46,011 research outputs found
Swarm behavior of self-propelled rods and swimming flagella
Systems of self-propelled particles are known for their tendency to aggregate
and to display swarm behavior. We investigate two model systems, self-propelled
rods interacting via volume exclusion, and sinusoidally-beating flagella
embedded in a fluid with hydrodynamic interactions. In the flagella system,
beating frequencies are Gaussian distributed with a non-zero average. These
systems are studied by Brownian-dynamics simulations and by mesoscale
hydrodynamics simulations, respectively. The clustering behavior is analyzed as
the particle density and the environmental or internal noise are varied. By
distinguishing three types of cluster-size probability density functions, we
obtain a phase diagram of different swarm behaviors. The properties of
clusters, such as their configuration, lifetime and average size are analyzed.
We find that the swarm behavior of the two systems, characterized by several
effective power laws, is very similar. However, a more careful analysis reveals
several differences. Clusters of self-propelled rods form due to partially
blocked forward motion, and are therefore typically wedge-shaped. At higher rod
density and low noise, a giant mobile cluster appears, in which most rods are
mostly oriented towards the center. In contrast, flagella become
hydrodynamically synchronized and attract each other; their clusters are
therefore more elongated. Furthermore, the lifetime of flagella clusters decays
more quickly with cluster size than of rod clusters
Help me describe my data: A demonstration of the Open PHACTS VoID Editor
Abstract. The Open PHACTS VoID Editor helps non-Semantic Web experts to create machine interpretable descriptions for their datasets. The web app guides the user, an expert in the domain of the data, through a series of questions to capture details of their dataset and then generates a VoID dataset description. The generated dataset description conforms to the Open PHACTS dataset description guidelines that en-sure suitable provenance information is available about the dataset to enable its discovery and reuse
Instant restore after a media failure
Media failures usually leave database systems unavailable for several hours
until recovery is complete, especially in applications with large devices and
high transaction volume. Previous work introduced a technique called
single-pass restore, which increases restore bandwidth and thus substantially
decreases time to repair. Instant restore goes further as it permits read/write
access to any data on a device undergoing restore--even data not yet
restored--by restoring individual data segments on demand. Thus, the restore
process is guided primarily by the needs of applications, and the observed mean
time to repair is effectively reduced from several hours to a few seconds.
This paper presents an implementation and evaluation of instant restore. The
technique is incrementally implemented on a system starting with the
traditional ARIES design for logging and recovery. Experiments show that the
transaction latency perceived after a media failure can be cut down to less
than a second and that the overhead imposed by the technique on normal
processing is minimal. The net effect is that a few "nines" of availability are
added to the system using simple and low-overhead software techniques
The magnetic fields of forming solar-like stars
Magnetic fields play a crucial role at all stages of the formation of low
mass stars and planetary systems. In the final stages, in particular, they
control the kinematics of in-falling gas from circumstellar discs, and the
launching and collimation of spectacular outflows. The magnetic coupling with
the disc is thought to influence the rotational evolution of the star, while
magnetised stellar winds control the braking of more evolved stars and may
influence the migration of planets. Magnetic reconnection events trigger
energetic flares which irradiate circumstellar discs with high energy particles
that influence the disc chemistry and set the initial conditions for planet
formation. However, it is only in the past few years that the current
generation of optical spectropolarimeters have allowed the magnetic fields of
forming solar-like stars to be probed in unprecedented detail. In order to do
justice to the recent extensive observational programs new theoretical models
are being developed that incorporate magnetic fields with an observed degree of
complexity. In this review we draw together disparate results from the
classical electromagnetism, molecular physics/chemistry, and the geophysics
literature, and demonstrate how they can be adapted to construct models of the
large scale magnetospheres of stars and planets. We conclude by examining how
the incorporation of multipolar magnetic fields into new theoretical models
will drive future progress in the field through the elucidation of several
observational conundrums.Comment: 55 pages, review article accepted for publication in Reports on
Progress in Physics. Astro-ph version includes additional appendice
Unified molecular field theory of nematic, smectic-A, and smectic-C phases
A unified mean-field molecular theory of nematic (NU), smectic A (SmA), and smectic C (SmC) liquid crystal phases, composed of uniaxial nonpolar molecules, is developed taking into account the variation of all orientational and translational order parameters in these phases. Numerical results, obtained by direct global minimization of the free energy, are presented in the form of three typical phase diagrams of different topology. Temperature variation of the relevant order parameters in different sequences of phases is analyzed for various cross sections of the phase diagrams. The present model enables one to reproduce all possible sequences of phase transitions between the given phases including isotropic (Iso)-NU-SmA-SmC, Iso-NU-SmC, Iso-SmA-SmC, and Iso-SmC. The properties of the NAC point, where the NU, SmA, and SmC structures coexist, are considered in detail and the shape of the phase diagram in the vicinity of the NAC point is compared with existing experimental data
Romania's evolving legal framework for private sector development
As the economies of Central and Eastern Europe move from central planning and state ownership to market-driven development of private sector activity, they are undertaking comprehensive change in the"rules of the game", the legal framework for economic activity. The authors analyze the evolving legal framework for private sector development in Romania. The government has worked intensively in the last two years to create a legal framework for a market economy. It has adopted not only a new constitution but also extensive new legislation covering real and intellectual property, companies, and foreign investment. It has revived the pre-war civil code as a basis for contract law, and is moving to modernize its bankruptcy code. The only area surveyed in which little legal reform has occurred is antimonopoly law. Challenges remain in both law and practice. The broad principles of private ownership, free market exchange, and equal treatment of public and private firms are well recognized and have been largely achieved. But a tendency towards centralized, bureaucratic control remains in excessive requirements for approval and uneconomic limits on certain activities. Moreover, implementation will take a long time because there is little or no institutional framework for enforcement and dispute resolution. Developing a body of regulation and case practice will take time.Environmental Economics&Policies,National Governance,Legal Products,Banks&Banking Reform,Real&Intellectual Property Law
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