10,067 research outputs found
Dynamics and Stability of Low-Reynolds-Number Swimming Near a Wall
The locomotion of microorganisms and tiny artificial swimmers is governed by low-Reynolds-number
hydrodynamics, where viscous effects dominate and inertial effects are negligible. While the theory
of low-Reynolds-number locomotion is well studied for unbounded fluid domains, the presence of a
boundary has a significant influence on the swimmer’s trajectories and poses problems of dynamic
stability of its motion. In this paper we consider a simple theoretical model of a microswimmer near
a wall, study its dynamics, and analyze the stability of its motion. We highlight the underlying
geometric structure of the dynamics, and establish a relation between the reversing symmetry of
the system and existence and stability of periodic and steady solutions of motion near the wall.
The results are demonstrated by numerical simulations and validated by motion experiments with
macroscale robotic swimmer prototypes
Progenitor constraints on the Type-Ia supernova SN2011fe from pre-explosion Hubble Space Telescope HeII narrow-band observations
We present Hubble Space Telescope (HST) imaging observations of the site of
the Type-Ia supernova SN2011fe in the nearby galaxy M101, obtained about one
year prior to the event, in a narrow band centred on the HeII 4686 \AA{}
emission line. In a "single-degenerate" progenitor scenario, the hard photon
flux from an accreting white dwarf (WD), burning hydrogen on its surface over
Myr should, in principle, create a HeIII Str\"{o}mgren sphere or shell
surrounding the WD. Depending on the WD luminosity, the interstellar density,
and the velocity of an outflow from the WD, the HeIII region could appear
unresolved, extended, or as a ring, with a range of possible surface
brightnesses. We find no trace of HeII 4686 \AA{} line emission in the HST
data. Using simulations, we set upper limits on the HeII 4686 \AA{}
luminosity of erg s for a point
source, corresponding to an emission region of radius pc. The upper
limit for an extended source is erg
s, corresponding to an extended region with pc. The largest
detectable shell, given an interstellar-medium density of 1 cm, has a
radius of pc. Our results argue against the presence, within the
yr prior to the explosion, of a supersoft X-ray source of luminosity
erg s, or of a super-Eddington
accreting WD that produces an outflowing wind capable of producing cavities
with radii of 2-6 pc.Comment: Accepted by MNRAS Letters; revised version following referee report
and readers' comment
Deep Functional Maps: Structured Prediction for Dense Shape Correspondence
We introduce a new framework for learning dense correspondence between
deformable 3D shapes. Existing learning based approaches model shape
correspondence as a labelling problem, where each point of a query shape
receives a label identifying a point on some reference domain; the
correspondence is then constructed a posteriori by composing the label
predictions of two input shapes. We propose a paradigm shift and design a
structured prediction model in the space of functional maps, linear operators
that provide a compact representation of the correspondence. We model the
learning process via a deep residual network which takes dense descriptor
fields defined on two shapes as input, and outputs a soft map between the two
given objects. The resulting correspondence is shown to be accurate on several
challenging benchmarks comprising multiple categories, synthetic models, real
scans with acquisition artifacts, topological noise, and partiality.Comment: Accepted for publication at ICCV 201
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