34,903 research outputs found
Aerodynamic stability analysis of NASA J85-13/planar pressure pulse generator installation
A digital computer simulation model for the J85-13/Planar Pressure Pulse Generator (P3 G) test installation was developed by modifying an existing General Electric compression system model. This modification included the incorporation of a novel method for describing the unsteady blade lift force. This approach significantly enhanced the capability of the model to handle unsteady flows. In addition, the frequency response characteristics of the J85-13/P3G test installation were analyzed in support of selecting instrumentation locations to avoid standing wave nodes within the test apparatus and thus, low signal levels. The feasibility of employing explicit analytical expression for surge prediction was also studied
Equivalence of the Falicov-Kimball and Brandt-Mielsch forms for the free energy of the infinite-dimensional Falicov-Kimball model
Falicov and Kimball proposed a real-axis form for the free energy of the
Falicov-Kimball model that was modified for the coherent potential
approximation by Plischke. Brandt and Mielsch proposed an imaginary-axis form
for the free energy of the dynamical mean field theory solution of the
Falicov-Kimball model. It has long been known that these two formulae are
numerically equal to each other; an explicit derivation showing this
equivalence is presented here.Comment: 4 pages, 1 figure, typeset with ReVTe
Ising films with surface defects
The influence of surface defects on the critical properties of magnetic films
is studied for Ising models with nearest-neighbour ferromagnetic couplings. The
defects include one or two adjacent lines of additional atoms and a step on the
surface. For the calculations, both density-matrix renormalization group and
Monte Carlo techniques are used. By changing the local couplings at the defects
and the film thickness, non-universal features as well as interesting crossover
phenomena in the magnetic exponents are observed.Comment: 8 pages, 12 figures included, submitted to European Physical Journal
A decentralized motion coordination strategy for dynamic target tracking
This paper presents a decentralized motion planning
algorithm for the distributed sensing of a noisy dynamical
process by multiple cooperating mobile sensor agents. This
problem is motivated by localization and tracking tasks of
dynamic targets. Our gradient-descent method is based on a
cost function that measures the overall quality of sensing. We
also investigate the role of imperfect communication between
sensor agents in this framework, and examine the trade-offs in
performance between sensing and communication. Simulations
illustrate the basic characteristics of the algorithms
Two paths of cluster evolution: global expansion versus core collapse
All gravitationally bound clusters expand, due to both gas loss from their
most massive members and binary heating. All are eventually disrupted tidally,
either by passing molecular clouds or the gravitational potential of their host
galaxies. However, their interior evolution can follow two very different
paths. Only clusters of sufficiently large initial population and size undergo
the combined interior contraction and exterior expansion that leads eventually
to core collapse. In all other systems, core collapse is frustrated by binary
heating. These clusters globally expand for their entire lives, up to the point
of tidal disruption.
Using a suite of direct N-body calculations, we trace the "collapse line" in
r_v-N space that separates these two paths. Here, r_v and N are the cluster's
initial virial radius and population, respectively. For realistic starting
radii, the dividing N-value is from 10^4 to over 10^5. We also show that there
exists a minimum population, N_min, for core collapse. Clusters with N < N_min
tidally disrupt before core collapse occurs. At the Sun's Galactocentric
radius, R_G = 8.5 kpc, we find N_min >~ 300. The minimum population scales with
Galactocentric radius as R_G^{-9/8}.
The position of an observed cluster relative to the collapse line can be used
to predict its future evolution. Using a small sample of open clusters, we find
that most lie below the collapse line, and thus will never undergo core
collapse. Most globular clusters, on the other hand, lie well above the line.
In such a case, the cluster may or may not go through core collapse, depending
on its initial size. We show how an accurate age determination can help settle
this issue.Comment: Accepted for publication in MNRAS. 14 Pages, 9 Figures, 2 Table
Lorentz and CPT Violating Chern-Simons Term in the Formulation of Functional Integral
We show that in the functional integral formalism the (finite) coefficient of
the induced, Lorentz- and CPT-violating Chern-Simons term, arising from the
Lorentz- and CPT-violating fermion sector, is undetermined.Comment: 5 pages, no figure, RevTe
Angular Normal Modes of a Circular Coulomb Cluster
We investigate the angular normal modes for small oscillations about an
equilibrium of a single-component coulomb cluster confined by a radially
symmetric external potential to a circle. The dynamical matrix for this system
is a Laplacian symmetrically circulant matrix and this result leads to an
analytic solution for the eigenfrequencies of the angular normal modes. We also
show the limiting dependence of the largest eigenfrequency for large numbers of
particles
A mathematical simulation model of the CH-47B helicopter, volume 1
A nonlinear simulation model of the CH-47B helicopter was adapted for use in the NASA Ames Research Center (ARC) simulation facility. The model represents the specific configuration of the ARC variable stability CH-47B helicopter and will be used in ground simulation research and to expedite and verify flight experiment design. Modeling of the helicopter uses a total force approach in six rigid body degrees of freedom. Rotor dynamics are simulated using the Wheatlely-Bailey equations including steady-state flapping dynamics. Also included in the model is the option for simulation of external suspension, slung-load equations of motion
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