722 research outputs found
Effects of leading-edge devices on the low-speed aerodynamic characteristics of a highly-swept arrow-wing
An investigation was conducted in the Texas A&M University 7 by 10 foot Low Speed Wind Tunnel to provide a direct comparison of the effect of several leading edge devices on the aerodynamic performance of a highly swept wing configuration. Analysis of the data indicates that for the configuration with undeflected leading edges, vortex separation first occurs on the outboard wing panel for angles of attack of approximately 2, and wing apex vorticies become apparent for alpha or = 4 deg. However, the occurrence of the leading edge vortex flow may be postponed with leading edge devices. Of the devices considered, the most promising were a simple leading edge deflection of 30 deg and a leading edge slat system. The trailing edge flap effectiveness was found to be essentially the same for the configuration employing either of these more promising leading edge devices. Analysis of the lateral directional data showed that for all of the concepts considered, deflecting leading edge downward in an attempt to postpone leading edge vortex flows, has the favorable effect of reducing the effective dihedral
Dimensional Reduction for Directed Branched Polymers
Dimensional reduction occurs when the critical behavior of one system can be
related to that of another system in a lower dimension. We show that this
occurs for directed branched polymers (DBP) by giving an exact relationship
between DBP models in D+1 dimensions and repulsive gases at negative activity
in D dimensions. This implies relations between exponents of the two models:
(the exponent describing the singularity of the
pressure), and (the correlation length exponent of
the repulsive gas). It also leads to the relation ,
where is the Yang-Lee edge exponent. We derive exact expressions
for the number of DBP of size N in two dimensions.Comment: 7 pages, 1 eps figure, ref 24 correcte
Critical temperature and density of spin-flips in the anisotropic random field Ising model
We present analytical results for the strongly anisotropic random field Ising
model, consisting of weakly interacting spin chains. We combine the mean-field
treatment of interchain interactions with an analytical calculation of the
average chain free energy (``chain mean-field'' approach). The free energy is
found using a mapping on a Brownian motion model. We calculate the order
parameter and give expressions for the critical random magnetic field strength
below which the ground state exhibits long range order and for the critical
temperature as a function of the random magnetic field strength. In the limit
of vanishing interchain interactions, we obtain corrections to the
zero-temperature estimate by Imry and Ma [Phys. Rev. Lett. 35, 1399 (1975)] of
the ground state density of domain walls (spin-flips) in the one-dimensional
random field Ising model. One of the problems to which our model has direct
relevance is the lattice dimerization in disordered quasi-one-dimensional
Peierls materials, such as the conjugated polymer trans-polyacetylene.Comment: 28 pages, revtex, 4 postscript figures, to appear in Phys. Rev.
Ice Age Epochs and the Sun's Path Through the Galaxy
We present a calculation of the Sun's motion through the Milky Way Galaxy
over the last 500 million years. The integration is based upon estimates of the
Sun's current position and speed from measurements with Hipparcos and upon a
realistic model for the Galactic gravitational potential. We estimate the times
of the Sun's past spiral arm crossings for a range in assumed values of the
spiral pattern angular speed. We find that for a difference between the mean
solar and pattern speed of Omega_Sun - Omega_p = 11.9 +/- 0.7 km/s/kpc the Sun
has traversed four spiral arms at times that appear to correspond well with
long duration cold periods on Earth. This supports the idea that extended
exposure to the higher cosmic ray flux associated with spiral arms can lead to
increased cloud cover and long ice age epochs on Earth.Comment: 14 pages, 3 figures, accepted for publication in Ap
Numerical study of the transition of the four dimensional Random Field Ising Model
We study numerically the region above the critical temperature of the four
dimensional Random Field Ising Model. Using a cluster dynamic we measure the
connected and disconnected magnetic susceptibility and the connected and
disconnected overlap susceptibility. We use a bimodal distribution of the field
with for all temperatures and a lattice size L=16. Through a
least-square fit we determine the critical exponents and . We find the magnetic susceptibility and the overlap
susceptibility diverge at two different temperatures. This is coherent with the
existence of a glassy phase above . Accordingly with other simulations
we find . In this case we have a scaling theory with
two indipendet critical exponentsComment: 10 pages, 2 figures, Late
Weighted Mean Field Theory for the Random Field Ising Model
We consider the mean field theory of the Random Field Ising Model obtained by
weighing the many solutions of the mean field equations with Boltzmann-like
factors. These solutions are found numerically in three dimensions and we
observe critical behavior arising from the weighted sum. The resulting
exponents are calculated.Comment: 15 pages of tex using harvmac. 8 postscript figures (fig3.ps is
large) in a separate .uu fil
On the thermodynamics of first-order phase transition smeared by frozen disorder
The simplified model of first-order transition in a media with frozen
long-range transition-temperature disorder is considered. It exhibits the
smearing of the transition due to appearance of the intermediate inhomogeneous
phase with thermodynamics described by the ground state of the short-range
random-field Ising model. Thus the model correctly reproduce the persistence of
first-order transition only in dimensions d > 2, which is found in more
realistic models. It also allows to estimate the behavior of thermodynamic
parameters near the boundaries of the inhomogeneous phase.Comment: 4 page
Modified Scaling Relation for the Random-Field Ising Model
We investigate the low-temperature critical behavior of the three dimensional
random-field Ising ferromagnet. By a scaling analysis we find that in the limit
of temperature the usual scaling relations have to be modified as far
as the exponent of the specific heat is concerned. At zero
temperature, the Rushbrooke equation is modified to , an equation which we expect to be valid also for other systems with
similar critical behavior. We test the scaling theory numerically for the three
dimensional random field Ising system with Gaussian probability distribution of
the random fields by a combination of calculations of exact ground states with
an integer optimization algorithm and Monte Carlo methods. By a finite size
scaling analysis we calculate the critical exponents , , and .Comment: 4 pages, Latex, Postscript Figures include
Dynamics of Domains in Diluted Antiferromagnets
We investigate the dynamics of two-dimensional site-diluted Ising
antiferromagnets. In an external magnetic field these highly disordered
magnetic systems have a domain structure which consists of fractal domains with
sizes on a broad range of length scales. We focus on the dynamics of these
systems during the relaxation from a long-range ordered initial state to the
disordered fractal-domain state after applying an external magnetic field. The
equilibrium state with applied field consists of fractal domains with a size
distribution which follows a power law with an exponential cut-off. The
dynamics of the system can be understood as a growth process of this
fractal-domain state in such a way that the equilibrium distribution of domains
develops during time. Following these ideas quantitatively we derive a simple
description of the time dependence of the order parameter. The agreement with
simulations is excellent.Comment: Revtex, 6 pages, 5 Postscript figure
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