1,610 research outputs found
A weakly nonlinear analysis of the magnetorotational instability in a model channel flow
We show by means of a perturbative weakly nonlinear analysis that the
axisymmetric magnetorotational instability (MRI) of a viscous, resistive,
incompressible rotating shear flow in a thin channel gives rise to a real
Ginzburg-Landau equation for the disturbance amplitude. For small magnetic
Prandtl number (), the saturation amplitude is and the resulting momentum transport scales as , where is the {\em hydrodynamic} Reynolds number. Simplifying
assumptions, such as linear shear base flow, mathematically expedient boundary
conditions and continuous spectrum of the vertical linear modes, are used to
facilitate this analysis. The asymptotic results are shown to comply with
numerical calculations using a spectral code. They suggest that the transport
due to the nonlinearly developed MRI may be very small in experimental setups
with .Comment: Accepted to Physical Review Letters - Nov. 30, 2006. In final for
What are the factors that contribute to road accidents? An assessment of law enforcement views, ordinary drivers’ opinions, and road accident records
What are the main contributing factors to road accidents? Factors such as inexperience, lack of skill, and risk-taking behaviors have been associated with the collisions of young drivers. In contrast, visual, cognitive, and mobility impairment have been associated with the collisions of older drivers. We investigated the main causes of road accidents by drawing on multiple sources: expert views of police officers, lay views of the driving public, and official road accident records. In Studies 1 and 2, police officers and the public were asked about the typical causes of road traffic collisions using hypothetical accident scenarios. In Study 3, we investigated whether the views of police officers and the public about accident causation influence their recall accuracy for factors reported to contribute to hypothetical road accidents. The results show that both expert views of police officers and lay views of the driving public closely approximated the typical factors associated with the collisions of young and older drivers, as determined from official accident records. The results also reveal potential underreporting of factors in existing accident records, identifying possible inadequacies in law enforcement practices for investigating driver distraction, drug and alcohol impairment, and uncorrected or defective eyesight. Our investigation also highlights a need for accident report forms to be continuously reviewed and updated to ensure that contributing factor lists reflect the full range of factors that contribute to road accidents. Finally, the views held by police officers and the public on accident causation influenced their memory recall of factors involved in hypothetical scenarios. These findings indicate that delay in completing accident report forms should be minimised, possibly by use of mobile reporting devices at the accident scene
How much measurement independence is needed in order to demonstrate nonlocality?
If nonlocality is to be inferred from a violation of Bell's inequality, an
important assumption is that the measurement settings are freely chosen by the
observers, or alternatively, that they are random and uncorrelated with the
hypothetical local variables. We study the case where this assumption is
weakened, so that measurement settings and local variables are at least
partially correlated. As we show, there is a connection between this type of
model and models which reproduce nonlocal correlations by allowing classical
communication between the distant parties, and a connection with models that
exploit the detection loophole. We show that even if Bob's choices are
completely independent, all correlations obtained from projective measurements
on a singlet can be reproduced, with the correlation (measured by mutual
information) between Alice's choice and local variables less than or equal to a
single bit.Comment: 5 pages, 1 figure. v2 Various improvements in presentation. Results
unchange
Effects of dissipation in an adiabatic quantum search algorithm
We consider the effect of two different environments on the performance of
the quantum adiabatic search algorithm, a thermal bath at finite temperature,
and a structured environment similar to the one encountered in systems coupled
to the electromagnetic field that exists within a photonic crystal. While for
all the parameter regimes explored here, the algorithm performance is worsened
by the contact with a thermal environment, the picture appears to be different
when considering a structured environment. In this case we show that, by tuning
the environment parameters to certain regimes, the algorithm performance can
actually be improved with respect to the closed system case. Additionally, the
relevance of considering the dissipation rates as complex quantities is
discussed in both cases. More particularly, we find that the imaginary part of
the rates can not be neglected with the usual argument that it simply amounts
to an energy shift, and in fact influences crucially the system dynamics.Comment: 18 pages, 9 figure
The Determination of induction and differentiation in grape vines
The induction and differentiation of 8-year-old Alphonse Lavallee and Sultana grape vines were studied.Defoliation methods enabled us to determine the induction time in grape vines as in other fruit species.Induction and differentiation in the tested varieties were not connected with temporary growth cessation; on the contrary, process took place during the most intensive growth.A correlation was found between the number of leaves and induction period. 18-21 leaves above the examined buds were needed in bot-h varieties to complete the induction.The leaf area needed for induction in a bud of Sultana was lYe times larger than that needed for Alphonse. The efficiency of the leaves of Alphonse to induce differentiation was thus greater.The primordia ,development from induction to detection under the microscope (differentiation) was connected with a constant vegetative development. The time needed for this development was determined by the growth rate of the variety (18 days in Sultana, 14 days in Alphonse).The translocation of materials inducing differentiation from the base of the shoot upwar,ds has not been proved in our work.In Alphonse a lag period of two days was found for the differentiation of each bud along the cane
The Hall instability of thin weakly-ionized stratified Keplerian disks
The stratification-driven Hall instability in a weakly ionized polytropic
plasma is investigated in the local approximation within an equilibrium
Keplerian disk of a small aspect ratio. The leading order of the asymptotic
expansions in the aspect ratio is applied to both equilibrium as well as the
perturbation problems. The equilibrium disk with an embedded purely toroidal
magnetic field is found to be stable to radial, and unstable to vertical
short-wave perturbations. The marginal stability surface is found in the space
of the local Hall and inverse plasma beta parameters, as well as the free
parameter of the model which is related to the total current through the disk.
To estimate the minimal values of the equilibrium magnetic field that leads to
instability, the latter is constructed as a sum of a current free magnetic
field and the simplest approximation for magnetic field created by a
distributed electric current.Comment: 13 pages, 7 figure
What is essential? – a pilot survey on views about the requirements metamodel of reqT.org
[Context & motivation] This research preview paper presents ongoing work on the metamodel of a free software requirements modeling tool called reqT that is developed in an educational context. The work aims to make an initial validation of a survey instrument that elicits views on the metamodel of the reqT tool, which aims to engage computer science students in Requirements Engineering (RE) through an open source DSL embedded in the Scala programming language. [Question] The research question is: Which RE concepts are essential to include in the metamodel for a requirements engineering tool in an educational context? [Principal ideas] A survey instrument is developed, with a list of 92 concepts (49 entities, 15 relations and 28 attributes) and a set of questions for each concept, to elicit the respondents’ views on the usage and interpretation of each concept. [Contribution] The survey is initially validated in a pilot study involving 14 Swedish RE scholars as subjects. The survey results indicate that the survey is feasible. The analysis of the responses suggest that many of the concepts in the metamodel are used frequently by the respondents and there is a large degree of agreement among the respondents about the meaning of the concepts. The results are encouraging for future work on empirical validation of the relevance of the reqT metamodel
An Introduction to Quantum Programming in Quipper
Quipper is a recently developed programming language for expressing quantum
computations. This paper gives a brief tutorial introduction to the language,
through a demonstration of how to make use of some of its key features. We
illustrate many of Quipper's language features by developing a few well known
examples of Quantum computation, including quantum teleportation, the quantum
Fourier transform, and a quantum circuit for addition.Comment: 15 pages, RC201
Perspective on Reversible to Irreversible Transitions in Periodic Driven Many Body Systems and Future Directions For Classical and Quantum Systems
Reversible to irreversible (R-IR) transitions arise in numerous periodically
driven collectively interacting systems that, after a certain number of driving
cycles, organize into a reversible state where the particle trajectories
repeat, or remain irreversible with chaotic motion. R-IR transitions were first
systematically studied for periodically sheared dilute colloids, and appear in
a wide variety of both soft and hard matter systems, including amorphous
solids, crystals, vortices in type-II superconductors, and magnetic textures.
In some cases, the reversible transition is an absorbing phase transition with
a critical divergence in the organization time scale. R-IR systems can store
multiple memories and exhibit return point memory. We give an overview of R-IR
transitions including recent advances in the field, and discuss how the general
framework of R-IR transitions could be applied to a much broader class of
periodically driven nonequilibrium systems, including soft and hard condensed
matter systems, astrophysics, biological systems, and social systems. Some
likely candidate systems are commensurate-incommensurate states, systems
exhibiting hysteresis or avalanches, and nonequilibrium pattern forming states.
Periodic driving could be applied to hard condensed matter systems to see if
R-IR transitions occur in metal-insulator transitions, semiconductors, electron
glasses, electron nematics, cold atom systems, or Bose-Einstein condensates.
R-IR transitions could also be examined in dynamical systems where
synchronization or phase locking occurs. We discuss the use of complex periodic
driving such as changing drive directions or multiple frequencies as a method
to retain complex multiple memories. Finally, we describe features of classical
and quantum time crystals that could suggest the occurrence of R-IR transitions
in these systems.Comment: 25 pages, 27 figure
Classical simulation of entanglement swapping with bounded communication
Entanglement appears under two different forms in quantum theory, namely as a
property of states of joint systems and as a property of measurement
eigenstates in joint measurements. By combining these two aspects of
entanglement, it is possible to generate nonlocality between particles that
never interacted, using the protocol of entanglement swapping. We show that
even in the more constraining bilocal scenario where distant sources of
particles are assumed to be independent, i.e. to share no prior randomness,
this process can be simulated classically with bounded communication, using
only 9 bits in total. Our result thus provides an upper bound on the
nonlocality of the process of entanglement swapping.Comment: 6 pages, 1 figur
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