1,339 research outputs found
Rotation of a spheroid in a simple shear at small Reynolds number
We derive an effective equation of motion for the orientational dynamics of a
neutrally buoyant spheroid suspended in a simple shear flow, valid for
arbitrary particle aspect ratios and to linear order in the shear Reynolds
number. We show how inertial effects lift the degeneracy of the Jeffery orbits
and determine the stabilities of the log-rolling and tumbling orbits at
infinitesimal shear Reynolds numbers. For prolate spheroids we find stable
tumbling in the shear plane, log-rolling is unstable. For oblate particles, by
contrast, log-rolling is stable and tumbling is unstable provided that the
aspect ratio is larger than a critical value. When the aspect ratio is smaller
than this value tumbling turns stable, and an unstable limit cycle is born.Comment: 25 pages, 5 figure
Signatures of Radiation Reaction in Ultra-Intense Laser Fields
We discuss radiation reaction effects on charges propagating in ultra-intense
laser fields. Our analysis is based on an analytic solution of the
Landau-Lifshitz equation. We suggest to measure radiation reaction in terms of
a symmetry breaking parameter associated with the violation of null translation
invariance in the direction opposite to the laser beam. As the Landau-Lifshitz
equation is nonlinear the energy transfer within the pulse is rather sensitive
to initial conditions. This is elucidated by comparing colliding and fixed
target modes in electron laser collisions.Comment: 8 pages, 6 figure
Performance and selection of winter durum wheat genotypes in different European conventional and organic fields
Sustainability is a key factor for the future of agriculture. Productivity in agriculture has more than tripled in developed countries since the 1950s. Beyond the success of plant breeding, the increased use of inorganic fertilizers, application of pesticides, and spread of irrigation also contributed to this success. However, impressive yield increases started to decline in the 1980s because of the lack of sustainability. One of the most beneficial ways to increase sustainability is organic agriculture. In such agro-ecosystem-based holistic production systems the prerequisite of successful farming is the availability of crop genotypes that perform well. However, selection of winter durum wheat for sub-optimal growing conditions is still mainly neglected, and the organic seed market also lacks of information on credibly tested winter durum varieties suitable for organic agriculture
Brownian motion and diffusion: from stochastic processes to chaos and beyond
One century after Einstein's work, Brownian Motion still remains both a
fundamental open issue and a continous source of inspiration for many areas of
natural sciences. We first present a discussion about stochastic and
deterministic approaches proposed in the literature to model the Brownian
Motion and more general diffusive behaviours. Then, we focus on the problems
concerning the determination of the microscopic nature of diffusion by means of
data analysis. Finally, we discuss the general conditions required for the
onset of large scale diffusive motion.Comment: RevTeX-4, 11 pages, 5 ps-figures. Chaos special issue "100 Years of
Brownian Motion
Bernstein-type polynomials on several intervals
We construct the analogues of Bernstein polynomials on the set Js of s finitely many intervals. Two cases are considered: first when there are no restrictions on Js, and then when Js has a so-called T-polynomial. On such sets we define approximating operators resembling the classic Bernstein polynomials. Reproducing and interpolation properties as well as estimates for the rate of convergence are given. © Springer International Publishing AG 2017
A Paradox of State-Dependent Diffusion and How to Resolve It
Consider a particle diffusing in a confined volume which is divided into two
equal regions. In one region the diffusion coefficient is twice the value of
the diffusion coefficient in the other region. Will the particle spend equal
proportions of time in the two regions in the long term? Statistical mechanics
would suggest yes, since the number of accessible states in each region is
presumably the same. However, another line of reasoning suggests that the
particle should spend less time in the region with faster diffusion, since it
will exit that region more quickly. We demonstrate with a simple microscopic
model system that both predictions are consistent with the information given.
Thus, specifying the diffusion rate as a function of position is not enough to
characterize the behaviour of a system, even assuming the absence of external
forces. We propose an alternative framework for modelling diffusive dynamics in
which both the diffusion rate and equilibrium probability density for the
position of the particle are specified by the modeller. We introduce a
numerical method for simulating dynamics in our framework that samples from the
equilibrium probability density exactly and is suitable for discontinuous
diffusion coefficients.Comment: 21 pages, 6 figures. Second round of revisions. This is the version
that will appear in Proc Roy So
Self-energy of a scalar charge near higher-dimensional black holes
We study the problem of self-energy of charges in higher dimensional static
spacetimes. Application of regularization methods of quantum field theory to
calculation of the classical self-energy of charges leads to model-independent
results. The correction to the self-energy of a scalar charge due to the
gravitational field of black holes of the higher dimensional
Majumdar-Papapetrou spacetime is calculated exactly. It proves to be zero in
even dimensions, but it acquires non-zero value in odd dimensional spacetimes.
The origin of the self-energy correction in odd dimensions is similar to the
origin the conformal anomalies in quantum field theory in even dimensional
spacetimes.Comment: 9 page
Advanced action in classical electrodynamics
The time evolution of a charged point particle is governed by a second-order
integro-differential equation that exhibits advanced effects, in which the
particle responds to an external force before the force is applied. In this
paper we give a simple physical argument that clarifies the origin and physical
meaning of these advanced effects, and we compare ordinary electrodynamics with
a toy model of electrodynamics in which advanced effects do not occur.Comment: 12 pages, 5 figure
Electromagnetic Simulation and Design of a Novel Waveguide RF Wien Filter for Electric Dipole Moment Measurements of Protons and Deuterons
The conventional Wien filter is a device with orthogonal static magnetic and
electric fields, often used for velocity separation of charged particles. Here
we describe the electromagnetic design calculations for a novel waveguide RF
Wien filter that will be employed to solely manipulate the spins of protons or
deuterons at frequencies of about 0.1 to 2 MHz at the COoler SYnchrotron COSY
at J\"ulich. The device will be used in a future experiment that aims at
measuring the proton and deuteron electric dipole moments, which are expected
to be very small. Their determination, however, would have a huge impact on our
understanding of the universe.Comment: 10 pages, 10 figures, 4 table
The role of inertia for the rotation of a nearly spherical particle in a general linear flow
We analyse the angular dynamics of a neutrally buoyant nearly spherical
particle immersed in a steady general linear flow. The hydrodynamic torque
acting on the particle is obtained by means of a reciprocal theorem, regular
perturbation theory exploiting the small eccentricity of the nearly spherical
particle, and assuming that inertial effects are small, but finite.Comment: 7 pages, 1 figur
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