11,813 research outputs found
Specimen Holder Design Improves Accuracy of X-Ray Powder Analysis
Specimen holder for X ray diffraction analysis presents the specimen to the incident X rays in a curvature. This permits the use of an X ray beam having a larger divergence angle, the beam intensity is increased, and the statistical accuracy of analysis is improved
A numerical method for the prediction of high-speed boundary-layer transition using linear theory
A method is described of estimating the location of transition in an arbitrary laminar boundary layer on the basis of linear stability theory. After an examination of experimental evidence for the relation between linear stability theory and transition, a discussion is given of the three essential elements of a transition calculation: (1) the interaction of the external disturbances with the boundary layer; (2) the growth of the disturbances in the boundary layer; and (3) a transition criterion. The computer program which carried out these three calculations is described. The program is first tested by calculating the effect of free-stream turbulence on the transition of the Blasius boundary layer, and is then applied to the problem of transition in a supersonic wind tunnel. The effects of unit Reynolds number and Mach number on the transition of an insulated flat-plate boundary layer are calculated on the basis of experimental data on the intensity and spectrum of free-stream disturbances. Reasonable agreement with experiment is obtained in the Mach number range from 2 to 4.5
SIMMUNE, a tool for simulating and analyzing immune system behavior
We present a new approach to the simulation and analysis of immune system
behavior. The simulations that can be done with our software package called
SIMMUNE are based on immunological data that describe the behavior of immune
system agents (cells, molecules) on a microscopial (i.e. agent-agent
interaction) scale by defining cellular stimulus-response mechanisms. Since the
behavior of the agents in SIMMUNE can be very flexibly configured, its
application is not limited to immune system simulations. We outline the
principles of SIMMUNE's multiscale analysis of emergent structure within the
simulated immune system that allow the identification of immunological contexts
using minimal a priori assumptions about the higher level organization of the
immune system.Comment: 23 pages, 10 figure
1.4-GHz observations of extended giant radio galaxies
This paper presents 1.4-GHz radio continuum observations of 15 very extended
radio galaxies. These sources are so large that most interferometers lose
partly their structure and total flux density. Therefore, single-dish
detections are required to fill in the central (u,v) gap of interferometric
data and obtain reliable spectral index patterns across the structures, and
thus also an integrated radio continuum spectrum. We have obtained such 1.4-GHz
maps with the 100-m Effelsberg telescope and combined them with the
corresponding maps available from the NVSS. The aggregated data allow us to
produce high-quality images, which can be used to obtain physical parameters of
the mapped sources. The combined images reveal in many cases extended low
surface-brightness cocoons.Comment: 39 pages, 19 figures, 3 tables. Published in Ac
Numerical Investigation of Second Mode Attenuation over Carbon/Carbon Surfaces on a Sharp Slender Cone
We have carried out axisymmetric numerical simulations of a spatially
developing hypersonic boundary layer over a sharp 7-half-angle cone
at inspired by the experimental investigations by Wagner (2015).
Simulations are first performed with impermeable (or solid) walls with a
one-time broadband pulse excitation applied upstream to determine the most
convectively-amplified frequencies resulting in the range 260kHz -- 400kHz,
consistent with experimental observations of second-mode instability waves.
Subsequently, we introduce harmonic disturbances via continuous periodic
suction and blowing at 270kHz and 350kHz. For each of these forcing frequencies
complex impedance boundary conditions (IBC), modeling the acoustic response of
two different carbon/carbon (C/C) ultrasonically absorptive porous surfaces,
are applied at the wall. The IBCs are derived as an output of a pore-scale
aeroacoustic analysis -- the inverse Helmholtz Solver (iHS) -- which is able to
return the broadband real and imaginary components of the surface-averaged
impedance. The introduction of the IBCs in all cases leads to a significant
attenuation of the harmonically-forced second-mode wave. In particular, we
observe a higher attenuation rate of the introduced waves with frequency of
350kHz in comparison with 270kHz, and, along with the iHS impedance results, we
establish that the C/C surfaces absorb acoustic energy more effectively at
higher frequencies.Comment: AIAA-SciTech 201
Limitations on wind-tunnel pressure signature extrapolation
Analysis of some recent experimental sonic boom data has revived the hypothesis that there is a closeness limit to the near-field separation distance from which measured wind tunnel pressure signatures can be extrapolated to the ground as though generated by a supersonic-cruise aircraft. Geometric acoustic theory is used to derive an estimate of this distance and the sample data is used to provide a preliminary indication of practical separation distance values
Deconfinement from Action Restriction
The effect of restricting the plaquette to be greater than a certain cutoff
value is studied. The action considered is the standard Wilson action with the
addition of a plaquette restriction, which should not affect the continuum
limit of the theory. In this investigation, the strong coupling limit is also
taken. It is found that a deconfining phase transition occurs as the cutoff is
increased, on all lattices studied (up to ). The critical cutoff on the
infinite lattice appears to be around 0.55. For cutoffs above this, a fixed
point behavior is observed in the normalized fourth cumulant of the Polyakov
loop, suggesting the existence of a line of critical points corresponding to a
massless gluon phase, not unlike the situation in compact U(1). The Polyakov
loop susceptibility also appears to be diverging with lattice size at these
cutoffs. A strong finite volume behavior is observed in the pseudo-specific
heat. It is discussed whether these results could still be consistent with the
standard crossover picture which precludes the existence of a deconfining phase
transition on an infinite symmetric lattice.Comment: 4 pages latex, 6 ps figures, uses espcrc2.sty (included). Poster
presented at LATTICE96(topology
Self-consistent Calculation of Real Space Renormalization Group Flows and Effective Potentials
We show how to compute real space renormalization group flows in lattice
field theory by a self-consistent method. In each step, the integration over
the fluctuation field (high frequency components of the field) is performed by
a saddle point method. The saddle point depends on the block-spin. Higher
powers of derivatives of the field are neglected in the actions, but no
polynomial approximation in the field is made. The flow preserves a simple
parameterization of the action. In this paper we treat scalar field theories as
an example.Comment: 52 pages, uses pstricks macro, three ps-figure
Neural multigrid for gauge theories and other disordered systems
We present evidence that multigrid works for wave equations in disordered
systems, e.g. in the presence of gauge fields, no matter how strong the
disorder, but one needs to introduce a "neural computations" point of view into
large scale simulations: First, the system must learn how to do the simulations
efficiently, then do the simulation (fast).
The method can also be used to provide smooth interpolation kernels which are
needed in multigrid Monte Carlo updates.Comment: 9 pages [2 figures appended in PostScript format], preprint DESY
92-126, Sept. 199
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