971,963 research outputs found
FTIR analysis of aviation fuel deposits
Five modes of operation of the Nicolet 7199 Fourier Transform Infrared Spectrophotometer have been evaluated for application in analysis of the chemical structure of accelerated storage/thermal deposits produced by jet fuels. Using primarily the absorption and emission modes, the effects of fuel type, stress temperature, stress time, type of spiking agent, spiking agent concentration, fuel flow, and post-depositional treatment on the chemical nature of fuel deposits have been determined
Modelling tidal energy extraction in a depth-averaged coastal domain
An extension of actuator disc theory is used to describe the properties of a tidal energy device, or row of tidal energy devices, within a depth-averaged numerical model. This approach allows a direct link to be made between an actual tidal device and its equivalent momentum sink in a depth-averaged domain. Extended actuator disc theory also leads to a measure of efficiency for an energy device in a tidal stream of finite Froude number, where efficiency is defined as the ratio of power extracted by one or more tidal devices to the total power removed from the tidal stream. To demonstrate the use of actuator disc theory in a depth-averaged model, tidal flow in a simple channel is approximated using the shallow water equations and the results are compared with the published analytical solutions. © 2010 © The Institution of Engineering and Technology
Energy potential of a tidal fence deployed near a coastal headland
Enhanced tidal streams close to coastal headlands appear to present ideal locations for the deployment of tidal energy devices. In this paper, the power potential of tidal streams near an idealized coastal headland with a sloping seabed is investigated using a near-field approximation to represent a tidal fence, i.e. a row of tidal devices, in a two-dimensional depth-averaged numerical model. Simulations indicate that the power extracted by the tidal fence is limited because the flow will bypass the fence, predominantly on the ocean side, as the thrust applied by the devices increases. For the dynamic conditions, fence placements and headland aspect ratios considered, the maximum power extracted at the fence is not related in any obvious way to the local undisturbed kinetic flux or the natural rate of energy dissipation due to bed friction (although both of these have been used in the past to predict the amount of power that may be extracted). The available power (equal to the extracted power net of vertical mixing losses in the immediate wake of devices) is optimized for devices with large area and small centre-to-centre spacing within the fence. The influence of energy extraction on the natural flow field is assessed relative to changes in the M2 component of elevation and velocity, and residual bed shear stress and tidal dispersion
Cosmological string models from Milne spaces and SL(2,Z) orbifold
The -dimensional Milne Universe with extra free directions is used to
construct simple FRW cosmological string models in four dimensions, describing
expansion in the presence of matter with , . We then
consider the n=2 case and make SL(2,Z) orbifold identifications. The model is
surprisingly related to the null orbifold with an extra reflection generator.
The study of the string spectrum involves the theory of harmonic functions in
the fundamental domain of SL(2,Z). In particular, from this theory one can
deduce a bound for the energy gap and the fact that there are an infinite
number of excitations with a finite degeneracy. We discuss the structure of
wave functions and give examples of physical winding states becoming light near
the singularity.Comment: 14 pages, harvma
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Accuracy and Mechanical Properties of Open-Cell Microstructures Fabricated by Selective Laser Sintering
This paper investigates the applicability of selective laser sintering (SLS) for the manufacture of
scaffold geometries for bone tissue engineering applications. Porous scaffold geometries with
open-cell structure and relative density of 10-60 v% were computationally designed and
fabricated by selective laser sintering using polyamide powder. Strut and pore sizes ranging from
0.4 - 1 mm and 1.2 -2 mm are explored. The effect of process parameters on compressive
properties and accuracy of scaffolds was examined and outline laser power and scan spacing
were identified as significant factors. In general, the designed scaffold geometry was not
accurately fabricated on the micron-scale. The smallest successfully fabricated strut and pore size
was 0.4 mm and 1.2 mm, respectively. It was found that selective laser sintering has the potential
to fabricate hard tissue engineering scaffolds. However the technology is not able to replicate
exact geometries on the micron-scale but by accounting for errors resulting from the diameter of
the laser and from the manufacturing induced geometrical deformations in different building
directions, the exact dimensions of the manufactured scaffolds can be predicted and controlled
indirectly, which corresponds favorably with its application in computer aided tissue engineering.Mechanical Engineerin
Gamma ray burst astrometry 2: Numerical tests
Since the announcement of the discovery of sources of gamma ray radiation in 1973, many more reports of such bursts have been published. Numerous artificial satellites have been equipped with gamma ray detectors including GRO. Unfortunately, almost no progress has been made in identifying the sources of this high energy radiation. Only one visible counterpart is known. It is suspected that this is a consequence of the methods currently used to define gamma ray burst source 'error boxes'. An alternative procedure was proposed in 1988 by Taff. Herein, Monte Carlo simulations are reported of the efficacy of this technique using realistic burst timing uncertainties and satellite location errors as well as a variety of satellite constellations. Since these are controlled numerical experiments, the dependence is examined of the statistics of the errors in the deduced burst wavefront normal as a function of the timing inconsistencies, detector location standard deviations, and especially the number and distribution of the detectors. The results clearly show that an arc minute prediction of a unique burst location is routinely obtainable once there are at least two interplanetary detectors
Advanced composites wing study program. Volume 1: Executive summary
The effort necessary to achieve a state of production readiness for the design and manufacturing of advanced composite wing structure is outlined. Technical assessment and program options are also reviewed for the wing study results
The Laser Astrometric Test of Relativity: Science, Technology, and Mission Design
The Laser Astrometric Test of Relativity (LATOR) experiment is designed to
explore general theory of relativity in the close proximity to the Sun -- the
most intense gravitational environment in the solar system. Using independent
time-series of highly accurate measurements of the Shapiro time-delay
(interplanetary laser ranging accurate to 3 mm at 2 AU) and interferometric
astrometry (accurate to 0.01 picoradian), LATOR will measure gravitational
deflection of light by the solar gravity with accuracy of 1 part in a billion
-- a factor ~30,000 better than currently available. LATOR will perform series
of highly-accurate tests in its search for cosmological remnants of scalar
field in the solar system. We present science, technology and mission design
for the LATOR mission.Comment: 12 pages, 4 figures. To appear in the proceedings of the
International Workshop "From Quantum to Cosmos: Fundamental Physics Research
in Space", 21-24 May 2006, Warrenton, Virginia, USA
http://physics.jpl.nasa.gov/quantum-to-cosmos
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