3,528 research outputs found
Development and fabrication of an advanced liquid cooling garment
The elastomeric film fin/tube concept which was developed is a composite of polyurethane film, fine expanded silver mesh, a serpentine pattern polyurethane transport tubing and an integral comfort liner, all bonded via adhesive application and vacuum-bagged for final cure. As demonstrated by thermal analysis, the composite garment material is capable of removing a 293 watt (1000 BTU/hr) metabolic load through a head and torso cooling area of .46 sq m (5 sq ft) with tube spacing of slightly under one inch. A total of 60 test elements, each .15m x .15m (6 in. x 6 in.) were fabricated in support of the liquid cooling garment concept development. In parallel with the fabrication of these elements a continuing series of laboratory tests to support the fabrication techniques was carried out. The elements and supporting tests are described
Decentralized Constraint Satisfaction
We show that several important resource allocation problems in wireless
networks fit within the common framework of Constraint Satisfaction Problems
(CSPs). Inspired by the requirements of these applications, where variables are
located at distinct network devices that may not be able to communicate but may
interfere, we define natural criteria that a CSP solver must possess in order
to be practical. We term these algorithms decentralized CSP solvers. The best
known CSP solvers were designed for centralized problems and do not meet these
criteria. We introduce a stochastic decentralized CSP solver and prove that it
will find a solution in almost surely finite time, should one exist, also
showing it has many practically desirable properties. We benchmark the
algorithm's performance on a well-studied class of CSPs, random k-SAT,
illustrating that the time the algorithm takes to find a satisfying assignment
is competitive with stochastic centralized solvers on problems with order a
thousand variables despite its decentralized nature. We demonstrate the
solver's practical utility for the problems that motivated its introduction by
using it to find a non-interfering channel allocation for a network formed from
data from downtown Manhattan
The influence of free-stream turbulence on separation of turbulent boundary layers in incompressible, two-dimensional flow
Experiments were conducted to determine if free-stream turbulence scale affects separation of turbulent boundary layers. In consideration of possible interrelation between scale and intensity of turbulence, the latter characteristic also was varied and its role was evaluated. Flow over a 2-dimensional airfoil in a subsonic wind tunnel was studied with the aid of hot-wire anemometry, liquid-film flow visualization, a Preston tube, and static pressure measurements. Profiles of velocity, relative turbulence intensity, and integral scale in the boundary layer were measured. Detachment boundary was determined for various angles of attack and free-stream turbulence. The free-stream turbulence intensity and scale were found to spread into the entire turbulent boundary layer, but the effect decreased as the airfoil surface was approached. When the changes in stream turbulence were such that the boundary layer velocity profiles were unchanged, detachment location was not significantly affected by the variations of intensity and scale. Pressure distribution remained the key factor in determining detachment location
Modelling 802.11 Wireless Links
Analysis of the 802.11 CSMA/CA mechanism has
received considerable attention recently. Bianchi [3] presents an
analytic model under a saturated traffic assumption. Bianchi’s
model is accurate, but typical network conditions are nonsaturated
heterogenous. We present an extension of his model
to a non-saturated environment. The model’s predictions, validated
against simulation, accurately capture many interesting
features of non-saturated operation. For example, the model
predicts that peak throughput occurs prior to saturation. Our
model also allows stations to have different traffic arrival
rates, enabling us to address the question of fairness between
competing flows
Modelling 802.11 Wireless Links
Analysis of the 802.11 CSMA/CA mechanism has
received considerable attention recently. Bianchi [3] presents an
analytic model under a saturated traffic assumption. Bianchi’s
model is accurate, but typical network conditions are nonsaturated
heterogenous. We present an extension of his model
to a non-saturated environment. The model’s predictions, validated
against simulation, accurately capture many interesting
features of non-saturated operation. For example, the model
predicts that peak throughput occurs prior to saturation. Our
model also allows stations to have different traffic arrival
rates, enabling us to address the question of fairness between
competing flows
Solution to the twin image problem in holography
While the invention of holography by Dennis Gabor truly constitutes an
ingenious concept, it has ever since been troubled by the so called twin image
problem limiting the information that can be obtained from a holographic
record. Due to symmetry reasons there are always two images appearing in the
reconstruction process. Thus, the reconstructed object is obscured by its
unwanted out of focus twin image. Especially for emission electron as well as
for x- and gamma-ray holography, where the source-object distances are small,
the reconstructed images of atoms are very close to their twin images from
which they can hardly be distinguished. In some particular instances only,
experimental efforts could remove the twin images. More recently, numerical
methods to diminish the effect of the twin image have been proposed but are
limited to purely absorbing objects failing to account for phase shifts caused
by the object. Here we show a universal method to reconstruct a hologram
completely free of twin images disturbance while no assumptions about the
object need to be imposed. Both, amplitude and true phase distributions are
retrieved without distortion
Investigating the validity of IEEE 802.11 MAC modeling hypotheses
As WLANs employing IEEE 802.11 have become pervasive, many analytic models for predicting their performance have been developed in recent years. Due to the complicated nature of the 802.11 MAC operation, approximations must be made to enable tractable mathematical models. In this article, through simulation we investigate the veracity of the approximations shared by many models that have been developed starting with the fundamental hypotheses in Bianchipsilas (1998 and 2000) seminal papers. We find that even for small numbers of station these assumptions that hold true for saturated stations (those that always have a packet to send) and for unsaturated stations with small buffers. However, despite their widespread adoption, we find that the commonly adopted assumptions that are used to incorporate station buffers are not appropriate. This raises questions about the predictive power of models based on these hypotheses
Stress corrosion in titanium alloys and other metallic materials
Multiple physical and chemical techniques including mass spectroscopy, atomic absorption spectroscopy, gas chromatography, electron microscopy, optical microscopy, electronic spectroscopy for chemical analysis (ESCA), infrared spectroscopy, nuclear magnetic resonance (NMR), X-ray analysis, conductivity, and isotopic labeling were used in investigating the atomic interactions between organic environments and titanium and titanium oxide surfaces. Key anhydrous environments studied included alcohols, which contain hydrogen; carbon tetrachloride, which does not contain hydrogen; and mixtures of alcohols and halocarbons. Effects of dissolved salts in alcohols were also studied. This program emphasized experiments designed to delineate the conditions necessary rather than sufficient for initiation processes and for propagation processes in Ti SCC
Interference in Exclusive Vector Meson Production in Heavy Ion Collisions
Photons emitted from the electromagnetic fields of relativistic heavy ions
can fluctuate into quark anti-quark pairs and scatter from a target nucleus,
emerging as vector mesons. These coherent interactions are identifiable by
final states consisting of the two nuclei and a vector meson with a small
transverse momentum. The emitters and targets can switch roles, and the two
possibilities are indistinguishable, so interference may occur. Vector mesons
are negative parity so the amplitudes have opposite signs. When the meson
transverse wavelength is larger than the impact parameter, the interference is
large and destructive.
The short-lived vector mesons decay before amplitudes from the two sources
can overlap, and so cannot interfere directly. However, the decay products are
emitted in an entangled state, and the interference depends on observing the
complete final state. The non-local wave function is an example of the
Einstein-Podolsky-Rosen paradox.Comment: 13 pages with 3 figures; submitted to Physical Review Letter
Decentralised Learning MACs for Collision-free Access in WLANs
By combining the features of CSMA and TDMA, fully decentralised WLAN MAC
schemes have recently been proposed that converge to collision-free schedules.
In this paper we describe a MAC with optimal long-run throughput that is almost
decentralised. We then design two \changed{schemes} that are practically
realisable, decentralised approximations of this optimal scheme and operate
with different amounts of sensing information. We achieve this by (1)
introducing learning algorithms that can substantially speed up convergence to
collision free operation; (2) developing a decentralised schedule length
adaptation scheme that provides long-run fair (uniform) access to the medium
while maintaining collision-free access for arbitrary numbers of stations
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