Graphite/polyimide laminates with near-zero thermal expansion

Composite structures can be laminated to have very low coefficients of thermal expansion. Such structures are light and strong and have many uses where expansion or contraction with temperature change is undesirable. One application is with instruments that measure thermal expansion

Remote Monitoring of Implantable Cardioverter Defibrillator

The rate of implantable cardioverter defibrillator (ICD) implantation has gone up as primary and secondary prevention trials have relatively consistently shown significant improvement in mortality and morbidity. Most patients with ICDs are followed routinely at intervals ranging from 3 to 6 months. Many patients require additional non-scheduled visits to investigate symptoms that may or may not relate to their cardiac disease or device. Appropriate and inappropriate therapies of implantable cardioverter defibrillators have a major impact on morbidity and quality of life in ICD recipients. Remote monitoring systems can substitute for routine follow-up visits and/ or deliver continuous diagnostic and device status information. Remote monitoring of ICDs can decrease the need for many patient visits and, thereby, probably reduce expense

A powerful and highly variable off-nuclear X-ray source in the composite starburst/Seyfert 2 galaxy NGC 4945

We report on a powerful and variable off-nuclear X-ray source in the nearby spiral galaxy NGC 4945. Two ROSAT PSPC observations show the source to brighten in 0.5--2.0 keV flux by a factor of about 9 on a time-scale of 11 months or less. It is seen by ASCA about one month after the second PSPC pointing, and is seen to have dimmed by a factor of > 7 in a ROSAT HRI pointing about one year after the second PSPC pointing. Its maximum observed 0.8--2.5 keV luminosity is about 8E38 erg/s, making it brighter than any known persistent X-ray binary in the Milky Way. Its total X-ray luminosity is probably larger than 1.2E39 erg/s. The observed variability argues against a superbubble interpretation, and the off-nuclear position argues against a low-luminosity active galactic nucleus. The source is therefore probably either an ultra-powerful X-ray binary or an ultra-powerful supernova remnant. Optical monitoring has not identified any supernovae in NGC 4945 during the time of the X-ray observations, and any supernova would have had to have been either very highly absorbed or intrinsically optically faint.Comment: 5 pages, uuencoded compressed tar file, MNRAS in pres

Large-eddy simulation of a turbulent mixing layer

The three dimensional, time dependent (incompressible) vorticity equations were used to simulate numerically the decay of isotropic box turbulence and time developing mixing layers. The vorticity equations were spatially filtered to define the large scale turbulence field, and the subgrid scale turbulence was modeled. A general method was developed to show numerical conservation of momentum, vorticity, and energy. The terms that arise from filtering the equations were treated (for both periodic boundary conditions and no stress boundary conditions) in a fast and accurate way by using fast Fourier transforms. Use of vorticity as the principal variable is shown to produce results equivalent to those obtained by use of the primitive variable equations

Microfocus Radiography

I should emphasize that what I am going to talk about is really the result of a very brief program which was funded at Harwell by David Godfrey of AMTE, who is probably well-known to ceramic practitioners here. We were very glad to have this contact because it gave us access to the TTCP specimens which were prepared in this country; specimens, in particular, of silicon nitride tiles containing seeded defects, which many of you have studied by various techniques

Implications of the X-ray Variability for the Mass of MCG-6-30-15

The bright Seyfert 1 galaxy \mcg shows large variability on a variety of time scales. We study the \aproxlt 3 day time scale variability using a set of simultaneous archival observations that were obtained from \rxte and the {\it Advanced Satellite for Cosmology and Astrophysics} (\asca). The \rxte\ observations span nearly $10^6$ sec and indicate that the X-ray Fourier Power Spectral Density has an rms variability of 16%, is flat from approximately 10^{-6} - 10^{-5} Hz, and then steepens into a power law $\propto f^{-\alpha}$ with \alpha\aproxgt 1. A further steepening to $\alpha \approx 2$ occurs between 10^{-4}-10^{-3} Hz. The shape and rms amplitude are comparable to what has been observed in \ngc and \cyg, albeit with break frequencies that differ by a factor of 10^{-2} and 10^{4}, respectively. If the break frequencies are indicative of the central black hole mass, then this mass may be as low as $10^6 {\rm M}_\odot$. An upper limit of $\sim 2$ ks for the relative lag between the 0.5-2 keV \asca band compared to the 8-15 keV \rxte band was also found. Again by analogy with \ngc and \cyg, this limit is consistent with a relatively low central black hole mass.Comment: 5 pages, 3 figures, LaTeX, uses emulateapj.sty and apjfonts.sty, revised version, accepted for publication in ApJ Letter

The Equilibrium Photoionized Absorber in 3C351

We present two ROSAT PSPC observations of the radio-loud, lobe-dominated quasar 3C 351, which shows an `ionized absorber' in its X-ray spectrum. The factor 1.7 change in flux in the $\sim$2~years between the observations allows a test of models for this ionized absorber. The absorption feature at ~0.7 keV (quasar frame) is present in both spectra but with a lower optical depth when the source intensity - and hence the ionizing flux at the absorber - is higher, in accordance with a simple, single-zone, equilibrium photoionization model. Detailed modeling confirms this agrement quantitatively. The maximum response time of 2 years allows us to limit the gas density: n_e > 2 x 10^4 cm^{-3}; and the distance of the ionized gas from the central source R < 19 pc. This produces a strong test for a photoionized absorber in 3C~351: a factor 2 flux change in ~1 week in this source must show non-equilibrium effects in the ionized absorber.Comment: 10 pages, 3 figures, accepted by Ap

Scalar entrainment in the mixing layer

New definitions of entrainment and mixing based on the passive scalar field in the plane mixing layer are proposed. The definitions distinguish clearly between three fluid states: (1) unmixed fluid, (2) fluid engulfed in the mixing layer, trapped between two scalar contours, and (3) mixed fluid. The difference betwen (2) and (3) is the amount of fluid which has been engulfed during the pairing process, but has not yet mixed. Trends are identified from direct numerical simulations and extensions to high Reynolds number mixing layers are made in terms of the Broadwell-Breidenthal mixing model. In the limit of high Peclet number (Pe = ReSc) it is speculated that engulfed fluid rises in steps associated with pairings, introducing unmixed fluid into the large scale structures, where it is eventually mixed at the Kolmogorov scale. From this viewpoint, pairing is a prerequisite for mixing in the turbulent plane mixing layer