12,029 research outputs found
Flow direction measurement with fixed probes
Fixed-position probes for determination of flow direction in one and two planes are tested over a wide range of Reynolds numbers and Mach numbers. The work is limited to tests of a single probe design for two dimensional flow and a single design for three dimensional flow
Total-pressure measurement in pulsating flows
Pneumatic-type probe was used as comparison instrument with total pressure tubes to determine true average pressure and, thus, to determine if nonlinear averaging effects were significant. Since pneumatic probe is more complicated to use than a total-pressure tube, it is used only as a comparison instrument to determine extent of averaging effects
Breakdown of Angular Momentum Selection Rules in High Pressure Optical Pumping Experiments
We present measurements, using two complementary methods, of the breakdown of
atomic angular momentum selection rules in He-broadened Rb vapor. Atomic dark
states are rendered weakly absorbing due to fine-structure mixing during Rb-He
collisions. The effect substantially increases the photon demand for optical
pumping of dense vapors
Reciprocal relationships in collective flights of homing pigeons
Collective motion of bird flocks can be explained via the hypothesis of many
wrongs, and/or, a structured leadership mechanism. In pigeons, previous studies
have shown that there is a well-defined hierarchical structure and certain
specific individuals occupy more dominant positions --- suggesting that
leadership by the few individuals drives the behavior of the collective.
Conversely, by analyzing the same data-sets, we uncover a more egalitarian
mechanism. We show that both reciprocal relationships and a stratified
hierarchical leadership are important and necessary in the collective movements
of pigeon flocks. Rather than birds adopting either exclusive averaging or
leadership strategies, our experimental results show that it is an integrated
combination of both compromise and leadership which drives the group's movement
decisions.Comment: 7 pages, 5 figure
Leading SU(3)-breaking corrections to the baryon magnetic moments in Chiral Perturbation Theory
We calculate the baryon magnetic moments using covariant Chiral Perturbation
Theory (PT) within the Extended-on-mass-shell (EOMS) renormalization
scheme. By fitting the two available low-energy constants (LECs), we improve
the Coleman-Glashow description of the data when we include the leading SU(3)
breaking effects coming from the lowest-order loops. This success is in
dramatic contrast with previous attempts at the same order using Heavy Baryon
(HB) PT and covariant Infrared (IR) PT. We also analyze the source
of this improvement with particular attention on the comparison between the
covariant results.Comment: 4 pages, 2 figures, accepted for publication in PR
Large-Amplitude, Pair-Creating Oscillations in Pulsar and Black Hole Magnetospheres
A time-dependent model for pair creation in a pulsar magnetosphere is
developed. It is argued that the parallel electric field that develops in a
charge-starved region (a gap) of a pulsar magnetosphere oscillates with large
amplitude. Electrons and positrons are accelerated periodically and the
amplitude of the oscillations is assumed large enough to cause creation of
upgoing and downgoing pairs at different phases of the oscillation. With a
charge-starved initial condition, we find that the oscillations result in
bursts of pair creation in which the pair density rises exponentially with
time. The pair density saturates at , where is the parallel electric field in the
charge-starved initial state, and is the Lorentz factor for
effec tive pair creation. The frequency of oscillations following the pair
creation burst is given roughly by . A positive feedback keeps the system stable, such that the average pair
creation rate balances the loss rate due to pairs escaping the magnetosphere.Comment: 21 pages, 6 figures, ApJ submitte
Electroweak Fermion-loop Contributions to the Muon Anomalous Magnetic Moment
The two-loop electroweak corrections to the anomalous magnetic moment of the
muon, generated by fermionic loops, are calculated. An interesting role of the
top quark in the anomaly cancellation is observed. New corrections, including
terms of order , are computed and a class of diagrams
previously thought to vanish are found to be important. The total fermionic
correction is which decreases the electroweak
effects on , predicted from one-loop calculations, by 12\%. We give an
updated theoretical prediction for of the muon.Comment: Corrected versio
Intrinsic peculiarities of real material realizations of a spin-1/2 kagome lattice
Spin-1/2 magnets with kagome geometry, being for years a generic object of
theoretical investigations, have few real material realizations. Recently, a
DFT-based microscopic model for two such materials, kapellasite Cu3Zn(OH)6Cl2
and haydeeite Cu3Mg(OH)6Cl2, was presented [O. Janson, J. Richter and H.
Rosner, arXiv:0806.1592]. Here, we focus on the intrinsic properties of real
spin-1/2 kagome materials having influence on the magnetic ground state and the
low-temperature excitations. We find that the values of exchange integrals are
strongly dependent on O--H distance inside the hydroxyl groups, present in most
spin-1/2 kagome compounds up to date. Besides the original kagome model,
considering only the nearest neighbour exchange, we emphasize the crucial role
of the exchange along the diagonals of the kagome lattice.Comment: 4 pages, 4 figures. A paper for the proceedings of the HFM 2008
conferenc
Wind measurement system
A system for remotely measuring vertical and horizontal winds present in discrete volumes of air at selected locations above the ground is described. A laser beam is optically focused in range by a telescope, and the output beam is conically scanned at an angle about a vertical axis. The backscatter, or reflected light, from the ambient particulates in a volume of air, the focal volume, is detected for shifts in wavelength, and from these, horizontal and vertical wind components are computed
Thermophysical and chemical characterization of charring ablative materials Final report
Thermophysical and chemical properties of charring ablative material
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