2,335 research outputs found
Stationary fluid models for the extra-planar gas in spiral galaxies
We show how to construct families of stationary hydrodynamical configurations
that reproduce the observed vertical gradient of the rotation velocity of the
extra-planar gas in spiral galaxies. We then present a simple model for the
lagging halo of the spiral galaxy NGC 891, which is in agreement with the HI
observations. Our method is based on well known properties of baroclinic
solutions, and it is an elementary application of a much more general and
flexible method.Comment: 8 pages, 2 figures. Proceedings of the Conference "Extra-planar gas",
ASP Conf. Series, R. Braun e
Structural Verification and Modeling of a Tension Cone Inflatable Aerodynamic Decelerator
Verification analyses were conducted on membrane structures pertaining to a tension cone inflatable aerodynamic decelerator using the analysis code LS-DYNA. The responses of three structures - a cylinder, torus, and tension shell - were compared against linear theory for various loading cases. Stress distribution, buckling behavior, and wrinkling behavior were investigated. In general, agreement between theory and LS-DYNA was very good for all cases investigated. These verification cases exposed the important effects of using a linear elastic liner in membrane structures under compression. Finally, a tension cone wind tunnel test article is modeled in LS-DYNA for which preliminary results are presented. Unlike data from supersonic wind tunnel testing, the segmented tension shell and torus experienced oscillatory behavior when subjected to a steady aerodynamic pressure distribution. This work is presented as a work in progress towards development of a fluid-structures interaction mechanism to investigate aeroelastic behavior of inflatable aerodynamic decelerators
Thermomechanical Design Criteria for Ceramic-Coated Surfaces
Some early history of ceramic applications is presented. Finite element modeling of components to determine service and fabrication loads found inelastic behavior and residual stresses to be significant to component life. Inelastic behavior mitigates peak strains but enhances residual strains. Results of furnace, Mach 0.3 burner, and engine tests are discussed and categorized into design criteria (loading, geometry, fabrication, materials, analysis, and testing). These design rules and finite element analyses are brought to bear on two test cases: turboshaft engine seals, and rocket thrust chambers
Spectrophotometry of HII Regions, Diffuse Ionized Gas and Supernova Remnants in M31: The Transition from Photo- to Shock-Ionization
We present results of KPNO 4-m optical spectroscopy of discrete emission-line
nebulae and regions of diffuse ionized gas (DIG) in M31. Long-slit spectra of
16 positions in the NE half of M31 were obtained over a 5-15 kpc range in
radial distance from the center of the galaxy. The spectra have been used to
confirm 16 supernova remnant candidates from the Braun & Walterbos (1993)
catalog. The slits also covered 46 HII regions which show significant
differences among the various morphological types (center-brightened, diffuse,
rings). Radial gradients in emission-line ratios such as [OIII]/H and
[OII]/[OIII] are observed most prominently in the center-brightened HII
regions. These line ratio trends are either much weaker or completely absent in
the diffuse and ring nebulae. The line ratio gradients previously seen in M31
SNRs (Blair, Kirshner, & Chevalier 1981; 1982) are well reproduced by our new
data. The spectra of center-brightened HII regions and SNRs confirm previous
determinations of the radial abundance gradient in M31. We use diagnostic
diagrams which separate photoionized gas from shock-ionized gas to compare the
spectral properties of HII regions, SNRs and DIG. This analysis strengthens
earlier claims (Greenawalt, Walterbos, & Braun 1997) that the DIG in the disk
of M31 is photoionized by a dilute radiation field.Comment: 45 pages, 9 figures, 7 tables, to appear in the Astronomical Journal
(December 1999
Subsonic and Transonic Wind Tunnel Testing of Two Inflatable Aerodynamic Decelerators
Two inflatable aerodynamic decelerator designs were tested in the Transonic Dynamics Tunnel at the NASA Langley Research Center: a tension cone and an isotensoid. The tension cone consists of a flexible tension shell attached to a torus and the isotensoid employs a ram-air inflated envelope. Tests were conducted at Mach numbers from 0.3 to 1.08 and Reynolds numbers from 0.59 to 2.46 million. The main objective of these tests was to obtain static aerodynamic coefficients at subsonic and transonic speeds to supplement supersonic aerodynamic data for these same two designs. The axial force coefficients of both designs increased smoothly from subsonic through transonic Mach numbers. Dynamic data show significant oscillation of the tension cone and minimal oscillation of the isotensoid. The transonic and subsonic data will be used to assemble an inflatable decelerator aerodynamic database for use in computational analyses and system studies
Response of Bose gases in time-dependent optical superlattices
The dynamic response of ultracold Bose gases in one-dimensional optical
lattices and superlattices is investigated based on exact numerical time
evolutions in the framework of the Bose-Hubbard model. The system is excited by
a temporal amplitude modulation of the lattice potential, as it was done in
recent experiments. For regular lattice potentials, the dynamic signatures of
the superfluid to Mott-insulator transition are studied and the position and
the fine-structure of the resonances is explained by a linear response
analysis. Using direct simulations and the perturbative analysis it is shown
that in the presence of a two-colour superlattice the excitation spectrum
changes significantly when going from the homogeneous Mott-insulator the quasi
Bose-glass phase. A characteristic and experimentally accessible signature for
the quasi Bose-glass is the appearance of low-lying resonances and a
suppression of the dominant resonance of the Mott-insulator phase.Comment: 20 pages, 9 figures; added references and corrected typo
Mean-Field- and Classical Limit of Many-Body Schr\"odinger Dynamics for Bosons
We present a new proof of the convergence of the N-particle Schroedinger
dynamics for bosons towards the dynamics generated by the Hartree equation in
the mean-field limit. For a restricted class of two-body interactions, we
obtain convergence estimates uniform in the Planck constant , up to an
exponentially small remainder. For h=0, the classical dynamics in the
mean-field limit is given by the Vlasov equation.Comment: Latex 2e, 18 page
Numerical Hermitian Yang-Mills Connections and Vector Bundle Stability in Heterotic Theories
A numerical algorithm is presented for explicitly computing the gauge
connection on slope-stable holomorphic vector bundles on Calabi-Yau manifolds.
To illustrate this algorithm, we calculate the connections on stable monad
bundles defined on the K3 twofold and Quintic threefold. An error measure is
introduced to determine how closely our algorithmic connection approximates a
solution to the Hermitian Yang-Mills equations. We then extend our results by
investigating the behavior of non slope-stable bundles. In a variety of
examples, it is shown that the failure of these bundles to satisfy the
Hermitian Yang-Mills equations, including field-strength singularities, can be
accurately reproduced numerically. These results make it possible to
numerically determine whether or not a vector bundle is slope-stable, thus
providing an important new tool in the exploration of heterotic vacua.Comment: 52 pages, 15 figures. LaTex formatting of figures corrected in
version 2
Form factor for a family of quantum graphs: An expansion to third order
For certain types of quantum graphs we show that the random-matrix form
factor can be recovered to at least third order in the scaled time from
periodic-orbit theory. We consider the contributions from pairs of periodic
orbits represented by diagrams with up to two self-intersections connected by
up to four arcs and explain why all other diagrams are expected to give
higher-order corrections only.
For a large family of graphs with ergodic classical dynamics the diagrams
that exist in the absence of time-reversal symmetry sum to zero. The mechanism
for this cancellation is rather general which suggests that it may also apply
at higher-orders in the expansion. This expectation is in full agreement with
the fact that in this case the linear- contribution, the diagonal
approximation, already reproduces the random-matrix form factor for .
For systems with time-reversal symmetry there are more diagrams which
contribute at third order. We sum these contributions for quantum graphs with
uniformly hyperbolic dynamics, obtaining , in agreement with
random-matrix theory. As in the previous calculation of the leading-order
correction to the diagonal approximation we find that the third order
contribution can be attributed to exceptional orbits representing the
intersection of diagram classes.Comment: 23 pages (including 4 fig.) - numerous typos correcte
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