12,828 research outputs found
Identification of vortexes obstructing the dynamo mechanism in laboratory experiments
The magnetohydrodynamic dynamo effect explains the generation of
self-sustained magnetic fields in electrically conducting flows, especially in
geo- and astrophysical environments. Yet the details of this mechanism are
still unknown, e.g., how and to which extent the geometry, the fluid topology,
the forcing mechanism and the turbulence can have a negative effect on this
process. We report on numerical simulations carried out in spherical geometry,
analyzing the predicted velocity flow with the so-called Singular Value
Decomposition, a powerful technique that allows us to precisely identify
vortexes in the flow which would be difficult to characterize with conventional
spectral methods. We then quantify the contribution of these vortexes to the
growth rate of the magnetic energy in the system. We identify an axisymmetric
vortex, whose rotational direction changes periodically in time, and whose
dynamics are decoupled from those of the large scale background flow, is
detrimental for the dynamo effect. A comparison with experiments is carried
out, showing that similar dynamics were observed in cylindrical geometry. These
previously unexpected eddies, which impede the dynamo effect, offer an
explanation for the experimental difficulties in attaining a dynamo in
spherical geometry.Comment: 25 pages, 12 figures, submitted to Physics of Fluid
Obtaining Maxwell's equations heuristically
Starting from the experimental fact that a moving charge experiences the
Lorentz force and applying the fundamental principles of simplicity (first
order derivatives only) and linearity (superposition principle), we show that
the structure of the microscopic Maxwell equations for the electromagnetic
fields can be deduced heuristically by using the transformation properties of
the fields under space inversion and time reversal. Using the experimental
facts of charge conservation and that electromagnetic waves propagate with the
speed of light together with Galileo invariance of the Lorentz force allows us
to introduce arbitrary electrodynamic units naturally.Comment: 11 page
Surface control system for the 15 meter hoop-column antenna
The 15-meter hoop-column antenna fabricated by the Harris Corporation under contract to the NASA Langley Research Center is described. The antenna is a deployable and restowable structure consisting of a central telescoping column, a 15-meter-diameter folding hoop, and a mesh reflector surface. The hoop is supported and positioned by 48 quartz cords attached to the column above the hoop, and by 24 graphite cords from the base of the antenna column. The RF reflective surface is a gold plated molybdenum wire mesh supported on a graphite cord truss structure which is attached between the hoop and the column. The surface contour is controlled by 96 graphite cords from the antenna base to the rear of the truss assembly. The antenna is actually a quadaperture reflector with each quadrant of the surface mesh shaped to produce an offset parabolic reflector. Results of near-field and structural tests are given. Controls structures and electromagnetics interaction, surface control system requirements, mesh control adjustment, surface control system actuator assembly, surface control system electronics, the system interface unit, and control stations are discussed
Nonlinearities and Parametric Amplification in Superconducting Coplanar Waveguide Resonators
Experimental investigations of the nonlinear properties of superconducting
niobium coplanar waveguide resonators are reported. The nonlinearity due to a
current dependent kinetic inductance of the center conductor is strong enough
to realize bifurcation of the nonlinear oscillator. When driven with two
frequencies near the threshold for bifurcation, parametric amplification with a
gain of +22.4 dB is observed.Comment: 4 pages, 5 figures, 1 table. In version 2: Added appendix with model
description and fits to measurements. Minor corrections and rephrasin
Evaluating Sweat Gland Activity with Imprint Techniques
Starch-iodine and bromphenol blue are standard indicators used to record the distribution and number of actively secreting sweat glands. In this report, techniques employing these indicators were compared with methods utilizing newer plastic and silicone impressions.1. Both the plastic and silicone impressions yielded consistently greater sweat gland droplet counts than either of the indicators (p < 0.01).2. A simple technique applying thin films of a silicone rubber monomer to obtain sharp, permanent and uniformly reproducible sweat droplet impressions with little technical skill was judged superior to all methods evaluated
Landscape Analysis of George W. Pirtle Scout Reservation
To assist in detecting change over time for the forested landscape of George W. Pirtle Scout Reservation, Panola County, Texas, digital ortho-photography imagery (acquired 1996 and 2004) were used to designate forest cover types and hazard rating for forest insects and diseases. Status of camp grounds was ascertained and recommendations made for management using the GIS database created for the camp
Roof Loads for Reliability Analysis of Lumber Properties Data
With load information reported in previous studies, distributions of maximum lifetime roof loads are developed in a form suitable for use in reliability analyses of lumber properties data. A lognormal distribution is chosen as best representing normalized maximum lifetime roof snow load.Examples are given in which contrasting lumber data sets are compared using the calculated load distributions and assuming that each set must provide equal reliability (equal safety) in the final design. A factor, k, resulting from this reliability analysis is shown to be a logical adjustment parameter for use in engineering design codes
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