1,403 research outputs found
On the decay of Burgers turbulence
This work is devoted to the decay ofrandom solutions of the unforced Burgers
equation in one dimension in the limit of vanishing viscosity. The initial
velocity is homogeneous and Gaussian with a spectrum proportional to at
small wavenumbers and falling off quickly at large wavenumbers. In physical
space, at sufficiently large distances, there is an ``outer region'', where the
velocity correlation function preserves exactly its initial form (a power law)
when is not an even integer. When the spectrum, at long times, has
three scaling regions : first, a region at very small \ms1 with a
time-independent constant, stemming from this outer region, in which the
initial conditions are essentially frozen; second, a region at
intermediate wavenumbers, related to a self-similarly evolving ``inner region''
in physical space and, finally, the usual region, associated to the
shocks. The switching from the to the region occurs around a wave
number , while the switching from to
occurs around (ignoring logarithmic
corrections in both instances). The key element in the derivation of the
results is an extension of the Kida (1979) log-corrected law for the
energy decay when to the case of arbitrary integer or non-integer .
A systematic derivation is given in which both the leading term and estimates
of higher order corrections can be obtained. High-resolution numerical
simulations are presented which support our findings.Comment: In LaTeX with 11 PostScript figures. 56 pages. One figure contributed
by Alain Noullez (Observatoire de Nice, France
Triangle percolation in mean field random graphs -- with PDE
We apply a PDE-based method to deduce the critical time and the size of the
giant component of the ``triangle percolation'' on the Erd\H{o}s-R\'enyi random
graph process investigated by Palla, Der\'enyi and VicsekComment: Summary of the changes made: We have changed a remark about k-clique
percolation in the first paragraph. Two new paragraphs are inserted after
equation (4.4) with two applications of the equation. We have changed the
names of some variables in our formula
Keeping mitochondria happy - benefits of a pore choice in acute pancreatitis
Mitochondrial dysfunction is a key feature of multiple diseases and thus protection of this organelle is an important therapeutic objective. The pancreatic acinar cell, which synthesises and stores digestive enzyme precursors, is the most abundant cell type in pancreatic tissue and considered to be the primary site of acute pancreatitis (AP) initiation. Early investigations at the University of Liverpool and by others discovered that precipitants of AP, including bile acids and alcohol non‐oxidative metabolites, disrupt calcium signalling in acinar cells leading to toxicity. Sustained cytosolic calcium elevations raise mitochondrial matrix calcium, triggering the opening of the mitochondrial permeability transition pore (MPTP), which results in a loss of membrane potential and ATP production vital for cellular processes (Criddle et al . 2006; Mukherjee et al . 2016) (Fig. 1). The prime consequence of pancreatic mitochondrial dysfunction in AP is necrotic cell death, the extent of which is a major determinant of clinical outcome. Subsequent studies have shown that calcium‐dependent mitochondrial dysfunction in response to AP precipitants also occurs in ductal cells, widening the cast of players implicated in the development of AP (Hegyi & Petersen, 2013). There is currently no specific therapy for the disease and protection of mitochondria by MPTP inhibition is considered a promising therapeutic approach
Lead and stable lead isotopes as tracers of soil pollution and human health risk assessment in former industrial cities of Hungary
Dynamical effects induced by long range activation in a nonequilibrium reaction-diffusion system
We both show experimentally and numerically that the time scales separation
introduced by long range activation can induce oscillations and excitability in
nonequilibrium reaction-diffusion systems that would otherwise only exhibit
bistability. Namely, we show that the Chlorite-Tetrathionate reaction, where
autocatalytic species diffuses faster than the substrates, the spatial
bistability domain in the nonequilibrium phase diagram is extended with
oscillatory and excitability domains. A simple model and a more realistic model
qualitatively account for the observed behavior. The latter model provides
quantitative agreement with the experiments.Comment: 19 pages + 9 figure
Hydrodynamics of topological defects in nematic liquid crystals
We show that back-flow, the coupling between the order parameter and the
velocity fields, has a significant effect on the motion of defects in nematic
liquid crystals. In particular the defect speed can depend strongly on the
topological strength in two dimensions and on the sense of rotation of the
director about the core in three dimensions.Comment: 4 pages including two figure
Exploring the circumstellar environment of the young eruptive star V2492 Cyg
Context. V2492 Cyg is a young eruptive star that went into outburst in 2010.
The near-infrared color changes observed since the outburst peak suggest that
the source belongs to a newly defined sub-class of young eruptive stars, where
time-dependent accretion and variable line-of-sight extinction play a combined
role in the flux changes.
Aims. In order to learn about the origin of the light variations and to
explore the circumstellar and interstellar environment of V2492 Cyg, we
monitored the source at ten different wavelengths, between 0.55 \mu m and 2.2
\mu m from the ground and between 3.6 \mu m and 160 \mu m from space.
Methods. We analyze the light curves and study the color-color diagrams via
comparison with the standard reddening path. We examine the structure of the
molecular cloud hosting V2492 Cyg by computing temperature and optical depth
maps from the far-infrared data.
Results. We find that the shapes of the light curves at different wavelengths
are strictly self-similar and that the observed variability is related to a
single physical process, most likely variable extinction. We suggest that the
central source is episodically occulted by a dense dust cloud in the inner
disk, and, based on the invariability of the far-infrared fluxes, we propose
that it is a long-lived rather than a transient structure. In some respects,
V2492 Cyg can be regarded as a young, embedded analog of UX Orionis-type stars.
Conclusions. The example of V2492 Cyg demonstrates that the light variations
of young eruptive stars are not exclusively related to changing accretion. The
variability provided information on an azimuthally asymmetric structural
element in the inner disk. Such an asymmetric density distribution in the
terrestrial zone may also have consequences for the initial conditions of
planet formation.Comment: 9 pages, 7 figures, 2 online tables, accepted for publication in A&
Effect of Solar Wind Drag on the Determination of the Properties of Coronal Mass Ejections from Heliospheric Images
The Fixed-\Phi (F\Phi) and Harmonic Mean (HM) fitting methods are two methods
to determine the average direction and velocity of coronal mass ejections
(CMEs) from time-elongation tracks produced by Heliospheric Imagers (HIs), such
as the HIs onboard the STEREO spacecraft. Both methods assume a constant
velocity in their descriptions of the time-elongation profiles of CMEs, which
are used to fit the observed time-elongation data. Here, we analyze the effect
of aerodynamic drag on CMEs propagating through interplanetary space, and how
this drag affects the result of the F\Phi and HM fitting methods. A simple drag
model is used to analytically construct time-elongation profiles which are then
fitted with the two methods. It is found that higher angles and velocities give
rise to greater error in both methods, reaching errors in the direction of
propagation of up to 15 deg and 30 deg for the F\Phi and HM fitting methods,
respectively. This is due to the physical accelerations of the CMEs being
interpreted as geometrical accelerations by the fitting methods. Because of the
geometrical definition of the HM fitting method, it is affected by the
acceleration more greatly than the F\Phi fitting method. Overall, we find that
both techniques overestimate the initial (and final) velocity and direction for
fast CMEs propagating beyond 90 deg from the Sun-spacecraft line, meaning that
arrival times at 1 AU would be predicted early (by up to 12 hours). We also
find that the direction and arrival time of a wide and decelerating CME can be
better reproduced by the F\Phi due to the cancellation of two errors:
neglecting the CME width and neglecting the CME deceleration. Overall, the
inaccuracies of the two fitting methods are expected to play an important role
in the prediction of CME hit and arrival times as we head towards solar maximum
and the STEREO spacecraft further move behind the Sun.Comment: Solar Physics, Online First, 17 page
Distorted TCR repertoires define multisystem inflammatory syndrome in children
While the majority of children infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) display mild or no symptoms, rare individuals develop severe disease presenting with multisystem inflammatory syndrome (MIS-C). The reason for variable clinical manifestations is not understood. Here, we carried out TCR sequencing and conducted comparative analyses of TCR repertoires between children with MIS-C (n = 12) and mild (n = 8) COVID-19. We compared these repertoires with unexposed individuals (samples collected pre-COVID-19 pandemic: n = 8) and with the Adaptive Biotechnologies MIRA dataset, which includes over 135,000 high-confidence SARS-CoV-2-specific TCRs. We show that the repertoires of children with MIS-C are characterised by the expansion of TRBV11-2 chains with high junctional and CDR3 diversity. Moreover, the CDR3 sequences of TRBV11-2 clones shift away from SARS-CoV-2 specific T cell clones, resulting in distorted TCR repertoires. In conclusion, our study reports that CDR3-independent expansion of TRBV11-2+ cells, lacking SARS-CoV-2 specificity, defines MIS-C in children
Laser-heated capillary discharge plasma waveguides for electron acceleration to 8 GeV
A plasma channel created by the combination of a capillary discharge and inverse Bremsstrahlung laser heating enabled the generation of electron bunches with energy up to 7.8 GeV in a laser-driven plasma accelerator. The capillary discharge created an initial plasma channel and was used to tune the plasma temperature, which optimized laser heating. Although optimized colder initial plasma temperatures reduced the ionization degree, subsequent ionization from the heater pulse created a fully ionized plasma on-axis. The heater pulse duration was chosen to be longer than the hydrodynamic timescale of ≈ 1 ns, such that later temporal slices were more efficiently guided by the channel created by the front of the pulse. Simulations are presented which show that this thermal self-guiding of the heater pulse enabled channel formation over 20 cm. The post-heated channel had lower on-axis density and increased focusing strength compared to relying on the discharge alone, which allowed for guiding of relativistically intense laser pulses with a peak power of 0.85 PW and wakefield acceleration over 15 diffraction lengths. Electrons were injected into the wake in multiple buckets and times, leading to several electron bunches with different peak energies. To create single electron bunches with low energy spread, experiments using localized ionization injection inside a capillary discharge waveguide were performed. A single injected bunch with energy 1.6 GeV, charge 38 pC, divergence 1 mrad, and relative energy spread below 2% full-width half-maximum was produced in a 3.3 cm-long capillary discharge waveguide. This development shows promise for mitigation of energy spread and future high efficiency staged acceleration experiments
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