14,295 research outputs found
Elastic turbulence in two-dimensional Taylor-Couette flows
We report the onset of elastic turbulence in a two-dimensional Taylor-Couette
geometry using numerical solutions of the Oldroyd-B model, also performed at
high Weissenberg numbers with the program OpenFOAM. Beyond a critical
Weissenberg number, an elastic instability causes a supercritical transition
from the laminar Taylor-Couette to a turbulent flow. The order parameter, the
time average of secondary-flow strength, follows the scaling law with and . The power spectrum of the velocity fluctuations shows a power-law decay
with a characteristic exponent, which strongly depends on the radial position.
It is greater than two, which we relate to the dimension of the geometry
Study of low gravity propellant transfer Quarterly progress report, 23 Dec. 1970 - 30 Apr. 1971
Bellows, metallic diaphragm, and paddle vortex subcritical transfer systems designs and high pressure systems analyses for orbital space station cryogen
Competence, specification and induction of Pax-3 in the trigeminal placode
Placodes are discrete regions of thickened ectoderm that contribute extensively to the peripheral nervous system in the vertebrate head. The paired-domain transcription factor Pax-3 is an early molecular marker for the avian ophthalmic trigeminal (opV) placode, which forms sensory neurons in the ophthalmic lobe of the trigeminal ganglion. Here, we use collagen gel cultures and heterotopic quail-chick grafts to examine the competence, specification and induction of Pax-3 in the opV placode. At the 3-somite stage, the whole head ectoderm rostral to the first somite is competent to express Pax-3 when grafted to the opV placode region, though competence is rapidly lost thereafter in otic-level ectoderm. Pax-3 specification in presumptive opV placode ectoderm occurs by the 8-somite stage, concomitant with robust Pax-3 expression. From the 8-somite stage onwards, significant numbers of cells are committed to express Pax-3. The entire length of the neural tube has the ability to induce Pax-3 expression in competent head ectoderm and the inductive interaction is direct. We propose a detailed model for Pax-3 induction in the opV placode
The effect of internal gravity waves on cloud evolution in sub-stellar atmospheres
Context. Sub-stellar objects exhibit photometric variability which is believed to be caused by a number of processes such as magnetically-driven spots or inhomogeneous cloud coverage. Recent sub-stellar models have shown that turbulent flows and waves, including internal gravity waves, may play an important role in cloud evolution.Aims. The aim of this paper is to investigate the effect of internal gravity waves on dust cloud nucleation and dust growth, and whether observations of the resulting cloud structures could be used to recover atmospheric density information.Methods. For a simplified atmosphere in two dimensions, we numerically solve the governing fluid equations to simulate the effect on dust nucleation and mantle growth as a result of the passage of an internal gravity wave. Furthermore, we derive an expression that relates the properties of the wave-induced cloud structures to observable parameters in order to deduce the atmospheric density.Results. Numerical simulations show that the density, pressure and temperature variations caused by gravity waves lead to an increase of dust nucleation by up to a factor 20, and dust mantle growth rate by up to a factor 1:6, compared to their equilibrium values. Through an exploration of the wider sub-stellar parameter space, we show that in absolute terms, the increase in dust nucleation due to internal gravity waves is stronger in cooler (T dwarfs) and TiO2-rich sub-stellar atmospheres. The relative increase however is greater in warm(L dwarf) and TiO2-poor atmospheres due to conditions less suited for efficient nucleation at equilibrium. These variations lead to banded areas in which dust formation is much more pronounced, and lead to banded cloud structures similar to those observed on Earth. Conclusions. Using the proposed method, potential observations of banded clouds could be used to estimate the atmospheric density of sub-stellar objects
Discovery of the Orbit of the Transient X ray Pulsar SAX J2103.5+4545
Using X-ray data from the Rossi X-Ray Timing Explorer (RXTE), we carried out
pulse timing analysis of the transient X-ray pulsar SAX J2103.5+4545. An
outburst was detected by All Sky Monitor (ASM) October 25 1999 and reached a
peak X-ray brightness of 27 mCrab October 28. Between November 19 and December
27, the RXTE/PCA carried out pointed observations which provided us with pulse
arrival times. These yield an eccentric orbit (e= 0.4 \pm 0.2) with an orbital
period of 12.68 \pm 0.25 days and light travel time across the projected
semimajor axis of 72 \pm 6 sec. The pulse period was measured to be 358.62171
\pm 0.00088 s and the spin-up rate (2.50 \pm 0.15) \times 10^{-13} Hz s^{-1}.
The ASM data for the February to September 1997 outburst in which BeppoSAX
discovered SAX J2103.5+4545 (Hulleman, in't Zand and Heise 1998) are modulated
at time scales close to the orbital period. Folded light curves of the 1997 ASM
data and the 1999 PCA data are similar and show that the intensity increases at
periastron passages.Comment: To appear in The Astrophysical Journal (Letters
Physical State of Molecular Gas in High Galactic Latitude Translucent Clouds
The rotational transitions of carbon monoxide (CO) are the primary means of
investigating the density and velocity structure of the molecular interstellar
medium. Here we study the lowest four rotational transitions of CO towards
high-latitude translucent molecular clouds (HLCs). We report new observations
of the J = (4-3), (2-1), and (1-0) transitions of CO towards eight
high-latitude clouds. The new observations are combined with data from the
literature to show that the emission from all observed CO transitions is
linearly correlated. This implies that the excitation conditions which lead to
emission in these transitions are uniform throughout the clouds. Observed
13CO/12CO (1-0) integrated intensity ratios are generally much greater than the
expected abundance ratio of the two species, indicating that the regions which
emit 12CO (1-0) radiation are optically thick. We develop a statistical method
to compare the observed line ratios with models of CO excitation and radiative
transfer. This enables us to determine the most likely portion of the physical
parameter space which is compatible with the observations. The model enables us
to rule out CO gas temperatures greater than 30K since the most likely
high-temperature configurations are 1 pc-sized structures aligned along the
line of sight. The most probable solution is a high density and low temperature
(HDLT) solution. The CO cell size is approximately 0.01 pc (2000 AU). These
cells are thus tiny fragments within the 100 times larger CO-emitting extent of
a typical high-latitude cloud. We discuss the physical implications of HDLT
cells, and we suggest ways to test for their existence.Comment: 19 pages, 13 figures, 2 tables, emulateapj To be published in The
Astrophysical Journa
Xenon Excimer Emission From Pulsed Microhollow Cathode Discharges
By applying electrical pulses of 20 ns duration to xenon microplasmas, generated by direct current microhollow cathode discharges, we were able to increase the xenon excimer emission by more than an order of magnitude over direct current discharge excimer emission. For pulsed voltages in excess of 500 V, the optical power at 172 nm was found to increase exponentially with voltage. Largest values obtained were 2.75 W of vacuum-ultraviolet optical power emitted from a single microhollow cathode discharge in 400 Torr xenon with a 750 V pulse applied to a discharge. Highest radiative emittance was 15.2 W/cm2. The efficiency for excimer emission was found to increase linearly with pulsed voltages above 500 V reaching values of 20% at 750 V
Resonant Energy Transfer From Argon Dimers to Atomic Oxygen in Microhollow Cathode Discharges
The emission of atomic oxygen lines at 130.2 and 130.5 nm from a microhollow cathode discharge in argon with oxygen added indicates resonant energy transfer from argon dimers to oxygen atoms. The internal efficiency of the vacuum-ultraviolet (VUV) radiation was measured as 0.7% for a discharge in 1100 Torr argon with 0.1% oxygen added. The direct current VUV point source operates at voltages below 300 V and at current levels of milliamperes
The azimuth structure of nuclear collisions -- I
We describe azimuth structure commonly associated with elliptic and directed
flow in the context of 2D angular autocorrelations for the purpose of precise
separation of so-called nonflow (mainly minijets) from flow. We extend the
Fourier-transform description of azimuth structure to include power spectra and
autocorrelations related by the Wiener-Khintchine theorem. We analyze several
examples of conventional flow analysis in that context and question the
relevance of reaction plane estimation to flow analysis. We introduce the 2D
angular autocorrelation with examples from data analysis and describe a
simulation exercise which demonstrates precise separation of flow and nonflow
using the 2D autocorrelation method. We show that an alternative correlation
measure based on Pearson's normalized covariance provides a more intuitive
measure of azimuth structure.Comment: 27 pages, 12 figure
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