35,238 research outputs found
Laser anemometry techniques for turbine applications
Laser anemometry offers a nonintrusive means for obtaining flow field information. Current research at NASA Lewis Research Center is focused on instrumenting a warm turbine facility with a laser anemometer system. In an effort to determine the laser anemometer system best qualified for the warm turbine environment, the performance of a conventional laser fringe anemometer and a two spot time of flight system were compared with a new, modified time of flight system, called a Four Spot laser anemometer. The comparison measurements were made in highly turbulent flows near walls. The Four Spot anemometer uses elliptical spots to increase the flow acceptance angle to be comparable to that of a Laser Fringe Anemometer. Also, the Four Spot uses an optical code that vastly simplifies the pulse detection processor. The results of the comparison measurements will exemplify which laser anemometer system is best suited to the hostile environment typically encountered in warm rotating turbomachinery
Dust in regions of massive star formation
It is suggested that protostars increase mass by accreting the surrounding gas and dust. Grains are destroyed as they near the central protostar creating a dust shell or cocoon. Radiation pressure acting on the grains can halt the inflow of material thereby limiting the amount of mass accumulated by the protostar. General constraints were considered on the initial dust-to-gas ratio and mass accretion rates that permit inflow. These results were constrained further by constructing a numerical model, including radiative deceleration on grains and grain destruction processes. Also the constraints on dust properties were investigated which allow the formation of massive stars. The obtained results seem to suggest that massive star formation requires rather extreme preconditioning of the grain and gas environment
High-order, Dispersionless "Fast-Hybrid" Wave Equation Solver. Part I: Sampling Cost via Incident-Field Windowing and Recentering
This paper proposes a frequency/time hybrid integral-equation method for the
time dependent wave equation in two and three-dimensional spatial domains.
Relying on Fourier Transformation in time, the method utilizes a fixed
(time-independent) number of frequency-domain integral-equation solutions to
evaluate, with superalgebraically-small errors, time domain solutions for
arbitrarily long times. The approach relies on two main elements, namely, 1) A
smooth time-windowing methodology that enables accurate band-limited
representations for arbitrarily-long time signals, and 2) A novel Fourier
transform approach which, in a time-parallel manner and without causing
spurious periodicity effects, delivers numerically dispersionless
spectrally-accurate solutions. A similar hybrid technique can be obtained on
the basis of Laplace transforms instead of Fourier transforms, but we do not
consider the Laplace-based method in the present contribution. The algorithm
can handle dispersive media, it can tackle complex physical structures, it
enables parallelization in time in a straightforward manner, and it allows for
time leaping---that is, solution sampling at any given time at
-bounded sampling cost, for arbitrarily large values of ,
and without requirement of evaluation of the solution at intermediate times.
The proposed frequency-time hybridization strategy, which generalizes to any
linear partial differential equation in the time domain for which
frequency-domain solutions can be obtained (including e.g. the time-domain
Maxwell equations), and which is applicable in a wide range of scientific and
engineering contexts, provides significant advantages over other available
alternatives such as volumetric discretization, time-domain integral equations,
and convolution-quadrature approaches.Comment: 33 pages, 8 figures, revised and extended manuscript (and now
including direct comparisons to existing CQ and TDIE solver implementations)
(Part I of II
Theory of photoferroelectric response in SmC* liquids
We are concerned with the modification of liquid crystalline and polar order
in SmC* liquids by illumination. In particular we show that non-uniformity due
to absorption and also dynamics, can be complex. The variation of polarization
with temperature, while illuminated, is modified from that assuming uniformity.
Apparent changes of polarization with illumination will be shown to be
underestimated due to non-uniformity. The dynamics is shown to depend on
propagating fronts of photo-conversion penetrating the sample.Comment: 6 pages, 7 figure
Generalized Uncertainty Principle Corrections to the Simple Harmonic Oscillator in Phase Space
We compute Wigner functions for the harmonic oscillator including corrections
from generalized uncertainty principles (GUPs), and study the corresponding
marginal probability densities and other properties. We show that the GUP
corrections to the Wigner functions can be significant, and comment on their
potential measurability in the laboratory.Comment: minor revisions; included journal referenc
Enhanced nonperturbative effects in jet distributions
We consider the triple differential distribution
d\Gamma/(dE_J)(dm_J^2)(d\Omega_J) for two-jet events at center of mass energy
M, smeared over the endpoint region m_J^2 << M^2, |2 E_J -M| ~ \Delta, \lqcd <<
\Delta << M. The leading nonperturbative correction, suppressed by
\lqcd/\Delta, is given by the matrix element of a single operator. A similar
analysis is performed for three jet events, and the generalization to any
number of jets is discussed. At order \lqcd/\Delta, non-perturbative effects in
four or more jet events are completely determined in terms of two matrix
elements which can be measured in two and three jet events.Comment: Significant changes made. The first moment does not vanish--the paper
has been modified to reflect this. Relations between different numbers of
jets still hol
Bridging the gap by shaking superfluid matter
In cold compact stars, Cooper pairing between fermions in dense matter leads
to the formation of a gap in their excitation spectrum and typically
exponentially suppresses transport properties. However, we show here that weak
Urca reactions become strongly enhanced and approach their ungapped level when
the star undergoes density oscillations of sufficiently large amplitude. We
study both the neutrino emissivity and the bulk viscosity due to direct Urca
processes in hadronic, hyperonic and quark matter and discuss different
superfluid and superconducting pairing patterns.Comment: 5 pages, 4 figure
Bispectrum signatures of a modified vacuum in single field inflation with a small speed of sound
Deviations from the Bunch-Davies vacuum during an inflationary period can
leave a testable imprint on the higher-order correlations of the CMB and large
scale structures in the Universe. The effect is particularly pronounced if the
statistical non-Gaussianity is inherently large, such as in models of inflation
with a small speed of sound, e.g. DBI. First reviewing the motivations for a
modified vacuum, we calculate the non-Gaussianity for a general action with a
small speed of sound. The shape of its bispectrum is found to most resemble the
'orthogonal' or 'local' templates depending on the phase of the Bogolyubov
parameter. In particular, for DBI models of inflation the bispectrum can have a
profound 'local' template feature, in contrast to previous results. Determining
the projection into the observational templates allows us to derive constraints
on the absolute value of the Bogolyubov parameter. In the small sound speed
limit, the derived constraints are generally stronger than the existing
constraint derived from the power spectrum. The bound on the absolute value of
the Bogolyubov parameter ranges from the 10^-6 to the 10^-3 level for
H/\Lambda_c = 10^-3, depending on the specific details of the model, the sound
speed and the phase of the Bogolyubov parameter.Comment: 34 pages, 8 figures, 2 appendices. New in this version: added
references, fixed typos, modified sentences. Version submitted to JCA
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