13,366 research outputs found
Charge Violation and Alice Behavior in Global and Textured Strings
Spontaneous breaking of global symmetries can produce ``Alice'' strings: line
defects which make unbroken symmetries multivalued, induce apparent charge
violation via Aharonov-Bohm interactions, and form point defects when twisted
into loops. We demonstrate this behavior for both divergent and textured global
Alice strings. Both adiabatically scatter charged particles via effective
Wilson lines. For textured Alice strings, such Wilson lines occur at all radii,
and are multivalued only inside the string. This produces measurable effects,
including path-dependent charge violation.Comment: 32 pages, 2 epsfigs, Revte
Mechanisms of fragmentation of Al-W granular composites under dynamic loading
Numerical simulations of Aluminum (Al) and Tungsten (W) granular composite
rings under various dynamic loading conditions caused by explosive loading were
examined. Three competing mechanisms of fragmentation were observed: a
continuum level mechanism generating large macrocracks described by the
Grady-Kipp fragmentation mechanism, a mesoscale mechanism generating voids and
microcracks near the initially unbonded Al/W interfaces due to tensile strains,
and a mesoscale jetting due to the development of large velocity gradients
between the W particles and adjacent Al. These mesoscale mechanisms can be used
to tailor the size of the fragments by selecting an appropriate initial
mesostructure for a given loading condition.Comment: 10 pages, 3 figures, submitted to AP
A Comparison of Semi-Analytic and Smoothed Particle Hydrodynamics Galaxy Formation
We compare the statistical properties of galaxies found in two different
models of hierarchical galaxy formation: the semi-analytic model of Cole et al.
and the smoothed particle hydrodynamics (SPH) simulations of Pearce et al.
Using a `stripped-down' version of the semi-analytic model which mimics the
resolution of the SPH simulations and excludes physical processes not included
in them, we find that the two models produce an ensemble of galaxies with
remarkably similar properties, although there are some differences in the gas
cooling rates and in the number of galaxies that populate halos of different
mass. The full semi-analytic model, which has effectively no resolution limit
and includes a treatment of star formation and supernovae feedback, produces
somewhat different (but readily understandable) results. Agreement is
particularly good for the present-day global fractions of hot gas, cold dense
(i.e. galactic) gas and uncollapsed gas, for which the SPH and stripped-down
semi-analytic calculations differ by at most 25%. In the most massive halos,
the stripped-down semi-analytic model predicts, on the whole, up to 50% less
gas in galaxies than is seen in the SPH simulations. The two techniques
apportion this cold gas somewhat differently amongst galaxies in a given halo.
This difference can be tracked down to the greater cooling rate in massive
halos in the SPH simulation compared to the semi-analytic model. (abridged)Comment: 19 pages, 13 figures, to appear in MNRAS. Significantly extended to
explore galaxy progenitor distributions and behaviour of models at high
redshift
Three dimensional viscous analysis of a hypersonic inlet
The flow fields in supersonic/hypersonic inlets are currently being studied at NASA Lewis Research Center using 2- and 3-D full Navier-Stokes and Parabolized Navier-Stokes solvers. These tools have been used to analyze the flow through the McDonnell Douglas Option 2 inlet which has been tested at Calspan in support of the National Aerospace Plane Program. Comparisons between the computational and experimental results are presented. These comparisons lead to better overall understanding of the complex flows present in this class of inlets. The aspects of the flow field emphasized in this work are the 3-D effects, the transition from laminar to turbulent flow, and the strong nonuniformities generated within the inlet
Kilohertz QPOs in Neutron Star Binaries modeled as Keplerian Oscillations in a Rotating Frame of Reference
Since the discovery of kHz quasi-periodic oscillations (QPO) in neutron star
binaries, the difference between peak frequencies of two modes in the upper
part of the spectrum, i.e. Delta (omega)=omega_h-omega_K has been studied
extensively. The idea that the difference Delta(omega) is constant and (as a
beat frequency) is related to the rotational frequency of the neutron star has
been tested previously. The observed decrease of Delta(omega) when omega_h and
omega_k increase has weakened the beat frequency interpretation. We put forward
a different paradigm: a Keplerian oscillator under the influence of the
Coriolis force. For such an oscillator, omega_h and the assumed Keplerian
frequency omega_k hold an upper hybrid frequency relation:
omega^2_h-omega^2_K=4*Omega^2, where Omega is the rotational frequency of the
star's magnetosphere near the equatorial plane. For three sources (Sco X-1, 4U
1608-52 and 4U 1702-429), we demonstrate that the solid body rotation
Omega=Omega_0=const. is a good first order approximation. Within the second
order approximation, the slow variation of Omega as a function of omega_K
reveals the structure of the magnetospheric differential rotation. For Sco X-1,
the QPO have frequencies approximately 45 and 90 Hz which we interpret as the
1st and 2nd harmonics of the lower branch of the Keplerian oscillations for the
rotator with vector Omega not aligned with the normal of the disk: omega_L/2
pi=(Omega/pi)(omega_K/omega_h)sin(delta) where delta is the angle between
vector Omega and the vector normal to the disk.Comment: 13 pages, 3 figures, accepted for publications in ApJ Letter
Calculating Farm Machinery Field Capacities
Calculating field capacities is just part of the overall concept of farm machinery management. Successful farm machinery management does not guarantee a profit, but machinery costs are a major expense and they must be monitored and managed. Therefore, the efficient use of farm machinery starts with determining working capacity in conjunction with the amount of work to be accomplished in a timely manner
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