21,447 research outputs found
Shock-Wave Heating Model for Chondrule Formation: Prevention of Isotopic Fractionation
Chondrules are considered to have much information on dust particles and
processes in the solar nebula. It is naturally expected that protoplanetary
disks observed in present star forming regions have similar dust particles and
processes, so study of chondrule formation may provide us great information on
the formation of the planetary systems.
Evaporation during chondrule melting may have resulted in depletion of
volatile elements in chondrules. However, no evidence for a large degree of
heavy-isotope enrichment has been reported in chondrules. In order to meet this
observed constraint, the rapid heating rate at temperatures below the silicate
solidus is required to suppress the isotopic fractionation.
We have developed a new shock-wave heating model taking into account the
radiative transfer of the dust thermal continuum emission and the line emission
of gas molecules and calculated the thermal history of chondrules. We have
found that optically-thin shock waves for the thermal continuum emission from
dust particles can meet the rapid heating constraint, because the dust thermal
emission does not keep the dust particles high temperature for a long time in
the pre-shock region and dust particles are abruptly heated by the gas drag
heating in the post-shock region. We have also derived the upper limit of
optical depth of the pre-shock region using the radiative diffusion
approximation, above which the rapid heating constraint is not satisfied. It is
about 1 - 10.Comment: 58 pages, including 5 tables and 15 figures, accepted for publication
in The Astrophysical Journa
Unzipping an adsorbed polymer in a dirty or random environment
The phase diagram of unzipping of an adsorbed directed polymer in two
dimensions in a random medium has been determined. Both the hard-wall and the
soft-wall cases are considered. Exact solutions for the pure problem with
different affinities on the two sides are given. The results obtained by the
numerical procedure adopted here are shown to agree with the exact results for
the pure case. The characteristic exponents for unzipping for the random
problem are different from the pure case. The distribution functions for the
unzipped length, first bubble, and the spacer are determined.Comment: Published version, uses revtex4, 14 page
Three-dimensional calculation of shuttle charging in polar orbit
The charged particles environment in polar orbit can be of sufficient intensity to cause spacecraft charging. In order to gain a quantitative understanding of such effects, the Air Force is developing POLAR, a computer code which simulates in three dimensions the electrical interaction of large space vehicles with the polar ionospheric plasma. It models the physical processes of wake generation, ambient ion collection, precipitating auroral electron fluxes, and surface interactions, including secondary electron generation and backscattering, which lead to vehicle charging. These processes may be followed dynamically on a subsecond timescale so that the rapid passage through intense auroral arcs can be simulated. POLAR models the ambient plasma as isotropic Maxwellian electrons and ions (0+, H+), and allows for simultaneous precipitation of power-law, energetic Maxwellian, and accelerated Gaussian distributions of electrons. Magnetic field effects will be modeled in POLAR but are currently ignored
Open Questions in Classical Gravity
We discuss some outstanding open questions regarding the validity and
uniqueness of the standard second order Newton-Einstein classical gravitational
theory. On the observational side we discuss the degree to which the realm of
validity of Newton's Law of Gravity can actually be extended to distances much
larger than the solar system distance scales on which the law was originally
established. On the theoretical side we identify some commonly accepted but
actually still open to question assumptions which go into the formulating of
the standard second order Einstein theory in the first place. In particular, we
show that while the familiar second order Poisson gravitational equation (and
accordingly its second order covariant Einstein generalization) may be
sufficient to yield Newton's Law of Gravity they are not in fact necessary. The
standard theory thus still awaits the identification of some principle which
would then make it necessary too. We show that current observational
information does not exclusively mandate the standard theory, and that the
conformal invariant fourth order theory of gravity considered recently by
Mannheim and Kazanas is also able to meet the constraints of data, and in fact
to do so without the need for any so far unobserved non-luminous or dark
matter.Comment: UCONN-93-1, plain TeX format, 22 pages (plus 7 figures - send
requests to [email protected]). To appear in a special issue of
Foundations of Physics honoring Professor Fritz Rohrlich on the occasion of
his retirement, L. P. Horwitz and A. van der Merwe Editors, Plenum Publishing
Company, N.Y., Fall 199
Consistent boundary conditions for Reduced Navier-Stokes (RNS) scheme applied to 3-dimensional internal viscous flows
A consistent and efficient set of boundary conditions is developed for the multi-sweep space-marching pressure-elliptic Reduced Navier-Stokes (RNS) scheme as applied for 3-D internal viscous flow problems. No-slip boundary conditions are directly imposed on the solid walls. There is no iteration procedure required in the cross plane to ensure mass conservation across each marching plane. The finite difference equations forming the coefficient matrix are ordered such that the surface normal velocity is specified on all the solid walls; unlike external flows, a pressure boundary condition in the cross plane is not required. Since continuity is directly satisfied at all points in the flow domain, the first order momentum equations can be solved directly for the pressure without the need for a Poisson pressure correction equation. The procedure developed herein can also be applied with periodic boundary conditions. The analysis is given for general compressible flows. Incompressible flow solutions are obtained, for straight and curved ducts of square cross section, to validate the procedure. These solutions are used to demonstrate the applicability of the RNS scheme, with the improved boundary conditions for internal flows with strong interaction, as would be encountered in ducts and turbomachinery geometries
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