1,807 research outputs found
Maximizing phonon thermal conductance for ballistic membranes
At low temperatures, phonon scattering can become so weak that phonon
transport becomes ballistic. We calculate the ballistic phonon conductance G
for membranes using elasticity theory, considering the transition from three to
two dimensions. We discuss the temperature and thickness dependence and
especially concentrate on the issue of material parameters. For all membrane
thicknesses, the best conductors have, counter-intuitively, the lowest speed of
sound.Comment: 4 pages, 4 figures, proceedings to phonons 2007 conferenc
Fall Migration of Ringed Seals (Phoca hispida) through the Beaufort and Chukchi Seas, 2001â02
In a study examining the range, distribution, and habitat use of the ringed seal, Phoca hispida, in Canadaâs Western Arctic, eight ringed seals were live-captured, instrumented with satellite-linked (SLTDR-16) transmitters, and released at Cape Parry, Northwest Territories, Canada, on 17 â 19 September 2001 and 7 â 8 September 2002. Locations accepted by the filtering process were received from seven of the eight tagged seals (5 subadults, 1 adult female, 1 pup) over periods ranging from 35 to 207 days (mean 99 d, SD 66). Mean rates of travel were 0.91 m/s (SE 0.011, n = 7) in the Canadian Beaufort Sea, 0.92 m/s (SE 0.014, n = 7) offshore of Alaskaâs North Slope, and 0.79 m/s (SE 0.008, n = 5) in the Chukchi Sea. On average, the seals took 32 days (range 19 â 56 d) to migrate between Cape Parry and Point Barrow, almost always remaining within 100 km of shore and over the continental shelf or slope, and covering an average migration distance of 2138 km. Dive depths for all groupings were mainly in the 4 â 80 m range (adult female: 63 â 73%; subadults: 54 â 73%; pup: 64 â 82%), with only the adult female diving deeper than 80 m on occasion, mainly in the Canadian Beaufort Sea (15.1% of her dives). The subadults and pup dove mainly for more than 1 to 5 min (60% and 55%, respectively), while a large proportion of the adult femaleâs dives were longer (34% for > 1 to 5 min; 31% for > 5 to 8 min; 5.4% for > 8 min). The tracks of the westward migrating seals revealed a routing through three political jurisdictions (including oil and gas industry lease areas in all three) over a period of about two months. This pattern highlights the importance of cooperation between the United States, Canada, and Russia in managing this species.Du 17 au 19 septembre 2001 et les 7 et 8 septembre 2002, dans le cadre dâune Ă©tude portant sur le parcours, la rĂ©partition et lâutilisation de lâhabitat du phoque annelĂ©, Phoca hispida, dans lâouest de lâArctique canadien, huit phoques annelĂ©s ont Ă©tĂ© capturĂ©s en vie, dotĂ©s de transmetteurs en liaison avec un satellite (SLTDR-16), puis relĂąchĂ©s au cap Parry, dans les Territoires du Nord-Ouest, au Canada. Des emplacements acceptĂ©s par filtrage ont Ă©tĂ© reçus de la part de sept des huit phoques marquĂ©s (5 prĂ©reproducteurs, 1 femelle adulte, 1 jeune) sur des pĂ©riodes variant entre 35 et 207 jours (moyenne 99 d, SD 66). Les taux de dĂ©placement moyens Ă©taient de 0,91 m/s (SE 0,011; n = 7) dans la mer canadienne de Beaufort, de 0,92 m/s (SE 0,014; n = 7) au large du versant nord de lâAlaska et de 0,79 m/s (SE 0,008; n = 5) dans la mer des Tchouktches. En moyenne, la migration des phoques entre le cap Parry et la pointe Barrow durait 32 jours (Ă©cart de 19 Ă 56 d), et les phoques restaient presque toujours en-dedans de 100 km de la cĂŽte et au-dessus de la pente ou du plateau continental. Ils couvraient en moyenne une distance de migration de 2 138 km. La profondeur des plongĂ©es de tous les groupements variait entre 4 et 80 m (femelle adulte : 63 % Ă 73 %; prĂ©reproducteurs : 54 % Ă 73 %; jeune : 64 % Ă 82 %), et seulement la femelle adulte plongeait Ă plus de 80 m de profondeur Ă lâoccasion, surtout dans la mer canadienne de Beaufort (15,1 % de ses plongĂ©es). Les prĂ©reproÂducteurs et le jeune phoque Ă©taient principalement en plongĂ©e pendant plus de 1 Ă 5 min (60 % et 55 %, respectivement), tandis quâune grande proportion des plongĂ©es de la femelle adulte durait plus longtemps (34 % pour > 1 Ă 5 min; 31 % pour > 5 Ă 8 min; 5,4 % pour > 8 min). Les pistes des phoques en migration vers lâouest rĂ©vĂ©laient des itinĂ©raires passant par trois compĂ©tences politiques (comprenant des concessions de lâindustrie pĂ©troliĂšre et gaziĂšre dans les trois cas) sur une pĂ©riode dâenviron deux mois. Cette tendance fait ressortir lâimportance dâune collaboration entre les Ătats-Unis, le Canada et la Russie en matiĂšre de gestion de cette espĂšce
Eddy Current Signal Calculations for Surface Breaking Cracks
This paper contains a brief status report on analytical modeling of the probe-flaw interactions for surface breaking cracks and some data on comparisons of theory and experiment for EDM notches and true fatigue cracks. The goal of the work reported here and in companion papers by Rummel and Rathke (1984), Auld, et al. (1984), and Martinez and Bahr (1984) is to improve the quantitative character of eddy current testing. In this joint effort, the role of probe-flaw interaction modeling is to provide engineering tools not previously available for: (1) setting design guidelines to optimize sensitivity and spatial resolution, (2) permitting analytic extrapolation of measured flaw response data, (3) defining a test basis for monitoring probe calibration, and (4) establishing a rational inversion procedure based on multifrequency measurements and the shape signature of a scanned flaw signal as a function of position
Scattering loss in electro-optic particulate composite materials
The effective permittivity dyadic of a composite material containing
particulate constituent materials with one constituent having the ability to
display the Pockels effect is computed, using an extended version of the
strong-permittivity-fluctuation theory which takes account of both the
distributional statistics of the constituent particles and their sizes.
Scattering loss, thereby incorporated in the effective electromagnetic response
of the homogenized composite material, is significantly affected by the
application of a low-frequency (dc) electric field
Possible impacts of climate change on freezing rain in south-central Canada using downscaled future climate scenarios
Freezing rain is a major atmospheric hazard in mid-latitude nations of the globe. Among all Canadian hydrometeorological hazards, freezing rain is associated with the highest damage costs per event. Using synoptic weather typing to identify the occurrence of freezing rain events, this study estimates changes in future freezing rain events under future climate scenarios for south-central Canada. Synoptic weather typing consists of principal components analysis, an average linkage clustering procedure (i.e., a hierarchical agglomerative cluster method), and discriminant function analysis (a nonhierarchical method). Meteorological data used in the analysis included hourly surface observations from 15 selected weather stations and six atmospheric levels of six-hourly National Centers for Environmental Prediction (NCEP) upper-air reanalysis weather variables for the winter months (November–April) of 1958/59–2000/01. A statistical downscaling method was used to downscale four general circulation model (GCM) scenarios to the selected weather stations. Using downscaled scenarios, discriminant function analysis was used to project the occurrence of future weather types. The within-type frequency of future freezing rain events is assumed to be directly proportional to the change in frequency of future freezing rain-related weather types <br><br> The results showed that with warming temperatures in a future climate, percentage increases in the occurrence of freezing rain events in the north of the study area are likely to be greater than those in the south. By the 2050s, freezing rain events for the three colder months (December–February) could increase by about 85% (95% confidence interval – CI: ±13%), 60% (95% CI: ±9%), and 40% (95% CI: ±6%) in northern Ontario, eastern Ontario (including Montreal, Quebec), and southern Ontario, respectively. The increase by the 2080s could be even greater: about 135% (95% CI: ±20%), 95% (95% CI: ±13%), and 45% (95% CI: ±9%). For the three warmer months (November, March, April), the percentage increases in future freezing rain events are projected to be much smaller with some areas showing either a decrease or little change in frequency of freezing rain. On average, northern Ontario could experience about 10% (95% CI: ±2%) and 20% (95% CI: ±4%) more freezing rain events by the 2050s and 2080s, respectively. However, future freezing rain events in southern Ontario could decrease about 10% (95% CI: ±3%) and 15% (95% CI: ±5%) by the 2050s and 2080s, respectively. In eastern Ontario (including Montreal, Quebec), the frequency of future freezing rain events is projected to remain the same as it is currently
Surface Flaw Detection with Ferromagnetic Resonance Probes
Small ferromagnetic resonators have been shown to provide effective electromagnetic detectors for surface flaws in magnetic and nonmagnetic metals. As such a resonator is moved along \u27the surface of a test piece it experiences a frequency shift when it passes over a flaw. lwo detection mechanisms are present: (1) an eddy current effect (2) a perturbation of the dc magnetic bias field used to tune the resonator. Results are given for experiments performed on machined slots in aluminum, titanium and steel and on tightly closed fatigue cracks in titanium. Results are also presented for some measurements on titanium aircraft fasteners
Characteristics of phonon transmission across epitaxial interfaces: a lattice dynamic study
Phonon transmission across epitaxial interfaces is studied within the lattice
dynamic approach. The transmission shows weak dependence on frequency for the
lattice wave with a fixed angle of incidence. The dependence on azimuth angle
is found to be related to the symmetry of the boundary interface. The
transmission varies smoothly with the change of the incident angle. A critical
angle of incidence exists when the phonon is incident from the side with large
group velocities to the side with low ones. No significant mode conversion is
observed among different acoustic wave branches at the interface, except when
the incident angle is near the critical value. Our theoretical result of the
Kapitza conductance across the Si-Ge (100) interface at temperature
K is 4.6\times10^{8} {\rm WK}^{-1}{\rmm}^{-2}. A scaling law at low temperature is also reported. Based on the features of
transmission obtained within lattice dynamic approach, we propose a simplified
formula for thermal conductanceacross the epitaxial interface. A reasonable
consistency is found between the calculated values and the experimentally
measured ones.Comment: 8 figure
Improved Probe-Flaw Interaction Modeling, Inversion Processing, and Surface Roughness Clutter
In Reference 1 a first comparison was made of measured eddy current signals with calculations based on nonuniform probe-field interaction theory. These calculations followed the basic analysis developed in Reference 2. They used interrogating field distributions calculated by Dodd and Deeds theory for the air core coils of Reference 3. (Note that Fig. 6 in Reference 1 and Fig. 7 in Reference 3 should be interchanged). In Reference 1 theoretical and experimental plots of the flaw profile curve (a plot of âłZ versus distance along the mouth of a surface breaking flaw) were found to be in good agreement, with regard to shape, for several selected EDM notch samples in aluminum. An iterative procedure was also developed for systematically varying the length, depth, and opening width to obtain a best fit to the experimental data.4 In the present paper a full inversion procedure is developed and illustrated for approximately rectangular-shaped EDM notches. The mathematical structure of the inversion problem is first examined and a solution is proposed. Physical reasoning, based on the form of the flaw profile curves, is then used to simplify the approach and to provide guidance in selection of the most suitable probe geometry. Other topics briefly addressed include, possible improvements in the theory for the region with a/§ close to unity and for more realistic flaw shapes (i.e., semi-elliptical, rather than rectangular), inaccuracies due to errors in the probe scan path, and background clutter due to surface roughness, machining marks, and micro-structure
Quantum chaos in nanoelectromechanical systems
We present a theoretical study of the electron-phonon coupling in suspended
nanoelectromechanical systems (NEMS) and investigate the resulting quantum
chaotic behavior. The phonons are associated with the vibrational modes of a
suspended rectangular dielectric plate, with free or clamped boundary
conditions, whereas the electrons are confined to a large quantum dot (QD) on
the plate's surface. The deformation potential and piezoelectric interactions
are considered. By performing standard energy-level statistics we demonstrate
that the spectral fluctuations exhibit the same distributions as those of the
Gaussian Orthogonal Ensemble (GOE) or the Gaussian Unitary Ensemble (GUE),
therefore evidencing the emergence of quantum chaos. That is verified for a
large range of material and geometry parameters. In particular, the GUE
statistics occurs only in the case of a circular QD. It represents an anomalous
phenomenon, previously reported for just a small number of systems, since the
problem is time-reversal invariant. The obtained results are explained through
a detailed analysis of the Hamiltonian matrix structure.Comment: 14 pages, two column
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