8,106 research outputs found
A laser velocimeter flow survey above a stalled wing
A laser velocimeter operating in the backscatter mode was used to survey the flow about a stalled wing installed in the Langley V/STOL tunnel. Mean velocities and magnitudes of velocity fluctuations were calculated from measurements of two orthogonal components of velocity. Free shear mixing layers above and below a large separated flow region were defined. Velocity power spectra were calculated at two points in the flow field. The flow-field survey was carried out about a rectangular aspect-ratio-8 wing with an airfoil section. The wing angle of attack was 19.4 deg, the Mach number was 0.148, and the nominal Reynolds number was 1 x 1 million
Correlation of laser velocimeter measurements over a wing with results of two prediction techniques
The flow field at the center line of an unswept wing with an aspect ratio of eight was determined using a two dimensional viscous flow prediction technique for the flow field calculation, and a three dimensional potential flow panel method to evaluate the degree of two dimensionality achieved at the wing center line. The analysis was made to provide an acceptable reference for comparison with velocity measurements obtained from a fringe type laser velocimeter optics systems operating in the backscatter mode in the Langley V/STOL tunnel. Good agreement between laser velocimeter measurements and theoretical results indicate that both methods provide a true representation of the velocity field about the wing at angles of attack of 0.6 and 4.75 deg
AN EVALUATION OF THE 1981 FARM PROGRAM FOR CROPS: IMPLICATIONS FOR THE 1985 FARM BILL
Agricultural and Food Policy,
Predicting the size and probability of epidemics in a population with heterogeneous infectiousness and susceptibility
We analytically address disease outbreaks in large, random networks with
heterogeneous infectivity and susceptibility. The transmissibility
(the probability that infection of causes infection of ) depends on the
infectivity of and the susceptibility of . Initially a single node is
infected, following which a large-scale epidemic may or may not occur. We use a
generating function approach to study how heterogeneity affects the probability
that an epidemic occurs and, if one occurs, its attack rate (the fraction
infected). For fixed average transmissibility, we find upper and lower bounds
on these. An epidemic is most likely if infectivity is homogeneous and least
likely if the variance of infectivity is maximized. Similarly, the attack rate
is largest if susceptibility is homogeneous and smallest if the variance is
maximized. We further show that heterogeneity in infectious period is
important, contrary to assumptions of previous studies. We confirm our
theoretical predictions by simulation. Our results have implications for
control strategy design and identification of populations at higher risk from
an epidemic.Comment: 5 pages, 3 figures. Submitted to Physical Review Letter
Effect of reheating on predictions following multiple-field inflation
We study the sensitivity of cosmological observables to the reheating phase
following inflation driven by many scalar fields. We describe a method which
allows semi-analytic treatment of the impact of perturbative reheating on
cosmological perturbations using the sudden decay approximation. Focusing on
-quadratic inflation, we show how the scalar spectral index and
tensor-to-scalar ratio are affected by the rates at which the scalar fields
decay into radiation. We find that for certain choices of decay rates,
reheating following multiple-field inflation can have a significant impact on
the prediction of cosmological observables.Comment: Published in PRD. 4 figures, 10 page
Exchange Rates, Trade Deficits, and U.S. Prices
Granger Causality tests are run to examine the causal relationships between the trade deficit, another macroeconomic variable, and U.S. prices. The results show that strong causal relationships run from the trade deficit to agricultural prices but not to nonagricultural prices. Results of causality tests also indicate that the huge trade deficit is caused by a strong U.S. dollar
Significance of low energy impact damage on modal parameters of composite beams by design of experiments
This paper presents an experimental study on the effects of multi-site damage on the vibration response of composite beams damaged by low energy impacts around the barely visible impact damage limit (BVID). The variation of the modal parameters with different levels of impact energy and density of damage is studied. Vibration tests have been carried out with both burst random and classical sine dwell excitations in order to compare that which of the methods among Polymax and Half Bandwidth Method is more suitable for damping estimation in the presence of damage. Design of experiments (DOE) performed on the experimental data show that natural frequency is a more sensitive parameter for damage detection than the damping ratio. It also highlighted energy of impact as the factor having a more significant effect on the modal parameters. Half Bandwidth Method is found to be unsuitable for damping estimation in the presence of damage
Incommensurate phonon anomaly and the nature of charge density waves in cuprates
While charge density wave (CDW) instabilities are ubiquitous to
superconducting cuprates, the different ordering wavevectors in various cuprate
families have hampered a unified description of the CDW formation mechanism.
Here we investigate the temperature dependence of the low energy phonons in the
canonical CDW ordered cuprate LaBaCuO. We discover
that the phonon softening wavevector associated with CDW correlations becomes
temperature dependent in the high-temperature precursor phase and changes from
a wavevector of 0.238 reciprocal space units (r.l.u.) below the ordering
transition temperature up to 0.3~r.l.u. at 300~K. This high-temperature
behavior shows that "214"-type cuprates can host CDW correlations at a similar
wavevector to previously reported CDW correlations in non-"214"-type cuprates
such as YBaCuO. This indicates that cuprate CDWs may
arise from the same underlying instability despite their apparently different
low temperature ordering wavevectors.Comment: Accepted in Phys. Rev. X; 9 pages; 5 figures; 3 pages of
supplementary materia
Heterogeneous Bond Percolation on Multitype Networks with an Application to Epidemic Dynamics
Considerable attention has been paid, in recent years, to the use of networks
in modeling complex real-world systems. Among the many dynamical processes
involving networks, propagation processes -- in which final state can be
obtained by studying the underlying network percolation properties -- have
raised formidable interest. In this paper, we present a bond percolation model
of multitype networks with an arbitrary joint degree distribution that allows
heterogeneity in the edge occupation probability. As previously demonstrated,
the multitype approach allows many non-trivial mixing patterns such as
assortativity and clustering between nodes. We derive a number of useful
statistical properties of multitype networks as well as a general phase
transition criterion. We also demonstrate that a number of previous models
based on probability generating functions are special cases of the proposed
formalism. We further show that the multitype approach, by naturally allowing
heterogeneity in the bond occupation probability, overcomes some of the
correlation issues encountered by previous models. We illustrate this point in
the context of contact network epidemiology.Comment: 10 pages, 5 figures. Minor modifications were made in figures 3, 4
and 5 and in the text. Explanations and references were adde
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