164,328 research outputs found
Resonant Acoustic Determination of Complex Elastic Moduli
A simple, inexpensive, yet accurate method for measuring the dynamic complex modulus of elasticity is described. Using a 'free-free' bar selectively excited in three independent vibrational modes, the shear modulus is obtained by measuring the frequency of the torsional resonant mode and the Young's modulus is determined from measurement of either the longitudinal or flexural mode. The damping properties are obtained by measuring the quality factor (Q) for each mode. The Q is inversely proportional to the loss tangent. The viscoelastic behavior of the sample can be obtained by tracking a particular resonant mode (and thus a particular modulus) using a phase locked loop (PLL) and by changing the temperature of the sample. The change in the damping properties is obtained by measuring the in-phase amplitude of the PLL which is proportional to the Q of the material. The real and imaginary parts or the complex modulus can be obtained continuously as a function of parameters such as temperature, pressure, or humidity. For homogeneous and isotropic samples only two independent moduli are needed in order to characterize the complete set of elastic constants, thus, values can be obtained for the dynamic Poisson's ratio, bulk modulus, Lame constants, etc
Cosmological constraints from Sunyaev-Zeldovich cluster counts: an approach to account for missing redshifts
The accumulation of redshifts provides a significant observational bottleneck
when using galaxy cluster surveys to constrain cosmological parameters. We
propose a simple method to allow the use of samples where there is a fraction
of the redshifts that are not known. The simplest assumption is that the
missing redshifts are randomly extracted from the catalogue, but the method
also allows one to take into account known selection effects in the
accumulation of redshifts. We quantify the reduction in statistical precision
of cosmological parameter constraints as a function of the fraction of missing
redshifts for simulated surveys, and also investigate the impact of making an
incorrect assumption for the distribution of missing redshifts.Comment: 6 pages, 5 figures, accepted by Ap
Fiber-optic push-pull sensor systems
Fiber-optic push-pull sensors are those which exploit the intrinsically differential nature of an interferometer with concommitant benefits in common-mode rejection of undesired effects. Several fiber-optic accelerometer and hydrophone designs are described. Additionally, the recent development at the Naval Postgraduate School of a passive low-cost interferometric signal demodulator permits the development of economical fiber-optic sensor systems
Flight investigation to evaluate the roll-rate stabilization system of the Naval Ordnance Test Station Sidewinder missile at Mach numbers from 0.9 to 2.3
Foreground removal requirements for measuring large-scale CMB B-modes in light of BICEP2
The most convincing confirmation that the B-mode polarization signal detected
at degree scales by BICEP2 is due to the Cosmic Microwave Background (CMB)
would be the measurement of its large-scale counterpart. We assess the
requirements for diffuse component separation accuracy over large portions of
the sky in order to measure the large-scale B-mode signal corresponding to a
tensor to scalar ratio of r=0.1-0.2.
We use the method proposed by Bonaldi & Ricciardi (2011) to forecast the
performances of different simulated experiments taking into account noise and
foreground removal issues. We do not consider instrumental systematics, and we
implicitly assume that they are not the dominant source of error. If this is
the case, the confirmation of an r=0.1-0.2 signal is achievable by Planck even
for conservative assumptions regarding the accuracy of foreground cleaning. Our
forecasts suggest that the combination of this experiment with BICEP2 will lead
to an improvement of 25-45% in the constraint on r.
A next-generation CMB polarization satellite, represented in this work by the
COrE experiment, can reduce dramatically (by almost another order of magnitude)
the uncertainty on r. In this case, however, the accuracy of foreground removal
becomes critical to fully benefit from the increase in sensitivity.Comment: 8 pages, 3 figures, 1 table. Accepted by MNRA
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