410 research outputs found
Characteristic eddy decomposition of turbulence in a channel
The proper orthogonal decomposition technique (Lumley's decomposition) is applied to the turbulent flow in a channel to extract coherent structures by decomposing the velocity field into characteristic eddies with random coefficients. In the homogeneous spatial directions, a generaliztion of the shot-noise expansion is used to determine the characteristic eddies. In this expansion, the Fourier coefficients of the characteristic eddy cannot be obtained from the second-order statistics. Three different techniques are used to determine the phases of these coefficients. They are based on: (1) the bispectrum, (2) a spatial compactness requirement, and (3) a functional continuity argument. Results from these three techniques are found to be similar in most respects. The implications of these techniques and the shot-noise expansion are discussed. The dominant eddy is found to contribute as much as 76 percent to the turbulent kinetic energy. In both 2D and 3D, the characteristic eddies consist of an ejection region straddled by streamwise vortices that leave the wall in the very short streamwise distance of about 100 wall units
Criticality of natural absorbing states
We study a recently introduced ladder model which undergoes a transition
between an active and an infinitely degenerate absorbing phase. In some cases
the critical behaviour of the model is the same as that of the branching
annihilating random walk with species both with and without hard-core
interaction. We show that certain static characteristics of the so-called
natural absorbing states develop power law singularities which signal the
approach of the critical point. These results are also explained using random
walk arguments. In addition to that we show that when dynamics of our model is
considered as a minimum finding procedure, it has the best efficiency very
close to the critical point.Comment: 6 page
Far Ultraviolet Spectroscopic Explorer Survey of Magellanic Cloud Supernova Remnants
We report the progress to date from an ongoing unbiased ultraviolet survey of
supernova remnants in the Magellanic Clouds using the Far Ultraviolet
Spectroscopic Explorer (FUSE) satellite. This survey is obtaining spectra of a
random large sample of Magellanic Cloud supernova remnants with a broad range
of radio, optical, and X-ray properties. To date, 39 objects have been observed
in the survey (38 in the LMC and one in the SMC) and 15 have been detected, a
detection rate of nearly 40%. Our survey has nearly tripled the number of
UV-detected SNRs in the Magellanic Clouds (from 8 to 22). Because of the
diffuse source sensitivity of FUSE, upper limits on non-detected objects are
quite sensitive in many cases. Estimated total luminosities in O~VI span a
broad range from considerably brighter to many times fainter than the inferred
soft X-ray luminosities, indicating that O~VI can be an important and largely
unrecognized coolant in certain objects. We compare the optical and X-ray
properties of the detected and non-detected objects but do not find a simple
indicator for ultraviolet detectability. Non-detections may be due to
clumpiness of the emission, high foreground extinction, slow shocks whose
emission gets attenuated by the Magellanic interstellar medium, or a
combination of these effects.Comment: 34 pages, 26 figures in 8 separate JPG figure files; the
characteristics of individual detected supernova remnants are summarized in
an Appendi
Cosmic Ray Acceleration at the Forward Shock in Tycho's Supernova Remnant: Evidence from Chandra X-ray Observations
We present evidence for cosmic ray acceleration at the forward shock in
Tycho's supernova remnant (SNR) from three X-ray observables: (1) the proximity
of the contact discontinuity to the forward shock, or blast wave, (2) the
morphology of the emission from the rim of Tycho, and (3) the spectral nature
of the rim emission. We determine the locations of the blast wave (BW), contact
discontinuity (CD), and reverse shock (RS) around the rim of Tycho's supernova
remnant using a principal component analysis and other methods applied to new
Chandra data. The azimuthal-angle-averaged radius of the BW is 251". For the CD
and RS we find average radii of 241" and 183", respectively. Taking account of
projection effects, we find ratios of 1:0.93:0.70 (BW:CD:RS). We show these
values to be inconsistent with adiabatic hydrodynamical models of SNR
evolution. The CD:BW ratio can be explained if cosmic ray acceleration of ions
is occurring at the forward shock. The RS:BW ratio, as well as the strong Fe Ka
emission from the Tycho ejecta, imply that the RS is not accelerating cosmic
rays. We also extract radial profiles from ~34% of the rim of Tycho and compare
them to models of surface brightness profiles behind the BW for a purely
thermal plasma with an adiabatic shock. The observed morphology of the rim is
much more strongly peaked than predicted by the model, indicating that such
thermal emission is implausible here. Spectral analysis also implies that the
rim emission is non-thermal in nature, lending further support to the idea that
Tycho's forward shock is accelerating cosmic rays.Comment: 39 pages, 10 figures, accepted by Ap
Numerical simulation of micromachined acoustic resonators
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76479/1/AIAA-2000-546-400.pd
Iron-Rich Ejecta in the Supernova Remnant DEM L71
Chandra X-ray observations of DEM L71, a supernova remnant (SNR) in the Large
Magellanic Cloud (LMC), reveal a clear double shock morphology consisting of an
outer blast wave shock surrounding a central bright region of reverse-shock
heated ejecta. The abundances of the outer shock are consistent with LMC
values, while the ejecta region shows enhanced abundances of Si, Fe, and other
species. However, oxygen is not enhanced in the ejecta; the Fe/O abundance
ratio there is more than 5 times the solar ratio. Based on the relative
positions of the blast wave shock and the contact discontinuity in the context
of SNR evolutionary models, we determine a total ejecta mass of approximately
1.5 solar masses. Ejecta mass estimates based on emission measures derived from
spectral fits are subject to considerable uncertainty due to lack of knowledge
of the true contribution of hydrogen continuum emission. Maximal mass
estimates, i.e., assuming no hydrogen, result in 1.5 solar masses of Fe and
0.24 solar masses of Si. Under the assumption that an equal quantity of
hydrogen has been mixed into the ejecta, we estimate 0.8 solar masses of Fe and
0.12 solar masses of Si. These characteristics support the view that in DEM L71
we see Fe-rich ejecta from a Type Ia SN several thousand years after explosion.Comment: 5 pages, including 3 postscript figs, LaTeX, to appear in ApJ Letters
2003 Jan 1
Systemic lobar shunting induces advanced pulmonary vasculopathy
AbstractObjectives: We characterized the morphology and vasomotor responses of a localized, high-flow model of pulmonary hypertension. Methods: An end-to-side anastomosis was created between the left lower lobe pulmonary artery and the aorta in 23 piglets. Control animals had a thoracotomy alone or did not have an operation. Eight weeks later, hemodynamic measurements were made. Then shunted and/or nonshunted lobes were removed for determination of vascular resistance and compliance by occlusion techniques under conditions of normoxia, hypoxia (FIO2 = 0.03), and inspired nitric oxide administration. Quantitative histologic studies of vessel morphology were performed. Results: Eighty-three percent of animals having a shunt survived to final study. Aortic pressure, main pulmonary artery and wedge pressures, cardiac output, blood gases, and weight gain were not different between control pigs and those receiving a shunt. Six of 9 shunted lobes demonstrated systemic levels of pulmonary hypertension in vivo. Arterial resistance was higher (24.3 ± 12.0 vs 1.3 ± 0.2 mm Hg · mL–1 · s–1, P =.04) and arterial compliance was lower (0.05 ± 0.01 vs 0.16 ± 0.03 mL/mm Hg, P =.02) in shunted compared with nonshunted lobes. Hypoxic vasoconstriction was blunted in shunted lobes compared with nonshunted lobes (31% ± 13% vs 452% ± 107% change in arterial resistance, during hypoxia, P <.001). Vasodilation to inspired nitric oxide was evident only in shunted lobes (34% ± 6% vs 1.8% ± 8.2% change in arterial resistance during administration of inspired nitric oxide, P =.008). Neointimal and medial proliferation was found in shunted lobes with approximately a 10-fold increase in wall/luminal area ratio. Conclusions: An aorta–lobar pulmonary artery shunt produces striking vasculopathy. The development of severe pulmonary hypertension within a short time frame, low mortality, and localized nature of the vasculopathy make this model highly attractive for investigation of mechanisms that underlie pulmonary hypertension. (J Thorac Cardiovasc Surg 2000; 120:88-98
The Detection of Far Ultraviolet Line Emission from Balmer-Dominated Supernova Remnants in the Large Magellanic Cloud
We present the first far ultraviolet (FUV) spectra of the four known
Balmer-dominated supernova remnants (SNRs) in the Large Magellanic Cloud,
acquired with the Far Ultraviolet Spectroscopic Explorer. The remnants DEM L 71
(0505-67.9), 0509-67.5, 0519-69.0 and 0548-70.4 are all in the non-radiative
stages of evolution and exhibit expansion speeds ranging from ~ 500 km/s to ~
5000 km/s. We have detected broad emission lines of Ly beta, Ly gamma, C III
and O VI in DEM L 71 (V(FWHM) ~ 1000 km/s) and have detected broad Ly beta and
O VI emission in 0519-69.0, (V(FWHM) ~ 3000 km/s). In addition, broad Ly beta
emission (V(FWHM) ~ 3700 km/s) has been observed in 0509-67.5, the first
detection of broad line emission from this SNR. No emission was detected in our
FUSE spectrum of 0548-70.4, allowing us to place only upper limits on the FUV
line fluxes. The spectra of these SNRs are unaffected by postshock cooling, and
provide valuable probes of collisionless heating efficiency in high Mach number
shocks. We have used the Ly beta / O VI flux ratio and relative widths of the
broad Ly beta and O VI lines to estimate the degree of electron-proton and
proton-oxygen ion equilibration in DEM L 71, 0509-67.5, and 0519-69.0. Although
our equilibration estimates are subject to considerable uncertainty due to the
faintness of the FUV lines and contributions from bulk Doppler broadening, our
results are consistent with a declining efficiency of electron- proton and
proton-oxygen ion equilibration with increasing shock speed. From our shock
velocity estimates we obtain ages of 295-585 years for 0509-67.5 and 520-900
years for 0519-69.0, respectively, in good agreement with the ages obtained
from SN light echo studies.Comment: 13 pages, 3 tables, 9 figures in emulateapj format, Accepted by Ap
Flow structure and performance of axisymmetric synthetic jets
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77189/1/AIAA-2001-1008-312.pd
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