26,723 research outputs found
CF6 jet engine performance improvement: High pressure turbine roundness
An improved high pressure turbine stator reducing fuel consumption in current CF6-50 turbofan engines was developed. The feasibility of the roundness and clearance response improvements was demonstrated. Application of these improvements will result in a cruise SFC reduction of 0.22 percent for new engines. For high time engines, the improved roundness and response characteristics results in an 0.5 percent reduction in cruise SFC. A basic life capability of the improved HP turbine stator in over 800 simulated flight cycles without any sign of significant distress is shown
CF6 jet engine diagnostics program. High pressure turbine roundness/clearance investigation
The effects of high pressure turbine clearance changes on engine and module performance was evaluated in addition to the measurement of CF6-50C high pressure turbine Stage 1 tip clearance and stator out-of-roundness during steady-state and transient operation. The results indicated a good correlation of the analytical model of round engine clearance response with measured data. The stator out-of-roundness measurements verified that the analytical technique for predicting the distortion effects of mechanical loads is accurate, whereas the technique for calculating the effects of certain circumferential thermal gradients requires some modifications. A potential for improvement in roundness was established in the order of 0.38 mm (0.015 in.), equivalent to 0.86 percent turbine efficiency which translates to a cruise SFC improvement of 0.36 percent. The HP turbine Stage 1 tip clearance performance derivative was established as 0.44 mm (17 mils) per percent of turbine efficiency at take-off power, somewhat smaller, therefore, more sensitive than predicted from previous investigations
Apparatus for reducing aerodynamic noise in a wind tunnel
An apparatus is described for reducing the background noise produced by the porous walls of the test section of a wind tunnel. A finely meshed screen member is placed over the perforations in the test section walls. The mesh wire screen attached to the interior wall provides a smoother surface for the air stream to flow against reducing the vorticies produced by the edges of the perforations in the test section walls
Solution of the inverse scattering problem by T-matrix completion. II. Simulations
This is Part II of the paper series on data-compatible T-matrix completion
(DCTMC), which is a method for solving nonlinear inverse problems. Part I of
the series contains theory and here we present simulations for inverse
scattering of scalar waves. The underlying mathematical model is the scalar
wave equation and the object function that is reconstructed is the medium
susceptibility. The simulations are relevant to ultrasound tomographic imaging
and seismic tomography. It is shown that DCTMC is a viable method for solving
strongly nonlinear inverse problems with large data sets. It provides not only
the overall shape of the object but the quantitative contrast, which can
correspond, for instance, to the variable speed of sound in the imaged medium.Comment: This is Part II of a paper series. Part I contains theory and is
available at arXiv:1401.3319 [math-ph]. Accepted in this form to Phys. Rev.
Nonlinear inverse problem by T-matrix completion. I. Theory
We propose a conceptually new method for solving nonlinear inverse scattering
problems (ISPs) such as are commonly encountered in tomographic ultrasound
imaging, seismology and other applications. The method is inspired by the
theory of nonlocality of physical interactions and utilizes the relevant
formalism. We formulate the ISP as a problem whose goal is to determine an
unknown interaction potential from external scattering data. Although we
seek a local (diagonally-dominated) as the solution to the posed problem,
we allow to be nonlocal at the intermediate stages of iterations. This
allows us to utilize the one-to-one correspondence between and the T-matrix
of the problem, . Here it is important to realize that not every
corresponds to a diagonal and we, therefore, relax the usual condition of
strict diagonality (locality) of . An iterative algorithm is proposed in
which we seek that is (i) compatible with the measured scattering data and
(ii) corresponds to an interaction potential that is as
diagonally-dominated as possible. We refer to this algorithm as to the
data-compatible T-matrix completion (DCTMC). This paper is Part I in a two-part
series and contains theory only. Numerical examples of image reconstruction in
a strongly nonlinear regime are given in Part II. The method described in this
paper is particularly well suited for very large data sets that become
increasingly available with the use of modern measurement techniques and
instrumentation.Comment: This is Part I of a paper series containing theory only. Part II
contains simulations and is available as arXiv:1505.06777 [math-ph]. Accepted
in this form to Phys. Rev.
Occurrence and core-envelope structure of 1--4x Earth-size planets around Sun-like stars
Small planets, 1-4x the size of Earth, are extremely common around Sun-like
stars, and surprisingly so, as they are missing in our solar system. Recent
detections have yielded enough information about this class of exoplanets to
begin characterizing their occurrence rates, orbits, masses, densities, and
internal structures. The Kepler mission finds the smallest planets to be most
common, as 26% of Sun-like stars have small, 1-2 R_e planets with orbital
periods under 100 days, and 11% have 1-2 R_e planets that receive 1-4x the
incident stellar flux that warms our Earth. These Earth-size planets are
sprinkled uniformly with orbital distance (logarithmically) out to 0.4 AU, and
probably beyond. Mass measurements for 33 transiting planets of 1-4 R_e show
that the smallest of them, R < 1.5 R_e, have the density expected for rocky
planets. Their densities increase with increasing radius, likely caused by
gravitational compression. Including solar system planets yields a relation:
rho = 2.32 + 3.19 R/R_e [g/cc]. Larger planets, in the radius range 1.5-4.0
R_e, have densities that decline with increasing radius, revealing increasing
amounts of low-density material in an envelope surrounding a rocky core,
befitting the appellation "mini-Neptunes." Planets of ~1.5 R_e have the highest
densities, averaging near 10 g/cc. The gas giant planets occur preferentially
around stars that are rich in heavy elements, while rocky planets occur around
stars having a range of heavy element abundances. One explanation is that the
fast formation of rocky cores in protoplanetary disks enriched in heavy
elements permits the gravitational accumulation of gas before it vanishes,
forming giant planets. But models of the formation of 1-4 R_e planets remain
uncertain. Defining habitable zones remains difficult, without benefit of
either detections of life elsewhere or an understanding of life's biochemical
origins.Comment: 11 pages, 6 figures, accepted for publication Proc. Natl. Acad. Sc
Prevalence of Earth-size planets orbiting Sun-like stars
Determining whether Earth-like planets are common or rare looms as a
touchstone in the question of life in the universe. We searched for Earth-size
planets that cross in front of their host stars by examining the brightness
measurements of 42,000 stars from National Aeronautics and Space
Administration's Kepler mission. We found 603 planets, including 10 that are
Earth size (1-2 Earth-radii) and receive comparable levels of stellar energy to
that of Earth (within a factor of four). We account for Kepler's imperfect
detectability of such planets by injecting synthetic planet-caused dimmings
into the Kepler brightness measurements and recording the fraction detected. We
find that of Sun-like stars harbor an Earth-size planet receiving
between one and four times the stellar intensity as Earth. We also find that
the occurrence of Earth-size planets is constant with increasing orbital period
(P), within equal intervals of logP up to d. Extrapolating, one finds
of Sun-like stars harbor an Earth-size planet with orbital
periods of 200-400 d.Comment: Main text: 6 pages, 5 figures, 1 table. Supporting information: 54
pages, 17 pages, 3 tables. Published in the Proceedings of the National
Academy of Sciences available at
http://www.pnas.org/cgi/doi/10.1073/pnas.131990911
Radial velocities from the N2K Project: 6 new cold gas giant planets orbiting HD 55696, HD 98736, HD 148164, HD 203473, and HD 211810
The N2K planet search program was designed to exploit the planet-metallicity
correlation by searching for gas giant planets orbiting metal-rich stars. Here,
we present the radial velocity measurements for 378 N2K target stars that were
observed with the HIRES spectrograph at Keck Observatory between 2004 and 2017.
With this data set, we announce the discovery of six new gas giant exoplanets:
a double-planet system orbiting HD 148164 ( of 1.23 and 5.16 M) and single planet detections around HD 55696 ( = 3.87 M), HD 98736 ( = 2.33 M), HD 203473 ( = 7.8
M), and HD 211810 ( = 0.67 M). These gas
giant companions have orbital semi-major axes between 1.0 and 6.2 AU and
eccentricities ranging from 0.13 to 0.71. We also report evidence for three
gravitationally bound companions with between 20 to 30 M, placing them in the mass range of brown dwarfs, around HD 148284, HD
214823, and HD 217850, and four low mass stellar companions orbiting HD 3404,
HD 24505, HD 98630, and HD 103459. In addition, we present updated orbital
parameters for 42 previously announced planets. We also report a nondetection
of the putative companion HD 73256 b. Finally, we highlight the most promising
candidates for direct imaging and astrometric detection, and find that many hot
Jupiters from our sample could be detectable by state-of-the-art telescopes
such as Gaia.Comment: Accepted by the Astronomical Journal. 75 pages, 49 figure
Collective force generation by groups of migrating bacteria
From biofilm and colony formation in bacteria to wound healing and embryonic
development in multicellular organisms, groups of living cells must often move
collectively. While considerable study has probed the biophysical mechanisms of
how eukaryotic cells generate forces during migration, little such study has
been devoted to bacteria, in particular with regard to the question of how
bacteria generate and coordinate forces during collective motion. This question
is addressed here for the first time using traction force microscopy. We study
two distinct motility mechanisms of Myxococcus xanthus, namely twitching and
gliding. For twitching, powered by type-IV pilus retraction, we find that
individual cells exert local traction in small hotspots with forces on the
order of 50 pN. Twitching of bacterial groups also produces traction hotspots,
however with amplified forces around 100 pN. Although twitching groups migrate
slowly as a whole, traction fluctuates rapidly on timescales <1.5 min. Gliding,
the second motility mechanism, is driven by lateral transport of substrate
adhesions. When cells are isolated, gliding produces low average traction on
the order of 1 Pa. However, traction is amplified in groups by a factor of ~5.
Since advancing protrusions of gliding cells push on average in the direction
of motion, we infer a long-range compressive load sharing among sub-leading
cells. Together, these results show that the forces generated during twitching
and gliding have complementary characters and both forces are collectively
amplified in groups
An Understanding of the Shoulder of Giants: Jovian Planets around Late K Dwarf Stars and the Trend with Stellar Mass
Analyses of exoplanet statistics suggest a trend of giant planet occurrence
with host star mass, a clue to how planets like Jupiter form. One missing piece
of the puzzle is the occurrence around late K dwarf stars (masses of
0.5-0.75Msun and effective temperatures of 3900-4800K). We analyzed four years
of Doppler radial velocities data of 110 late K dwarfs, one of which hosts two
previously reported giant planets. We estimate that 4.0+/-2.3% of these stars
have Saturn-mass or larger planets with orbital periods <245d, depending on the
planet mass distribution and RV variability of stars without giant planets. We
also estimate that 0.7+/-0.5% of similar stars observed by Kepler have giant
planets. This Kepler rate is significantly (99% confidence) lower than that
derived from our Doppler survey, but the difference vanishes if only the single
Doppler system (HIP 57274) with completely resolved orbits is considered. The
difference could also be explained by the exclusion of close binaries (without
giant planets) from the Doppler but not Kepler surveys, the effect of
long-period companions and stellar noise on the Doppler data, or an intrinsic
difference between the two populations. Our estimates for late K dwarfs bridge
those for solar-type stars and M dwarfs and support a positive trend with
stellar mass. Small sample size precludes statements about finer structure,
e.g. a "shoulder" in the distribution of giant planets with stellar mass.
Future surveys such as the Next Generation Transit Survey and the Transiting
Exoplanet Satellite Survey will ameliorate this deficiency.Comment: Accepted to The Astrophysical Journa
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