10,297 research outputs found
Effect of hole geometry and Electric-Discharge Machining (EDM) on airflow rates through small diameter holes in turbine blade material
The effects of two design parameters, electrode diameter and hole angle, and two machine parameters, electrode current and current-on time, on air flow rates through small-diameter (0.257 to 0.462 mm) electric-discharge-machined holes were measured. The holes were machined individually in rows of 14 each through 1.6 mm thick IN-100 strips. The data showed linear increase in air flow rate with increases in electrode cross sectional area and current-on time and little change with changes in hole angle and electrode current. The average flow-rate deviation (from the mean flow rate for a given row) decreased linearly with electrode diameter and increased with hole angle. Burn time and finished hole diameter were also measured
Searching for Planets in the Hyades V: Limits on Planet Detection in the Presence of Stellar Activity
We present the results of a radial velocity survey of a sample of Hyades
stars, and discuss the effects of stellar activity on radial velocity
measurements. The level of radial velocity scatter due to rotational modulation
of stellar surface features for the Hyades is in agreement with the predictions
of Saar & Donahue (1997)- the maximum radial velocity rms of up to ~50 m/s,
with an average rms of ~16 m/s. In this sample of 94 stars, we find 1 new
binary, 2 stars with linear trends indicative of binary companions, and no
close-in giant planets. We discuss the limits on extrasolar planet detection in
the Hyades and the constraints imposed on radial velocity surveys of young
stars.Comment: To appear in the June 2004 issue of A
Temperature driven to phase-transformation in Ti, Zr and Hf from first principles theory combined with lattice dynamics
Lattice dynamical methods used to predict phase transformations in crystals
typically deal with harmonic phonon spectra and are therefore not applicable in
important situations where one of the competing crystal structures is unstable
in the harmonic approximation, such as the bcc structure involved in the hcp to
bcc martensitic phase transformation in Ti, Zr and Hf. Here we present an
expression for the free energy that does not suffer from such shortcomings, and
we show by self consistent {\it ab initio} lattice dynamical calculations
(SCAILD), that the critical temperature for the hcp to bcc phase transformation
in Ti, Zr and Hf, can be effectively calculated from the free energy difference
between the two phases. This opens up the possibility to study quantitatively,
from first principles theory, temperature induced phase transitions.Comment: 4 pages, 3 figure
Primary decomposition and the fractal nature of knot concordance
For each sequence of polynomials, P=(p_1(t),p_2(t),...), we define a
characteristic series of groups, called the derived series localized at P.
Given a knot K in S^3, such a sequence of polynomials arises naturally as the
orders of certain submodules of the sequence of higher-order Alexander modules
of K. These group series yield new filtrations of the knot concordance group
that refine the (n)-solvable filtration of Cochran-Orr-Teichner. We show that
the quotients of successive terms of these refined filtrations have infinite
rank. These results also suggest higher-order analogues of the p(t)-primary
decomposition of the algebraic concordance group. We use these techniques to
give evidence that the set of smooth concordance classes of knots is a fractal
set. We also show that no Cochran-Orr-Teichner knot is concordant to any
Cochran-Harvey-Leidy knot.Comment: 60 pages, added 4 pages to introduction, minor corrections otherwise;
Math. Annalen 201
Confirmation of the Planet Hypothesis for the Long-period Radial Velocity Variations of Beta Geminorum
We present precise stellar radial velocity measurements for the K giant star
Beta Gem spanning over 25 years. These data show that the long period low
amplitude radial velocity variations found by Hatzes & Cochran (1993) are
long-lived and coherent. An examination of the Ca II K emission, spectral line
shapes from high resolution data (R = 210,000), and Hipparcos photometry show
no significant variations of these quantities with the RV period. These data
confirm the planetary companion hypothesis suggested by Hatzes & Cochran
(1993). An orbital solution assuming a stellar mass of 1.7 M_sun yields a
period, P = 589.6 days, a minimum mass of 2.3 M_Jupiter, and a semi-major axis,
and a = 1.6 AU. The orbit is nearly circular (e = 0.02). Beta Gem is the
seventh intermediate mass star shown to host a sub-stellar companion and
suggests that planet-formation around stars much more massive than the sun may
common.Comment: 10 pages, 9 figures, Astronomy and Astrophysics, in pres
Observing Strategies for the Detection of Jupiter Analogs
To understand the frequency, and thus the formation and evolution, of planetary systems like our own solar system, it is critical to detect Jupiter-like planets in Jupiter-like orbits. For long-term radial-velocity monitoring, it is useful to estimate the observational effort required to reliably detect such objects, particularly in light of severe competition for limited telescope time. We perform detailed simulations of observational campaigns, maximizing the realism of the sampling of a set of simulated observations. We then compute the detection limits for each campaign to quantify the effect of increasing the number of observational epochs and varying their time coverage. We show that once there is sufficient time baseline to detect a given orbital period, it becomes less effective to add further time coverage-rather, the detectability of a planet scales roughly as the square root of the number of observations, independently of the number of orbital cycles included in the data string. We also show that no noise floor is reached, with a continuing improvement in detectability at the maximum number of observations N = 500 tested here.Peer reviewe
A Search for Variability in the Spectral Line Shapes of tau Bootis: Does this Star Really Have a Planet?
An analysis is made of the spectral line shapes of tau Bootis using high
resolution (0.026 A) and high signal-to-noise (S/N~400) data in an effort to
confirm the planet hypothesis for this star. Changes in the line shape are
quantified using spectral line bisectors and line residuals. We detect no
variations in either of these quantities above the level of the noise in the
data. One spectral line, Fe I 6213 A, does show a hint of sinusoidal variations
in the bisector velocity span when phased to the radial velocity period of 3.3
days, but this is not seen in the bisectors for two other lines, nor in the
line residuals. Comparisons of the data to the bisector and residual variations
expected for nonradial pulsations indicate that we can exclude those sectoral
nonradial modes having m>2 and all sectoral modes with k>1, where k is the
ratio of the horizontal to vertical velocities for the pulsations. The lack of
line shape variability and the 469 m/s radial velocity amplitude is still
consistent with nonradial sectoral modes m=1, and possibly m=2, but with k~1,
which is at least 3 orders of magnitude less than the predicted value given the
3.3 day period of tau Bootis. Such low values of k can probably be excluded
given the lack of photometric variations for this star. Although the
measurements presented here do not prove, without any doubt, that tau Boo has a
planetary companion, they do add significantly to the increasing body of
evidence in favor of this hypothesis.Comment: LaTeX. 22 pages, 9 figures. Accepted in the Astrophysical Journa
Recidivism and Time Served in Prison
A justification for lengthier stays in prison stems from the belief that spending more time in prison reduces recidivism. Extant studies, however, have provided limited evidence for that belief and, indeed, suggest the effect of time served may be minimal. Few studies have employed rigorous methodological approaches, examined time spans of more than one to two years, or investigated the potential for the relationship between recidivism and time served to be curvilinear. Drawing on prior scholarship, this paper identifies three sets of hypotheses about the functional form of the time served and recidivism relationship. Using generalized propensity score analysis to examine data on 90,423 inmates released from Florida prisons, we find three patterns: greater time served initially increases recidivism but then, after approximately one year, decreases it, and, after approximately two years, exerts no effect; estimation of the effects associated with durations of more than five years are uncertain. The results point to potential criminogenic and beneficial effects of time served and underscore the need to identify how varying durations of incarceration affect recidivism
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