37,834 research outputs found
Free wake analysis of hover performance using a new influence coefficient method
A new approach to the prediction of helicopter rotor performance using a free wake analysis was developed. This new method uses a relaxation process that does not suffer from the convergence problems associated with previous time marching simulations. This wake relaxation procedure was coupled to a vortex-lattice, lifting surface loads analysis to produce a novel, self contained performance prediction code: EHPIC (Evaluation of Helicopter Performance using Influence Coefficients). The major technical features of the EHPIC code are described and a substantial amount of background information on the capabilities and proper operation of the code is supplied. Sample problems were undertaken to demonstrate the robustness and flexibility of the basic approach. Also, a performance correlation study was carried out to establish the breadth of applicability of the code, with very favorable results
New Regime of MHD Turbulence: Cascade Below Viscous Cutoff
In astrophysical situations, e.g. in the interstellar medium (ISM), neutrals
can provide viscous damping on scales much larger than the magnetic diffusion
scale. Through numerical simulations, we have found that the magnetic field can
have a rich structure below the dissipation cutoff scale. This implies that
magnetic fields in the ISM can have structures on scales much smaller than
parsec scales. Our results show that the magnetic energy contained in a
wavenumber band is independent of the wavenumber and magnetic structures are
intermittent and extremely anisotropic. We discuss the relation between our
results and the formation of the tiny-scale atomic structure (TSAS).Comment: ApJ Letters, accepted (Feb. 10, 2002; ApJ, 566, L...); 10 pages, 3
figure
Discontinuous percolation transitions in real physical systems
We study discontinuous percolation transitions (PT) in the diffusion-limited
cluster aggregation model of the sol-gel transition as an example of real
physical systems, in which the number of aggregation events is regarded as the
number of bonds occupied in the system. When particles are Brownian, in which
cluster velocity depends on cluster size as with
, a larger cluster has less probability to collide with other
clusters because of its smaller mobility. Thus, the cluster is effectively more
suppressed in growth of its size. Then the giant cluster size increases
drastically by merging those suppressed clusters near the percolation
threshold, exhibiting a discontinuous PT. We also study the tricritical
behavior by controlling the parameter , and the tricritical point is
determined by introducing an asymmetric Smoluchowski equation.Comment: 5 pages, 5 figure
Schwinger Effect in Non-parallel D1-branes: A Path Integral Approach
We study the Schwinger effect in a system of non-parallel D1-branes for the
bosonic strings using the path integral formalism. We drive the string pair
creation rate by calculating the one loop vacuum amplitude of the setup in
presence of the background electric filed defined along one of the D1-branes.
We find an angle dependent minimum value for the background field and show that
the decaying of vacuum into string pairs takes place for the field above this
value. It is shown that in limit the vacuum
becomes stable and thus no pair creation occurs
Vacuum defects without a vacuum
Topological defects can arise in symmetry breaking models where the scalar
field potential has no minima and is a monotonically decreasing
function of . The properties of such vacuumless defects are quite
different from those of the ``usual'' strings and monopoles. In some models
such defects can serve as seeds for structure formation, or produce an
appreciable density of mini-black holes.Comment: 11 pages, REVTeX, 1 Postscript figure. Minor changes. Final version,
to appear in Phys. Rev.
Polarization of Prompt J/psi at the Tevatron
The polarization of prompt J/psi at the Fermilab Tevatron is calculated
within the nonrelativistic QCD factorization framework. The contribution from
radiative decays of P-wave charmonium states decreases, but does not eliminate,
the transverse polarization at large transverse momentum. The angular
distribution parameter alpha for leptonic decays of the J/\psi is predicted to
increase from near 0 at p_T = 5 GeV to about 0.5 at p_T = 20 GeV. The
prediction is consistent with measurements by the CDF Collaboration at
intermediate values of p_T, but disagrees by about 3 standard deviations at the
largest values of p_T measured.Comment: 4 pages, 2 figures, one reference added, accepted for publication in
Phys. Rev.
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