3,536 research outputs found
Vortex Noise from Rotating Cylindrical Rods
A series of round rods of the some diameter were rotated individually about the mid-point of each rod. Vortices are shed from the rods when in motion, giving rise to the emission of sound. With the rotating system placed in the open air, the distribution of sound in space, the acoustical power output, and the spectral distribution have been studied. The frequency of emission of vortices from any point on the rod is given by the formula von Karman. From the spectrum estimates are made of the distribution of acoustical power along the rod, the amount of air concerned in sound production, the "equivalent size" of the vortices, and the acoustical energy content for each vortex
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Modification of Poly(5,6-epoxy-l-norleucine) Gives Functional Polypeptides with Alternative Side-Chain Linkages.
The preparation and characterization of a new epoxide containing polypeptide, poly(5,6-epoxy-l-norleucine), via postpolymerization modification of poly(l-homoallylglycine) is described. Addition of thiols to the epoxide groups in poly(5,6-epoxy-l-norleucine) was studied as a means to prepare side-chain functional polypeptides. The solution properties of the derivatized polypeptides were studied in water and compared to similar thioether containing functional polypeptides prepared via different routes. Subtle differences in side-chain linkage chemistry were found to influence polypeptide solubility, chain conformation in solution, and thermoresponsive behavior. Poly(5,6-epoxy-l-norleucine) was found to be useful as a readily prepared intermediate that can be reacted with thiols to give a variety of functional polypeptides
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Self-Sorting Microscale Compartmentalized Block Copolypeptide Hydrogels
Multicomponent interpenetrating network hydrogels possessing enhanced mechanical stiffness compared to their individual components were prepared via physical mixing of diblock copolypeptides that assemble by either hydrophobic association or polyion complexation in aqueous media. Optical microscopy analysis of fluorescent-probe-labeled multicomponent hydrogels revealed that the diblock copolypeptide components rapidly and spontaneously self-sort to form distinct hydrogel networks that interpenetrate at micron length scales. These materials represent a class of microscale compartmentalized hydrogels composed of degradable, cell-compatible components, which possess rapid self-healing properties and independently tunable domains for downstream applications in biology and additive manufacturing
A Ground-Based Search for Thermal Emission from the Exoplanet TrES-1
Eclipsing planetary systems give us an important window on extrasolar planet
atmospheres. By measuring the depth of the secondary eclipse, when the planet
moves behind the star, we can estimate the strength of the thermal emission
from the day side of the planet. Attaining a ground-based detection of one of
these eclipses has proven to be a significant challenge, as time-dependent
variations in instrument throughput and atmospheric seeing and absorption
overwhelm the small signal of the eclipse at infrared wavelengths. We gathered
a series of simultaneous L grism spectra of the transiting planet system TrES-1
and a nearby comparison star of comparable brightness, allowing us to correct
for these effects in principle. Combining the data from two eclipses, we
demonstrate a detection sensitivity of 0.15% in the eclipse depth relative to
the stellar flux. This approaches the sensitivity required to detect the
planetary emission, which theoretical models predict should lie between
0.05-0.1% of the stellar flux in our 2.9-4.3 micron bandpass. We explore the
factors that ultimately limit the precision of this technique, and discuss
potential avenues for future improvements.Comment: 10 pages, 1 table, four figures, accepted for publication in PAS
Theoretical Spectral Models of the Planet HD 209458b with a Thermal Inversion and Water Emission Bands
We find that a theoretical fit to all the HD 209458b data at secondary
eclipse requires that the dayside atmosphere of HD 209458b have a thermal
inversion and a stratosphere. This inversion is caused by the capture of
optical stellar flux by an absorber of uncertain origin that resides at
altitude. One consequence of stratospheric heating and temperature inversion is
the flipping of water absorption features into emission features from the near-
to the mid-infrared and we see evidence of such a water emission feature in the
recent HD 209458b IRAC data of Knutson et al. In addition, an upper-atmosphere
optical absorber may help explain both the weaker-than-expected Na D feature
seen in transit and the fact that the transit radius at 24 m is smaller
than the corresponding radius in the optical. Moreover, it may be a factor in
why HD 209458b's optical transit radius is as large as it is. We speculate on
the nature of this absorber and the planets whose atmospheres may, or may not,
be affected by its presence.Comment: Accepted to the Astrophysical Journal Letters on August 28, 2007, six
pages in emulateapj forma
Marginalising instrument systematics in HST WFC3 transit lightcurves
Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) infrared observations
at 1.1-1.7m probe primarily the HO absorption band at 1.4m, and
has provided low resolution transmission spectra for a wide range of
exoplanets. We present the application of marginalisation based on Gibson
(2014) to analyse exoplanet transit lightcurves obtained from HST WFC3, to
better determine important transit parameters such as R/R, important
for accurate detections of HO. We approximate the evidence, often referred
to as the marginal likelihood, for a grid of systematic models using the Akaike
Information Criterion (AIC). We then calculate the evidence-based weight
assigned to each systematic model and use the information from all tested
models to calculate the final marginalised transit parameters for both the
band-integrated, and spectroscopic lightcurves to construct the transmission
spectrum. We find that a majority of the highest weight models contain a
correction for a linear trend in time, as well as corrections related to HST
orbital phase. We additionally test the dependence on the shift in spectral
wavelength position over the course of the observations and find that
spectroscopic wavelength shifts , best describe the
associated systematic in the spectroscopic lightcurves for most targets, while
fast scan rate observations of bright targets require an additional level of
processing to produce a robust transmission spectrum. The use of
marginalisation allows for transparent interpretation and understanding of the
instrument and the impact of each systematic evaluated statistically for each
dataset, expanding the ability to make true and comprehensive comparisons
between exoplanet atmospheres.Comment: 19 pages, 13 figures, 8 tables, Accepted to Ap
Infrared Observations During the Secondary Eclipse of HD 209458 b II. Strong Limits on the Infrared Spectrum Near 2.2 Microns
We report observations of the transiting extrasolar planet, HD 209458 b,
designed to detect the secondary eclipse. We employ the method of `occultation
spectroscopy', which searches in combined light (star and planet) for the
disappearance and reappearance of weak infrared spectral features due to the
planet as it passes behind the star and reappears. Our observations cover two
predicted secondary eclipse events, and we obtained 1036 individual spectra of
the HD 209458 system using the SpeX instrument at the NASA IRTF in September
2001. Our spectra extend from 1.9 to 4.2 microns with a spectral resolution of
1500. We have searched for a continuum peak near 2.2 microns (caused by CO and
water absorption bands), as predicted by some models of the planetary
atmosphere to be approximately 6E-4 of the stellar flux, but no such peak is
detected at a level of about 3E-4 of the stellar flux. Our results represent
the strongest limits on the infrared spectrum of the planet to date and carry
significant implications for understanding the planetary atmosphere. In
particular, some models that assume the stellar irradiation is re-radiated
entirely on the sub-stellar hemisphere predict a flux peak inconsistent with
our observations. Several physical mechanisms can improve agreement with our
observations, including the re-distribution of heat by global circulation, a
nearly isothermal atmosphere, and/or the presence of a high cloud.Comment: Accepted to the Astrophysical Journal 17 pages, 6 figure
Noise from propellers with symmetrical sections at zero blade angle, II
In a previous paper (Technical Note No. 605), a theory was developed that required an empirical relation to calculate sound pressures for the higher harmonics. Further investigation indicated that the modified theory agrees with experiment and that the empirical relation was due to an interference phenomenon peculiar to the test arrangement used. Comparison is made between the test results for a two-blade arrangement and the theory. The comparison is made for sound pressures in the plane of the revolving blades for varying values of tip velocity. Comparison is also made at constant tip velocity for all values of azimuth angle B. A further check is made between the theory and the experimental results for the fundamental of a four-blade arrangement with blades of the same dimensions as those used in the two-blade arrangement
Noise from Propellers with Symmetrical Sections at Zero Blade Angle
A theory has been deduced for the "rotation noise" from a propeller with blades of symmetrical section about the chord line and set at zero blade angle. Owing to the limitation of the theory, the equations give without appreciable error only the sound pressure for cases where the wave lengths are large compared with the blade lengths. With the aid of experimental data obtained from a two-blade arrangement, an empirical relation was introduced that permitted calculation of higher harmonics. The generality of the final relation given is indicated by the fundamental and second harmonic of a four-blade arrangement
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