15,123 research outputs found
Performance characteristics of wind profiling radars
Doppler radars used to measure winds in the troposphere and lower stratosphere for weather analysis and forecasting are lower-sensitivity versions of mesosphere-stratosphere-troposphere radars widely used for research. The term wind profiler is used to denote these radars because measurements of vertical profiles of horizontal and vertical wind are their primary function. It is clear that wind profilers will be in widespread use within five years: procurement of a network of 30 wind profilers is underway. The Wave Propagation Laboratory (WPL) has operated a small research network of radar wind profilers in Colorado for about two and one-half years. The transmitted power and antenna aperture for these radars is given. Data archiving procedures have been in place for about one year, and this data base is used to evaluate the performance of the radars. One of the prime concerns of potential wind profilers users is how often and how long wind measurements are lacking at a given height. Since these outages constitute an important part of the performance of the wind profilers, they are calculated at three radar frequencies, 50-, 405-, and 915-MHz, (wavelengths of 6-, 0.74-, and 0.33-m) at monthly intervals to determine both the number of outages at each frequency and annual variations in outages
"Locally homogeneous turbulence" Is it an inconsistent framework?
In his first 1941 paper Kolmogorov assumed that the velocity has increments
which are homogeneous and independent of the velocity at a suitable reference
point. This assumption of local homogeneity is consistent with the nonlinear
dynamics only in an asymptotic sense when the reference point is far away. The
inconsistency is illustrated numerically using the Burgers equation.
Kolmogorov's derivation of the four-fifths law for the third-order structure
function and its anisotropic generalization are actually valid only for
homogeneous turbulence, but a local version due to Duchon and Robert still
holds. A Kolomogorov--Landau approach is proposed to handle the effect of
fluctuations in the large-scale velocity on small-scale statistical properties;
it is is only a mild extension of the 1941 theory and does not incorporate
intermittency effects.Comment: 4 pages, 2 figure
Statistics of mixing in three-dimensional Rayleigh--Taylor turbulence at low Atwood number and Prandtl number one
Three-dimensional miscible Rayleigh--Taylor (RT) turbulence at small Atwood
number and at Prandtl number one is investigated by means of high resolution
direct numerical simulations of the Boussinesq equations. RT turbulence is a
paradigmatic time-dependent turbulent system in which the integral scale grows
in time following the evolution of the mixing region. In order to fully
characterize the statistical properties of the flow, both temporal and spatial
behavior of relevant statistical indicators have been analyzed.
Scaling of both global quantities ({\it e.g.}, Rayleigh, Nusselt and Reynolds
numbers) and scale dependent observables built in terms of velocity and
temperature fluctuations are considered. We extend the mean-field analysis for
velocity and temperature fluctuations to take into account intermittency, both
in time and space domains. We show that the resulting scaling exponents are
compatible with those of classical Navier--Stokes turbulence advecting a
passive scalar at comparable Reynolds number. Our results support the scenario
of universality of turbulence with respect to both the injection mechanism and
the geometry of the flow
On the von Karman-Howarth equations for Hall MHD flows
The von Karman-Howarth equations are derived for three-dimensional (3D) Hall
magnetohydrodynamics (MHD) in the case of an homogeneous and isotropic
turbulence. From these equations, we derive exact scaling laws for the
third-order correlation tensors. We show how these relations are compatible
with previous heuristic and numerical results. These multi-scale laws provide a
relevant tool to investigate the non-linear nature of the high frequency
magnetic field fluctuations in the solar wind or, more generally, in any plasma
where the Hall effect is important.Comment: 11 page
Rough-conduit flows and the existence of fully developed turbulence
It is widely believed that at high Reynolds number (Re) all turbulent flows
approach a state of "fully developed turbulence" defined by a unique,
Re-independent statistics of the velocity fluctuations. Yet direct measurements
of the velocity fluctuations have failed to yield clear-cut empirical evidence
of the existence of fully developed turbulence. Here we relate the friction
coefficient () of rough-conduit flows to the statistics of the velocity
fluctuations. In light of experimental measurements of , our results yield
unequivocal evidence of the existence of fully developed turbulence.Comment: 5 pages, 2 figure
Consequences of a Change in the Galactic Environment of the Sun
The interaction of the heliosphere with interstellar clouds has attracted
interest since the late 1920's, both with a view to explaining apparent
quasi-periodic climate "catastrophes" as well as periodic mass extinctions.
Until recently, however, models describing the solar wind - local interstellar
medium (LISM) interaction self-consistently had not been developed. Here, we
describe the results of a two-dimensional (2D) simulation of the interaction
between the heliosphere and an interstellar cloud with the same properties as
currently, except that the neutral H density is increased from the present
value of n(H) ~ 0.2 cm^-3 to 10 cm^-3. The mutual interaction of interstellar
neutral hydrogen and plasma is included. The heliospheric cavity is reduced
considerably in size (approximately 10 - 14 AU to the termination shock in the
upstream direction) and is highly dynamical. The interplanetary environment at
the orbit of the Earth changes markedly, with the density of interstellar H
increasing to ~2 cm^-3. The termination shock itself experiences periods where
it disappears, reforms and disappears again. Considerable mixing of the shocked
solar wind and LISM occurs due to Rayleigh-Taylor-like instabilities at the
nose, driven by ion-neutral friction. Implications for two anomalously high
concentrations of 10Be found in Antarctic ice cores 33 kya and 60 kya, and the
absence of prior similar events, are discussed in terms of density enhancements
in the surrounding interstellar cloud. The calculation presented here supports
past speculation that the galactic environment of the Sun moderates the
interplanetary environment at the orbit of the Earth, and possibly also the
terrestrial climate.Comment: 23 pages, 2 color plates (jpg), 3 figures (eps
Lagrangian Velocity Statistics in Turbulent Flows: Effects of Dissipation
We use the multifractal formalism to describe the effects of dissipation on
Lagrangian velocity statistics in turbulent flows. We analyze high Reynolds
number experiments and direct numerical simulation (DNS) data. We show that
this approach reproduces the shape evolution of velocity increment probability
density functions (PDF) from Gaussian to stretched exponentials as the time lag
decreases from integral to dissipative time scales. A quantitative
understanding of the departure from scaling exhibited by the magnitude
cumulants, early in the inertial range, is obtained with a free parameter
function D(h) which plays the role of the singularity spectrum in the
asymptotic limit of infinite Reynolds number. We observe that numerical and
experimental data are accurately described by a unique quadratic D(h) spectrum
which is found to extend from to , as
the signature of the highly intermittent nature of Lagrangian velocity
fluctuations.Comment: 5 pages, 3 figures, to appear in PR
Kolmogorov's law for two-dimensional electron-magnetohydrodynamic turbulence
The analogue of the Kolmogorov's four-fifths law is derived for
two-dimensional, homogeneous, isotropic EMHD turbulence in the energy cascade
inertial range. Direct numerical simulations for the freely decaying case show
that this relation holds true for different values of the adimensional electron
inertial length scale, . The energy spectrum is found to be close to the
expected Kolmogorov spectrum.Comment: 9 pages RevTeX, 3 PostScript figure
On Making Good Games - Using Player Virtue Ethics and Gameplay Design Patterns to Identify Generally Desirable Gameplay Features
This paper uses a framework of player virtues to perform a
theoretical exploration of what is required to make a game
good. The choice of player virtues is based upon the view
that games can be seen as implements, and that these are
good if they support an intended use, and the intended use
of games is to support people to be good players. A collection of gameplay design patterns, identified through
their relation to the virtues, is presented to provide specific starting points for considering design options for this type of good games. 24 patterns are identified supporting the virtues, including RISK/REWARD, DYNAMIC ALLIANCES, GAME MASTERS, and PLAYER DECIDED RESULTS, as are 7 countering three or more virtues, including ANALYSIS
PARALYSIS, EARLY ELIMINATION, and GRINDING. The paper concludes by identifying limitations of the approach as well as by showing how it can be applied using other views of what are preferable features in games
Bose-Einstein Condensation of Erbium
We report on the achievement of Bose-Einstein condensation of erbium atoms
and on the observation of magnetic Feshbach resonances at low magnetic field.
By means of evaporative cooling in an optical dipole trap, we produce pure
condensates of Er, containing up to atoms. Feshbach
spectroscopy reveals an extraordinary rich loss spectrum with six loss
resonances already in a narrow magnetic-field range up to 3 G. Finally, we
demonstrate the application of a low-field Feshbach resonance to produce a
tunable dipolar Bose-Einstein condensate and we observe its characteristic
d-wave collapse.Comment: 4 pages, 3 figure
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