106,669 research outputs found
Shock wave induced vaporization of porous solids
Strong shock waves generated by hypervelocity impact can induce vaporization in solid materials. To pursue knowledge of the chemical species in the shock-induced vapors, one needs to design experiments that will drive the system to such thermodynamic states that sufficient vapor can be generated for investigation. It is common to use porous media to reach high entropy, vaporized states in impact experiments. We extended calculations by Ahrens [J. Appl. Phys. 43, 2443 (1972)] and Ahrens and O'Keefe [The Moon 4, 214 (1972)] to higher distentions (up to five) and improved their method with a different impedance match calculation scheme and augmented their model with recent thermodynamic and Hugoniot data of metals, minerals, and polymers. Although we reconfirmed the competing effects reported in the previous studies: (1) increase of entropy production and (2) decrease of impedance match, when impacting materials with increasing distentions, our calculations did not exhibit optimal entropy-generating distention. For different materials, very different impact velocities are needed to initiate vaporization. For aluminum at distention (m)<2.2, a minimum impact velocity of 2.7 km/s is required using tungsten projectile. For ionic solids such as NaCl at distention <2.2, 2.5 km/s is needed. For carbonate and sulfate minerals, the minimum impact velocities are much lower, ranging from less than 1 to 1.5 km/s
Crescent Singularities in Crumpled Sheets
We examine the crescent singularity of a developable cone in a setting
similar to that studied by Cerda et al [Nature 401, 46 (1999)]. Stretching is
localized in a core region near the pushing tip and bending dominates the outer
region. Two types of stresses in the outer region are identified and shown to
scale differently with the distance to the tip. Energies of the d-cone are
estimated and the conditions for the scaling of core region size R_c are
discussed. Tests of the pushing force equation and direct geometrical
measurements provide numerical evidence that core size scales as R_c ~ h^{1/3}
R^{2/3}, where h is the thickness of sheet and R is the supporting container
radius, in agreement with the proposition of Cerda et al. We give arguments
that this observed scaling law should not represent the asymptotic behavior.
Other properties are also studied and tested numerically, consistent with our
analysis.Comment: 13 pages with 8 figures, revtex. To appear in PR
Determination of intercontinental baselines and Earth orientation using VLBI
A series of experiments was conducted during the last decade to explore the capability of very long baseline interferometry (VLBI) to measure the crustal and rotational motions of the Earth with accuracies at the centimeter level. The observing stations are those of NASA's Deep Space Network in California, Spain and Australia. A multiparameter fit to the observed values of delay and delay rate yields radio source positions, polar motion, universal time, the precession constant, baseline vectors, and solid Earth tides. Source positions are obtained with formal errors of the order of 0''.01. UT1-UTC and polar motion are determined at 49 epochs, with formal error estimates for the more recent data of 0.5 msec for UT1-UTC and 2 to 6 mas for polar motion. Intercontinental baseline lengths are determined with formal errors of 5 to 10 cm. The Love numbers and Earth tide phase lag agree with the commonly accepted values
Spontaneous curvature cancellation in forced thin sheets
In this paper we report numerically observed spontaneous vanishing of mean
curvature on a developable cone made by pushing a thin elastic sheet into a
circular container. We show that this feature is independent of thickness of
the sheet, the supporting radius and the amount of deflection. Several variants
of developable cone are studied to examine the necessary conditions that lead
to the vanishing of mean curvature. It is found that the presence of
appropriate amount of radial stress is necessary. The developable cone geometry
somehow produces the right amount of radial stress to induce just enough radial
curvature to cancel the conical azimuthal curvature. In addition, the circular
symmetry of supporting container edge plays an important role. With an
elliptical supporting edge, the radial curvature overcompensates the azimuthal
curvature near the minor axis and undercompensates near the major axis. Our
numerical finding is verified by a crude experiment using a reflective plastic
sheet. We expect this finding to have broad importance in describing the
general geometrical properties of forced crumpling of thin sheets.Comment: 13 pages, 12 figures, revtex
KPP reaction-diffusion equations with a non-linear loss inside a cylinder
We consider in this paper a reaction-diffusion system in presence of a flow
and under a KPP hypothesis. While the case of a single-equation has been
extensively studied since the pioneering Kolmogorov-Petrovski-Piskunov paper,
the study of the corresponding system with a Lewis number not equal to 1 is
still quite open. Here, we will prove some results about the existence of
travelling fronts and generalized travelling fronts solutions of such a system
with the presence of a non-linear spacedependent loss term inside the domain.
In particular, we will point out the existence of a minimal speed, above which
any real value is an admissible speed. We will also give some spreading results
for initial conditions decaying exponentially at infinity
Energy-level pinning and the 0.7 spin state in one dimension: GaAs quantum wires studied using finite-bias spectroscopy
We study the effects of electron-electron interactions on the energy levels
of GaAs quantum wires (QWs) using finite-bias spectroscopy. We probe the energy
spectrum at zero magnetic field, and at crossings of opposite-spin-levels in
high in-plane magnetic field B. Our results constitute direct evidence that
spin-up (higher energy) levels pin to the chemical potential as they populate.
We also show that spin-up and spin-down levels abruptly rearrange at the
crossing in a manner resembling the magnetic phase transitions predicted to
occur at crossings of Landau levels. This rearranging and pinning of subbands
provides a phenomenological explanation for the 0.7 structure, a
one-dimensional (1D) nanomagnetic state, and its high-B variants.Comment: 6 pages, 4 figure
Nondestructive testing techniques used in analysis of honeycomb structure bond strength
DOT /Driver-Displacement Oriented Transducer/, applicable to both lap shear type application and honeycomb sandwich structures, measures the displacement of the honeycomb composite face sheet. It incorporates an electromagnetic driver and a displacement measuring system into a single unit to provide noncontact bond strength measurements
VLBI measurements of radio source positions at the Jet Propulsion Laboratory
The results of approximately 1300 observations of 67 radio sources are presented. Most of the measurements were made at the stations of the Deep Space Network in California, Spain, and Australia at wavelengths of 13.1 and 3.6 cm, between 1971 and 1978. The formal errors in the derived source positions are generally in the neighborhood of 0.01 seconds of arc and the positions agree fairly well with those previously published
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