41,539 research outputs found
Two-dimensional polymer networks at a mixed boundary: Surface and wedge exponents
We provide general formulae for the configurational exponents of an arbitrary
polymer network connected to the surface of an arbitrary wedge of the
two-dimensional plane, where the surface is allowed to assume a general mixture
of boundary conditions on either side of the wedge. We report on a
comprehensive study of a linear chain by exact enumeration, with various
attachments of the walk's ends to the surface, in wedges of angles and
, with general mixed boundary conditions.Comment: 4 pages, Latex2e, 3 figures, Eur. Phys. J. B macro
Simulations of satellite Doppler wind observations
This study will involve two objectives: (1) to develop, through computer simulations, optimal satellite-based sensor scanning techniques for direct measurement of tropospheric winds on the meso- and synoptic scales; and (2) to construct simulations of remotely measured wind fields for assessing impact of such fields on the diagnosis and prognosis of atmospheric phenomena through the use of Observing System Simulation Experiments (OSSE). Using the LAWS Simulation Model (LSM), various global coverage scenarios have been investigated as part of an effort to define the optimal orbit, configuration and sampling strategies for observations of winds for use in global circulation models. Simulated data sets have been provided to GSFC, FSU and several LAWS team members. Particular emphasis has been on providing realistic cloud cover, cirrus backscatter, aerosol distribution and wind variance on scales less than 600 km. Progress is currently being made to incorporate other remote sensors (AIRS/AMSU, STIKSCAT) into the global OSSEs
Effect of area ratio on the performance of a 5.5:1 pressure ratio centrifugal impeller
A centrifugal impeller which was initially designed for a pressure ratio of approximately 5.5 and a mass flow rate of 0.959 kg/sec was tested with a vaneless diffuser for a range of design point impeller area ratios from 2.322 to 2.945. The impeller area ratio was changed by successively cutting back the impeller exit axial width from an initial value of 7.57 mm to a final value of 5.97 mm. In all, four separate area ratios were tested. For each area ratio a series of impeller exit axial clearances was also tested. Test results are based on impeller exit surveys of total pressure, total temperature, and flow angle at a radius 1.115 times the impeller exit radius. Results of the tests at design speed, peak efficiency, and an exit tip clearance of 8 percent of exit blade height show that the impeller equivalent pressure recovery coefficient peaked at a design point area ratio of approximately 2.748 while the impeller aerodynamic efficiency peaked at a lower value of area ratio of approximately 2.55. The variation of impeller efficiency with clearance showed expected trends with a loss of approximately 0.4 points in impeller efficiency for each percent increase in exit axial tip clearance for all impellers tested
Mixing with the radiofrequency single-electron transistor
By configuring a radio-frequency single-electron transistor as a mixer, we
demonstrate a unique implementation of this device, that achieves good charge
sensitivity with large bandwidth about a tunable center frequency. In our
implementation we achieve a measurement bandwidth of 16 MHz, with a tunable
center frequency from 0 to 1.2 GHz, demonstrated with the transistor operating
at 300 mK. Ultimately this device is limited in center frequency by the RC time
of the transistor's center island, which for our device is ~ 1.6 GHz, close to
the measured value. The measurement bandwidth is determined by the quality
factor of the readout tank circuit.Comment: Submitted to APL september 200
Two blowing concepts for roll and lateral control of aircraft
Two schemes to modulate aerodynamic forces for roll and lateral control of aircraft have been investigated. The first scheme, called the lateral blowing concept, consists of thin jets of air exiting spanwise, or at small angle with the spanwise direction, from slots at the tips of straight wings. For this scheme, in addition to experimental measurements, a theory was developed showing the analytical relationship between aerodynamic forces and jet and wing parameters. Experimental results confirmed the theoretically derived scaling laws. The second scheme, which was studied experimentally, is called the jet spoiler concept and consists of thin jets exiting normally to the wing surface from slots aligned with the spanwise direction
Evolutionary calculations of phase separation in crystallizing white dwarf stars
We present an exploration of the significance of Carbon/Oxygen phase
separation in white dwarf stars in the context of self-consistent evolutionary
calculations. Because phase separation can potentially increase the calculated
ages of the oldest white dwarfs, it can affect the age of the Galactic disk as
derived from the downturn in the white dwarf luminosity function. We find that
the largest possible increase in ages due to phase separation is 1.5 Gyr, with
a most likely value of approximately 0.6 Gyr, depending on the parameters of
our white dwarf models.
The most important factors influencing the size of this delay are the total
stellar mass, the initial composition profile, and the phase diagram assumed
for crystallization. We find a maximum age delay in models with masses of 0.6
solar masses, which is near the peak in the observed white dwarf mass
distribution. We find that varying the opacities (via the metallicity) has
little effect on the calculated age delays.
In the context of Galactic evolution, age estimates for the oldest Galactic
globular clusters range from 11.5 to 16 Gyr, and depend on a variety of
parameters. In addition, a 4 to 6 Gyr delay is expected between the formation
of the globular clusters and that of the Galactic thin disk, while the observed
white dwarf luminosity function gives an age estimate for the thin disk of 9.5
+/-1.0 Gyr, without including the effect of phase separation. Using the above
numbers, we see that phase separation could add between 0 to 3 Gyr to the white
dwarf ages and still be consistent with the overall picture of Galaxy
formation. Our calculated maximum value of 1.5 Gyr fits within these bounds, as
does our best guess value of 0.6 Gyr.Comment: 13 total pages, 8 figures, 3 tables, accepted for publication in the
Astrophysical Journal on May 25, 199
An Audible Demonstration Of The Speed Of Sound In Bubbly Liquids
The speed of sound in a bubbly liquid is strongly dependent upon the volume fraction of the gas phase, the bubble size distribution, and the frequency of the acoustic excitation. At sufficiently low frequencies, the speed of sound depends primarily on the gas volume fraction. This effect can be audibly demonstrated using a one-dimensional acoustic waveguide, in which the flow rate of air bubbles injected into a water-filled tube is varied by the user. The normal modes of the waveguide are excited by the sound of the bubbles being injected into the tube. As the flow rate is varied, the speed of sound varies as well, and hence, the resonance frequencies shift. This can be clearly heard through the use of an amplified hydrophone and the user can create aesthetically pleasing and even musical sounds. In addition, the apparatus can be used to verify a simple mathematical model known as Wood's equation that relates the speed of sound of a bubbly liquid to its void fraction. (c) 2008 American Association of Physics Teachers.Mechanical Engineerin
The lead discrepancy in intrinsically s-process enriched post-AGB stars in the Magellanic Clouds
Context: Our understanding of the s-process nucleosynthesis in asymptotic
giant branch (AGB) stars is incomplete. AGB models predict, for example, large
overabundances of lead (Pb) compared to other s-process elements in metal-poor
low-mass AGB stars. This is indeed observed in some extrinsically enhanced
metal-poor stars, but not in all. An extensive study of intrinsically s-process
enriched objects is essential for improving our knowledge of the AGB third
dredge-up and associated s-process nucleosynthesis. Aims: We compare the
spectral abundance analysis of the SMC post-AGB star J004441.04-732136.4 with
state-of-the-art AGB model predictions with a main focus on Pb. The low S/N in
the Pb line region made the result of our previous study inconclusive. We
acquired additional data covering the region of the strongest Pb line. Methods:
By carefully complementing re-reduced previous data, with newly acquired UVES
optical spectra, we improve the S/N of the spectrum around the strongest Pb
line. Therefore, an upper limit for the Pb abundance is estimated from a merged
weighted mean spectrum using synthetic spectral modeling. We then compare the
abundance results from the combined spectra to predictions of tailored AGB
evolutionary models from two independent evolution codes. In addition, we
determine upper limits for Pb abundances for three previously studied LMC
post-AGB objects. Results: Although theoretical predictions for
J004441.04-732136.4 match the s-process distribution up to tungsten (W), the
predicted very high Pb abundance is clearly not detected. The three additional
LMC post-AGB stars show a similar lack of a very high Pb abundance. Conclusion:
From our study, we conclude that none of these low-mass, low-metallicity
post-AGB stars of the LMC and SMC are strong Pb producers. This conflicts with
current theoretical predictions.Comment: 4 pages, 3 figure
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