5,434 research outputs found
Wi-Fi Sensing Algorithms Utilizing Zigbee RF Receiver for Use in Emergency Communications Mesh
This thesis examines a low-power Wi-Fi sensing wake-up controller for an emergency communications mesh network; the goal of the research study is to progressively develop a prototype system that could be used in a live environment. Wireless network protocols are reviewed, in addition to a limited view of cluster analysis, in order to introduce relevant receiver concepts crucial to understanding this study. Algorithms for system implementation are developed, and pseudocode, designed to be configurable and platform-independent, is given for each. The system’s design goals are identified, followed by a discussion on approaches and optimizations in order to maximize the system’s usefulness. An example hardware configuration is given, in conjunction with an analysis of benefits and a discussion of drawbacks for several design options. Finally, the prototype is tested according to design goals in order to establish its feasibility. The results demonstrate that the prototype meets the proposed design goals. The implications of these findings include low power optimization for wireless technologies and machine learning techniques for wireless detection
The visual binary AG Tri in Pictoris Association: can a debris disc cause very different rotation periods of its components?
We measure the photometric rotation periods of the components of multiple
systems in young stellar associations to investigate the causes of the observed
rotation period dispersion. We present the case of the wide binary AG Tri in
the 23-Myr young beta Pictoris Association consisting of K4 + M1 dwarfs. Our
multi-band, multi-season photometric monitoring allowed us to measure the
rotation periods of both components P_A = 12.4d and P_B = 4.66d, to detect a
prominent magnetic activity in the photosphere, likely responsible for the
measured radial velocity variations, and for the first time, a flare event on
the M1 component AG Tri B. We investigate either the possibility that the
faster rotating component may have suffered an enhanced primordial disc
dispersal, starting its PMS spin-up earlier than the slower rotating component,
or the possibility that the formation of a debris disc may have prevented AG
Tri A from gaining part of the angular momentum from the accreting disc.Comment: 28 pages, 7 figures, accepted for publication in Astrophysics and
Space Science 2015, (ASTR-D-15-00445R2
Raw and Count Data Comparability of Hip-Worn ActiGraph GT3X+ and Link Accelerometers
To enable inter- and intrastudy comparisons it is important to ascertain comparability among accelerometer models.
Purpose: The purpose of this study was to compare raw and count data between hip-worn ActiGraph GT3X+ and GT9X Link accelerometers.
Methods: Adults (n = 26 (n = 15 women); age, 49.1 T 20.0 yr) wore GT3X+ and Link accelerometers over the right hip for an 80-min protocol involving 12–21 sedentary, household, and ambulatory/exercise activities lasting 2–15 min each. For each accelerometer, mean and variance of the raw (60 Hz) data for each axis and vector magnitude (VM) were extracted in 30-s epochs. A machine learning model (Montoye 2015) was used to predict energy expenditure in METs from the raw data. Raw data were also processed into activity counts in 30-s epochs for each axis and VM, with Freedson 1998 and 2011 count-based regression models used to predictMETs. Time spent in sedentary, light, moderate, and vigorous intensities was derived from predicted METs from each model. Correlations were calculated to compare raw and count data between accelerometers, and percent agreement was used to compare epoch-by-epoch activity intensity.
Results: For raw data, correlations for mean acceleration were 0.96 T 0.05, 0.89 T 0.16, 0.71 T 0.33, and 0.80 T 0.28, and those for variance were 0.98 T 0.02, 0.98 T 0.03, 0.91 T 0.06, and 1.00 T 0.00 in the X, Y, and Z axes and VM, respectively. For count data, corresponding correlations were 1.00 T 0.01, 0.98 T 0.02, 0.96 T 0.04, and 1.00 T 0.00, respectively. Freedson 1998 and 2011 count-based models had significantly higher percent agreement for activity intensity (95.1% T 5.6% and 95.5% T 4.0%) compared with theMontoye 2015 raw data model (61.5% T 27.6%; P G 0.001).
Conclusions: Count data were more highly comparable than raw data between accelerometers. Data filtering and/or more robust raw data models are needed to improve raw data comparability between ActiGraph GT3X+ and Link accelerometers
Hamiltonian structure for dispersive and dissipative dynamical systems
We develop a Hamiltonian theory of a time dispersive and dissipative
inhomogeneous medium, as described by a linear response equation respecting
causality and power dissipation. The proposed Hamiltonian couples the given
system to auxiliary fields, in the universal form of a so-called canonical heat
bath. After integrating out the heat bath the original dissipative evolution is
exactly reproduced. Furthermore, we show that the dynamics associated to a
minimal Hamiltonian are essentially unique, up to a natural class of
isomorphisms. Using this formalism, we obtain closed form expressions for the
energy density, energy flux, momentum density, and stress tensor involving the
auxiliary fields, from which we derive an approximate, ``Brillouin-type,''
formula for the time averaged energy density and stress tensor associated to an
almost mono-chromatic wave.Comment: 68 pages, 1 figure; introduction revised, typos correcte
Vortices and 2D bosons: A Path-Integral Monte Carlo Study
The vortex system in a high-T_c superconductor has been studied numerically
using the mapping to 2D bosons and the path-integral Monte Carlo method. We
find a single first-order transition from an Abrikosov lattice to an entangled
vortex liquid. The transition is characterized by an entropy jump dS = 0.4 k_B
per vortex and layer (parameters for YBCO) and a Lindemann number c_L = 0.25.
The increase in density at melting is given by d\rho = 6.0*10^{-4} /
\lambda(T)^2. The vortex liquid corresponds to a bosonic superfluid, with
\rho_s = \rho even in the limit \lambda -> \infty.Comment: 9 pages, RevTeX, 4 PostScript figures. The entropy jump at the
transition has been recomputed and is now in agreement with experiments on
YBCO. Some minor modifications were made in the tex
Fractal Scales in a Schwarzschild Atmosphere
Recently, Glass and Krisch have extended the Vaidya radiating metric to
include both a radiation fluid and a string fluid [1999 Class. Quantum Grav.
vol 16, 1175]. Mass diffusion in the extended Schwarzschild atmosphere was
studied. The continuous solutions of classical diffusive transport are believed
to describe the envelope of underlying fractal behavior. In this work we
examine the classical picture at scales on which fractal behavior might be
evident.Comment: to appear in Class. Quantum Gra
Light Induced Melting of Colloidal Crystals in Two Dimensions
We demonstrate that particles confined to two dimensions (2d) and subjected
to a one-dimensional (1d) periodic potential exhibit a rich phase diagram, with
both ``locked floating solids'' and smectic phases. The resulting phases and
phase transitions are studied as a function of temperature and potential
strength. We find reentrant melting as a function of the potential strength.
Our results lead to universal predictions consistent with recent experiments on
2d colloids in the presence of a laser-induced 1d periodic potential.Comment: 4 pages, 3 figures, also available at http://cmtw.harvard.edu/~fre
Simulations of Direct Collisions of Gas Clouds with the Central Black Hole
We perform numerical simulations of clouds in the Galactic Centre (GC)
engulfing the nuclear super-massive black hole and show that this mechanism
leads to the formation of gaseous accretion discs with properties that are
similar to the expected gaseous progenitor discs that fragmented into the
observed stellar disc in the GC. As soon as the cloud hits the black hole, gas
with opposite angular momentum relative to the black hole collides downstream.
This process leads to redistribution of angular momentum and dissipation of
kinetic energy, resulting in a compact gaseous accretion disc. A parameter
study using thirteen high resolution simulations of homogeneous clouds falling
onto the black hole and engulfing it in parts demonstrates that this mechanism
is able to produce gaseous accretion discs that could potentially be the
progenitor of the observed stellar disc in the GC. A comparison of simulations
with different equations of state (adiabatic, isothermal and full cooling)
demonstrates the importance of including a detailed thermodynamical
description. However the simple isothermal approach already yields good results
on the radial mass transfer and accretion rates, as well as disc eccentricities
and sizes. We find that the cloud impact parameter strongly influences the
accretion rate whereas the impact velocity has a small affect on the accretion
rate.Comment: 21 pages, 18 figures, Accepted for publication in MNRA
Buckling Instabilities of a Confined Colloid Crystal Layer
A model predicting the structure of repulsive, spherically symmetric,
monodisperse particles confined between two walls is presented. We study the
buckling transition of a single flat layer as the double layer state develops.
Experimental realizations of this model are suspensions of stabilized colloidal
particles squeezed between glass plates. By expanding the thermodynamic
potential about a flat state of confined colloidal particles, we derive
a free energy as a functional of in-plane and out-of-plane displacements. The
wavevectors of these first buckling instabilities correspond to three different
ordered structures. Landau theory predicts that the symmetry of these phases
allows for second order phase transitions. This possibility exists even in the
presence of gravity or plate asymmetry. These transitions lead to critical
behavior and phases with the symmetry of the three-state and four-state Potts
models, the X-Y model with 6-fold anisotropy, and the Heisenberg model with
cubic interactions. Experimental detection of these structures is discussed.Comment: 24 pages, 8 figures on request. EF508
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