26,076 research outputs found
Summary of all cycle II.5 shear and boundary layer measurements, aerodynamics
The two measurement systems were used to measure mean velocity and velocity, mass flux, and total temperature fluctuations in the turbulent boundary on the fuselage of a KC-135 aircraft. The boundary layer thickness ranged between about 20 and 30 cm for the range of flight Mach numbers from about 0.25 to 0.85 and Reynolds numbers between 3 and 6 x 10 to the 6th power/m. The adaptation of each system for use in airborne applications is discussed. The data obtained from each system are given and compared with each other and they indicate that the two systems represent viable ones for use in future airborne turbulence experiments
On the calculation of supersonic, separating, and reattaching flows
A method is developed for solving the laminar and turbulent compressible boundary-layer equations for separating and reattaching flows. Results of this method are compared with experimental data for two laminar and three turbulent boundary-layer, shock-wave interactions. Several Navier-Stokes solutions were obtained for each of the laminar boundary-layer, shock-wave interactions considered. Comparison of these solutions indicates a first-order sensitivity in C sub f to the computational mesh selected in both the viscous and inviscid portions of the flow
Correction, improvement and model verification of CARE 3, version 3
An independent verification of the CARE 3 mathematical model and computer code was conducted and reported in NASA Contractor Report 166096, Review and Verification of CARE 3 Mathematical Model and Code: Interim Report. The study uncovered some implementation errors that were corrected and are reported in this document. The corrected CARE 3 program is called version 4. Thus the document, correction. improvement, and model verification of CARE 3, version 3 was written in April 1984. It is being published now as it has been determined to contain a more accurate representation of CARE 3 than the preceding document of April 1983. This edition supercedes NASA-CR-166122 entitled, 'Correction and Improvement of CARE 3,' version 3, April 1983
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Observations of the J = 2→1 transitions of <sup>12</sup>C<sup>16</sup>O and <sup>12</sup>C<sup>18</sup>O towards galactic H II regions
Observations are reported of the J = 2→1 transitions of CO and 12C18O at 230 and 219 GHz respectively from a number of galactic sources. A map of the central 1/2° × 1/2° of the Orion A molecular cloud is presented. The spectra are interpreted to derive molecular densities and abundance ratios in the molecular clouds observed
Reexamining Black-Body Shifts for Hydrogenlike Ions
We investigate black-body induced energy shifts for low-lying levels of
atomic systems, with a special emphasis on transitions used in current and
planned high-precision experiments on atomic hydrogen and ionized helium.
Fine-structure and Lamb-shift induced black-body shifts are found to increase
with the square of the nuclear charge number, whereas black-body shifts due to
virtual transitions decrease with increasing nuclear charge as the fourth power
of the nuclear charge. We also investigate the decay width acquired by the
ground state of atomic hydrogen, due to interaction with black-body photons.
The corresponding width is due to an instability against excitation to higher
excited atomic levels, and due to black-body induced ionization. These effects
limit the lifetime of even the most fundamental, a priori absolutely stable,
"asymptotic" state of atomic theory, namely the ground state of atomic
hydrogen.Comment: 11 pages; LaTe
Zitterbewegung of nearly-free and tightly bound electrons in solids
We show theoretically that nonrelativistic nearly-free electrons in solids
should experience a trembling motion
(Zitterbewegung, ZB) in absence of external fields, similarly to relativistic
electrons in vacuum.
The Zitterbewegung is directly related to the influence of periodic potential
on the free electron motion.
The frequency of ZB is , where is the energy
gap. The amplitude of ZB is determined by the strength of periodic potential
and the lattice period and it can be of the order of nanometers. We show that
the amplitude of ZB does not depend much on the width of the wave packet
representing an electron in real space.
An analogue of the Foldy-Wouthuysen transformation, known from relativistic
quantum mechanics, is introduced in order to decouple electron states in
various bands. We demonstrate that, after the bands are decoupled, electrons
should be treated as particles of a finite size.
In contrast to nearly-free electrons we consider a two-band model of tightly
bound electrons.
We show that also in this case the electrons should experience the trembling
motion. It is concluded that the phenomenon of Zitterbewegung of electrons in
crystalline solids is a rule rather than an exception.Comment: 22 pages, 6 figures Published version, minor changes mad
Hot Populations in M87 Globular Clusters
We have obtained HST/STIS far- and near-UV photometry of globular clusters in
four fields in the gE galaxy M87. To a limit of m(FUV) = 25 we detect a total
of 66 globular clusters (GCs) in common with the deep HST optical-band study of
Kundu et al. (1999). Despite strong overlap in V- and I-band properties, the
M87 GCs have UV/optical properties that are distinct from clusters in the Milky
Way and in M31. M87 clusters, especially metal-poor ones, produce larger hot HB
populations than do Milky Way analogues. Cluster mass is probably not a factor
in these distinctions. The most metal-rich M87 GCs in our sample are near Z_sun
and overlap the local E galaxy sample in estimated Mg_2 line indices.
Nonetheless, the clusters produce much more UV light at a given Mg_2, being up
to 1 mag bluer than any gE galaxy in (FUV-V) color. The M87 GCs do not appear
to represent a transition between Milky Way-type clusters and E galaxies. The
differences are in the correct sense if the clusters are significantly older
than the E galaxies. Comparisons with Galactic open clusters indicate that the
hot stars lie on the extreme horizontal branch, rather than being blue
stragglers, and that the EHB becomes well populated for ages > 5 Gyr. We find
that 43 of our UV detections have no optical-band counterparts. Most appear to
be UV-bright background galaxies, seen through M87. Eleven NUV variable sources
detected at only one epoch in the central field are probably classical novae.
[Abridged]Comment: 70 pages, 25 figures (including 4 jpgs), 7 tables. To appear in AJ.
Full resolution version available at
http://www.astro.virginia.edu/~rwo/m87/m87-hotpops.pd
Non-locality of Foldy-Wouthuysen and related transformations for the Dirac equation
Non-localities of Foldy-Wouthuysen and related transformations, which are
used to separate positive and negative energy states in the Dirac equation, are
investigated. Second moments of functional kernels generated by the
transformations are calculated, the transformed functions and their variances
are computed. It is shown that all the transformed quantities are smeared in
the coordinate space by the amount comparable to the Compton wavelength
.Comment: 7 pages, two figure
Critical Charges on Strange Quark Nuggets and Other Extended Objects
We investigate the behavior of the critical charge for spontaneous pair
production, , defined as the charge at which the total energy of a
-shell electron is , as a function of the radius of the charge
distribution. Our approach is to solve the Dirac equation for a potential
consisting of a spherically symmetrical charge distribution of radius
and a Coulomb tail. For a spherical shell distribution of the type usually
associated with color-flavor locked strange quark nuggets, we confirm the
relation for sufficiently large obtained by Madsen,
who used an approach based on the Thomas-Fermi model. We also present results
for a uniformly charged sphere and again find that for large enough
. Also discussed is the behavior of when simple {\it ad hoc}
modifications are made to the potential for .Comment: 7 pages, 6 figure
A simple description of the states and in
A sixth-order quadrupole boson Hamiltonian is used to describe 26 states
and 67 states which have been recently identified in .
Two closed expressions are alternatively used for energy levels. One
corresponds to a semi-classical approach while the other one represents the
exact eigenvalue of the model Hamiltonian. The semi-classical expression
involves four parameters, while the exact eigenvalue is determined by five
parameters. In each of the two descriptions a least square fit procedure is
adopted.
Both expressions provide a surprisingly good agreement with the experimental
data.Comment: 9 pages, 5 figure
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