264 research outputs found
Development of singularities for the compressible Euler equations with external force in several dimensions
We consider solutions to the Euler equations in the whole space from a
certain class, which can be characterized, in particular, by finiteness of
mass, total energy and momentum. We prove that for a large class of right-hand
sides, including the viscous term, such solutions, no matter how smooth
initially, develop a singularity within a finite time. We find a sufficient
condition for the singularity formation, "the best sufficient condition", in
the sense that one can explicitly construct a global in time smooth solution
for which this condition is not satisfied "arbitrary little". Also compactly
supported perturbation of nontrivial constant state is considered. We
generalize the known theorem by Sideris on initial data resulting in
singularities. Finally, we investigate the influence of frictional damping and
rotation on the singularity formation.Comment: 23 page
Chaotic mixing in noisy Hamiltonian systems
This paper summarises an investigation of the effects of low amplitude noise
and periodic driving on phase space transport in 3-D Hamiltonian systems, a
problem directly applicable to systems like galaxies, where such perturbations
reflect internal irregularities and.or a surrounding environment. A new
diagnsotic tool is exploited to quantify how, over long times, different
segments of the same chaotic orbit can exhibit very different amounts of chaos.
First passage time experiments are used to study how small perturbations of an
individual orbit can dramatically accelerate phase space transport, allowing
`sticky' chaotic orbits trapped near regular islands to become unstuck on
suprisingly short time scales. Small perturbations are also studied in the
context of orbit ensembles with the aim of understanding how such
irregularities can increase the efficacy of chaotic mixing. For both noise and
periodic driving, the effect of the perturbation scales roughly in amplitude.
For white noise, the details are unimportant: additive and multiplicative noise
tend to have similar effects and the presence or absence of a friction related
to the noise by a Fluctuation- Dissipation Theorem is largely irrelevant.
Allowing for coloured noise can significantly decrease the efficacy of the
perturbation, but only when the autocorrelation time, which vanishes for white
noise, becomes so large that t here is little power at frequencies comparable
to the natural frequencies of the unperturbed orbit. This suggests strongly
that noise-induced extrinsic diffusion, like modulational diffusion associated
with periodic driving, is a resonance phenomenon. Potential implications for
galaxies are discussed.Comment: 15 pages including 18 figures, uses MNRAS LaTeX macro
Chaos and the continuum limit in the gravitational N-body problem II. Nonintegrable potentials
This paper continues a numerical investigation of orbits evolved in `frozen,'
time-independent N-body realisations of smooth time-independent density
distributions corresponding to both integrable and nonintegrable potentials,
allowing for N as large as 300,000. The principal focus is on distinguishing
between, and quantifying, the effects of graininess on initial conditions
corresponding, in the continuum limit, to regular and chaotic orbits. Ordinary
Lyapunov exponents X do not provide a useful diagnostic for distinguishing
between regular and chaotic behaviour. Frozen-N orbits corresponding in the
continuum limit to both regular and chaotic characteristics have large positive
X even though, for large N, the `regular' frozen-N orbits closely resemble
regular characteristics in the smooth potential. Viewed macroscopically both
`regular' and `chaotic' frozen-N orbits diverge as a power law in time from
smooth orbits with the same initial condition. There is, however, an important
difference between `regular' and `chaotic' frozen-N orbits: For regular orbits,
the time scale associated with this divergence t_G ~ N^{1/2}t_D, with t_D a
characteristic dynamical time; for chaotic orbits t_G ~ (ln N) t_D. At least
for N>1000 or so, clear distinctions exist between phase mixing of initially
localised orbit ensembles which, in the continuum limit, exhibit regular versus
chaotic behaviour. For both regular and chaotic ensembles, finite-N effects are
well mimicked, both qualitatively and quantitatively, by energy-conserving
white noise with amplitude ~ 1/N. This suggests strongly that earlier
investigations of the effects of low amplitude noise on phase space transport
in smooth potentials are directly relevant to real physical systems.Comment: 20 pages, including 21 FIGURES, uses RevTeX macro
Evaluating the educational environment of an international animal model-based wet lab course for undergraduate students
publisher: Elsevier articletitle: Evaluating the educational environment of an international animal model-based wet lab course for undergraduate students journaltitle: Annals of Medicine and Surgery articlelink: http://dx.doi.org/10.1016/j.amsu.2016.10.004 content_type: article copyright: © 2016 The Author(s). Published by Elsevier Ltd on behalf of IJS Publishing Group Ltd
Measurement of inclusive D*+- and associated dijet cross sections in photoproduction at HERA
Inclusive photoproduction of D*+- mesons has been measured for photon-proton
centre-of-mass energies in the range 130 < W < 280 GeV and a photon virtuality
Q^2 < 1 GeV^2. The data sample used corresponds to an integrated luminosity of
37 pb^-1. Total and differential cross sections as functions of the D*
transverse momentum and pseudorapidity are presented in restricted kinematical
regions and the data are compared with next-to-leading order (NLO) perturbative
QCD calculations using the "massive charm" and "massless charm" schemes. The
measured cross sections are generally above the NLO calculations, in particular
in the forward (proton) direction. The large data sample also allows the study
of dijet production associated with charm. A significant resolved as well as a
direct photon component contribute to the cross section. Leading order QCD
Monte Carlo calculations indicate that the resolved contribution arises from a
significant charm component in the photon. A massive charm NLO parton level
calculation yields lower cross sections compared to the measured results in a
kinematic region where the resolved photon contribution is significant.Comment: 32 pages including 6 figure
Angular and Current-Target Correlations in Deep Inelastic Scattering at HERA
Correlations between charged particles in deep inelastic ep scattering have
been studied in the Breit frame with the ZEUS detector at HERA using an
integrated luminosity of 6.4 pb-1. Short-range correlations are analysed in
terms of the angular separation between current-region particles within a cone
centred around the virtual photon axis. Long-range correlations between the
current and target regions have also been measured. The data support
predictions for the scaling behaviour of the angular correlations at high Q2
and for anti-correlations between the current and target regions over a large
range in Q2 and in the Bjorken scaling variable x. Analytic QCD calculations
and Monte Carlo models correctly describe the trends of the data at high Q2,
but show quantitative discrepancies. The data show differences between the
correlations in deep inelastic scattering and e+e- annihilation.Comment: 26 pages including 10 figures (submitted to Eur. J. Phys. C
Plastisol Foaming Process. Decomposition of the Foaming Agent, Polymer Behavior in the Corresponding Temperature Range and Resulting Foam Properties
The decomposition of azodicarbonamide, used as foaming agent in PVC - plasticizer (1/1) plastisols was studied by DSC. Nineteen different plasticizers, all belonging to the ester family, two being polymeric (polyadipates), were compared. The temperature of maximum decomposition rate (in anisothermal regime at 5 K min-1 scanning rate), ranges between 434 and 452 K. The heat of decomposition ranges between 8.7 and 12.5 J g -1. Some trends of variation of these parameters appear significant and are discussed in terms of solvent (matrix) and viscosity effects on the decomposition reactions. The shear modulus at 1 Hz frequency was determined at the temperature of maximum rate of foaming agent decomposition, and differs significantly from a sample to another. The foam density was determined at ambient temperature and the volume fraction of bubbles was used as criterion to judge the efficiency of the foaming process. The results reveal the existence of an optimal shear modulus of the order of 2 kPa that corresponds roughly to plasticizer molar masses of the order of 450 ± 50 g mol-1. Heavier plasticizers, especially polymeric ones are too difficult to deform. Lighter plasticizers such as diethyl phthalate (DEP) deform too easily and presumably facilitate bubble collapse
Observation of Scaling Violations in Scaled Momentum Distributions at HERA
Charged particle production has been measured in deep inelastic scattering
(DIS) events over a large range of and using the ZEUS detector. The
evolution of the scaled momentum, , with in the range 10 to 1280
, has been investigated in the current fragmentation region of the Breit
frame. The results show clear evidence, in a single experiment, for scaling
violations in scaled momenta as a function of .Comment: 21 pages including 4 figures, to be published in Physics Letters B.
Two references adde
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