epochs is a non-negligible fraction of the orbital
period of the motion causing the line displacement. We discuss the implications
of our results for the identification of sub-pc BBH candidates in offset-line
quasars and for the constraints on their frequency and orbital parameters.
[Abridged]Comment: 23 pages, 18 figures, ApJ in pres
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
Simulation and evaluation of 2-m temperature over Antarctica in polar regional climate model
The European Centre for Medium-Range Weather Forecasts Reanalysis ERA40, National Centers for Environmental Prediction (NCEP) 20th-century reanalysis, and three station observations along an Antarctic traverse from Zhongshan to Dome-A stations are used to assess 2-m temperature simulation skill of a regional climate model. This model (HIRHAM) is from the Alfred Wegener Institute for Polar and Marine Research in Germany. Results show: (1) The simulated multiyear averaged 2-m temperature field pattern is close to that of ERA40 and NCEP; (2) the cold bias relative to ERA40 over all of Antarctic regions is 1.8°C, and that to NCEP reaches 5.1°C; (3) bias of HIRHAM relative to ERA40 has seasonal variation, with a cold bias mainly in the summer, as much as 3.4°C. There is a small inland warm bias in autumn of 0.3°C. Further analysis reveals that the reason for the cold bias of 2-m temperature is that physical conditions of the near-surface boundary layer simulated by HIRHAM are different from observations: (1) During the summer, observations show that near-surface atmospheric stability conditions have both inversions and non-inversions, which is due to the existence of both positive and negative sensible heat fluxes, but HIRHAM almost always simulates a situation of inversion and negative sensible heat flux; (2) during autumn and winter, observed near-surface stability is almost always that of inversions, consistent with HIRHAM simulations. This partially explains the small bias during autumn and winter
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