30,159 research outputs found
Sound propagation from a simple source in a wind tunnel
The nature of the acoustic field of a simple source in a wind tunnel under flow conditions was examined theoretically and experimentally. The motivation of the study was to establish aspects of the theoretical framework for interpreting acoustic data taken (in wind) tunnels using in wind microphones. Three distinct investigations were performed and are described in detail
Investigation of the effects of a moving acoustic medium on jet noise measurements
Noise from an unheated sonic jet in the presence of an external flow is measured in a free-jet wind tunnel using microphones located both inside and outside the flow. Comparison of the data is made with results of similar studies. The results are also compared with theoretical predictions of the source strength for jet noise in the presence of flow and of the effects of sound propagation through a shear layer
Cosmological redshift distortion: deceleration, bias and density parameters from future redshift surveys of galaxies
The observed two-point correlation functions of galaxies in redshift space
become anisotropic due to the geometry of the universe as well as due to the
presence of the peculiar velocity field. On the basis of linear perturbation
theory, we expand the induced anisotropies of the correlation functions with
respect to the redshift , and obtain analytic formulae to infer the
deceleration parameter , the density parameter and the
derivative of the bias parameter at in terms of the
observable statistical quantities. The present method does not require any
assumption of the shape and amplitude of the underlying fluctuation spectrum,
and thus can be applied to future redshift surveys of galaxies including the
Sloan Digital Sky Survey. We also evaluate quantitatively the systematic error
in estimating the value of from a galaxy
redshift survey on the basis of a conventional estimator for which
neglects both the geometrical distortion effect and the time evolution of the
parameter . If the magnitude limit of the survey is as faint as 18.5
(in B-band) as in the case of the Sloan Digital Sky Survey, the systematic
error ranges between -20% and 10% depending on the cosmological parameters.
Although such systematic errors are smaller than the statistical errors in the
current surveys, they will dominate the expected statistical error for future
surveys.Comment: 9 pages, 5 figs, aastex, ApJ in press, replaced version includes
minor correction
Interference Phenomena in Medium Induced Radiation
We consider the interference pattern for the medium-induced gluon radiation
produced by a color singlet quark-antiquark antenna embedded in a QCD medium
with size and `jet quenching' parameter . Within the BDMPS-Z
regime, we demonstrate that, for a dipole opening angle , the interference between the
medium--induced gluon emissions by the quark and the antiquark is suppressed
with respect to the direct emissions. This is so since direct emissions are
delocalized throughout the medium and thus yield contributions proportional to
while interference occurs only between emissions at early times, when both
sources remain coherent. Thus, for \tqq \gg\theta_c, the medium-induced
radiation is the sum of the two spectra individually produced by the quark and
the antiquark, without coherence effects like angular ordering. For \tqq
\ll\theta_c, the medium--induced radiation vanishes.Comment: 4 pages, 2 figures; Proceedings of the "Quark Matter 2011" conferenc
Addressing Problems in Evaluating Health-Relevant Programs through Systematic Planning and Evaluation
The authors argue that inconsistent terminology is often a hindrance in assessing health program implementation, effectiveness and efficiency. Attending closely to this, they propose a model scheme for conducting such evaluations
Mean age gradient and asymmetry in the star formation history of the Small Magellanic Cloud
We derive the star formation history in four regions of the Small Magellanic
Cloud (SMC) using the deepest VI color-magnitude diagrams (CMDs) ever obtained
for this galaxy. The images were obtained with the Advanced Camera for Surveys
onboard the Hubble Space Telescope and are located at projected distances of
0.5-2 degrees from the SMC center, probing the main body and the wing of the
galaxy. We derived the star-formation histories (SFH) of the four fields using
two independent procedures to fit synthetic CMDs to the data. We compare the
SFHs derived here with our earlier results for the SMC bar to create a deep
pencil-beam survey of the global history of the central SMC. We find in all the
six fields observed with HST a slow star formation pace from 13 to 5-7 Gyr ago,
followed by a ~ 2-3 times higher activity. This is remarkable because dynamical
models do not predict a strong influence of either the LMC or the Milky Way
(MW) at that time. The level of the intermediate-age SFR enhancement
systematically increases towards the center, resulting in a gradient in the
mean age of the population, with the bar fields being systematically younger
than the outer ones. Star formation over the most recent 500 Myr is strongly
concentrated in the bar, the only exception being the area of the SMC wing. The
strong current activity of the latter is likely driven by interaction with the
LMC. At a given age, there is no significant difference in metallicity between
the inner and outer fields, implying that metals are well mixed throughout the
SMC. The age-metallicity relations we infer from our best fitting models are
monotonically increasing with time, with no evidence of dips. This may argue
against the major merger scenario proposed by Tsujimoto and Bekki 2009,
although a minor merger cannot be ruled out.Comment: 30 pages, 16 figures, accepted for publication in Ap
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