4,359 research outputs found
From gyroscopic to thermal motion: a crossover in the dynamics of molecular superrotors
Localized heating of a gas by intense laser pulses leads to interesting
acoustic, hydrodynamic and optical effects with numerous applications in
science and technology, including controlled wave guiding and remote atmosphere
sensing. Rotational excitation of molecules can serve as the energy source for
raising the gas temperature. Here, we study the dynamics of energy transfer
from the molecular rotation to heat. By optically imaging a cloud of molecular
superrotors, created with an optical centrifuge, we experimentally identify two
separate and qualitatively different stages of its evolution. The first
non-equilibrium "gyroscopic" stage is characterized by the modified optical
properties of the centrifuged gas - its refractive index and optical
birefringence, owing to the ultrafast directional molecular rotation, which
survives tens of collisions. The loss of rotational directionality is found to
overlap with the release of rotational energy to heat, which triggers the
second stage of thermal expansion. The crossover between anisotropic rotational
and isotropic thermal regimes is in agreement with recent theoretical
predictions and our hydrodynamic calculations
Science for Global Ubiquitous Computing
This paper describes an initiative to provide theories that can underlie the development of the Global Ubiquitous Computer, the network of ubiquitous computing devices that will pervade the civilised world in the course of the next few decades. We define the goals of the initiative and the criteria for judging whether they are achieved; we then propose a strategy for the exercise. It must combine a bottom-up development of theories in directions that are currently pursued with success, together with a top-down approach in the form of collaborative projects relating these theories to engineered systems that exist or are imminent
Some aspects of upwelling in the Gulf of Panama
ENGLISH: Strong coastal upwelling occurs in the Gulf of Panama regularly each year during the season, from about January through April, when strong northerly winds are blowing offshore. Because of the evident importance of upwelling to the ecology of the Gulf of Panama, we commenced in the fall of 1954 a study of various physical, chemical, and biological phenomena associated therewith. Observations were taken at bi-weekly intervals at a fixed location in the Gulf (approximately 10 miles SE of Taboga Island) to supplement the serial observations of sea level, sea temperature, and winds that have been gathered for many years by the Panama Canal Company. SPANISH: Cado año, en la estación de enero a abril, cuando los vientos del norte
soplan vigorosamente frente a la costa, ocurre en el Golfo de Panamá un fuerte afloramiento costanero. Se cree que este afloramiento periódico en el Golfo de Panamá es responsable de la alta productividad biológica que sostiene considerables cantidades de organismos de importancia comercial. Esta región, por ejemplo, es una fuente importante de la especie Cetengraulis mysticetus) pez de carnada para el atún, (Alverson y Shimada, 1957) y mantiene una considerable pesca de camarones llamados langostinos (Burkenroad, Obarrio y Mendoza,1955).
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Dynamics of molecular rotors in bulk superfluid helium
Molecules immersed in liquid helium are excellent probes of superfluidity.
Their electronic, vibrational and rotational dynamics provide valuable clues
about the superfluid at the nanoscale. Here we report on the experimental study
of the laser-induced rotation of helium dimers inside the superfluid
bath at variable temperature. The coherent rotational dynamics
of is initiated in a controlled way by ultrashort laser
pulses, and tracked by means of time-resolved laser-induced fluorescence. We
detect the decay of rotational coherence on the nanosecond timescale and
investigate the effects of temperature on the decoherence rate. The observed
temperature dependence suggests a non-equilibrium evolution of the quantum
bath, accompanied by the emission of the wave of second sound. The method
offers new ways of studying superfluidity with molecular nano-probes under
variable thermodynamic conditions
Time-partitioning simulation models for calculation on parallel computers
A technique allowing time-staggered solution of partial differential equations is presented in this report. Using this technique, called time-partitioning, simulation execution speedup is proportional to the number of processors used because all processors operate simultaneously, with each updating of the solution grid at a different time point. The technique is limited by neither the number of processors available nor by the dimension of the solution grid. Time-partitioning was used to obtain the flow pattern through a cascade of airfoils, modeled by the Euler partial differential equations. An execution speedup factor of 1.77 was achieved using a two processor Cray X-MP/24 computer
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