210 research outputs found
A Comparison of Optimized Link State Routing with Traditional Ad-hoc Routing Protocols
The performance of mobile ad-hoc networks
(MANET) is related to the efficiency of the routing protocols in
adapting to frequently changing network topology and link
status. This paper addresses the issue by comparing the
relative performance of three key ad-hoc routing protocols:
Destination-sequenced Distance Vector (DSDV), Ad-hoc Ondemand
Distance Vector (AODV) and Optimized Link State
Routing (OLSR). The protocols are tested based on two
scenarios, namely, tactical networks for ships and sensor-based
network nodes. Four performance metrics were measured by
varying the maximum speed of mobile hosts, network size and
traffic load, to assess the routing capability and protocol
efficiency. The simulation results indicate that AODV
performs better than OSLR and DSDV in the first scenario.
Although OLSR also performed relatively well, the associated
high routing overhead is the dominant reason for not choosing
it. On the other hand, OLSR emerged as the protocol of choice
for sensor networks, where the high routing overhead is
counteracted by consistently better performance in all other
metrics. Due to the slow evolution of the sensor network
topology, OLSR performed satisfactorily for best effort traffic
but needed subtle adjustments to balance between latency and
bandwidth to meet the requirements of delay-sensitive
applications
Feelings about Math and Science: Reciprocal Determinism and Catholic School Education
Applying Bandura’s reciprocal determinism model, differences in math and science experiences influenced by individual, gender, and school variables were investigated within 1,368 elementary students who attended 21 Catholic schools. Math and Science were evaluated positively and favored more than other academic subjects. However, advantages were found for boys by lowered math anxiety levels and favoring of math, and for large schools by lowered math anxiety levels and higher student ratings of science. No advantages were found for small schools. However, school poverty rate appeared to have a confounding effect on school size. Discussion is presented pertaining to the specific need to study Catholic school systems regarding student perceptions in light of distinguishing Catholic school factors
Dynamics of confined water reconstructed from inelastic x-ray scattering measurements of bulk response functions
Nanoconfined water and surface-structured water impacts a broad range of fields. For water confined between hydrophilic surfaces, measurements and simulations have shown conflicting results ranging from “liquidlike” to “solidlike” behavior, from bulklike water viscosity to viscosity orders of magnitude higher. Here, we investigate how a homogeneous fluid behaves under nanoconfinement using its bulk response function: The Green's function of water extracted from a library of S(q,ω) inelastic x-ray scattering data is used to make femtosecond movies of nanoconfined water. Between two confining surfaces, the structure undergoes drastic changes as a function of surface separation. For surface separations of ≈9 Å, although the surface-associated hydration layers are highly deformed, they are separated by a layer of bulklike water. For separations of ≈6 Å, the two surface-associated hydration layers are forced to reconstruct into a single layer that modulates between localized “frozen’ and delocalized “melted” structures due to interference of density fields. These results potentially reconcile recent conflicting experiments. Importantly, we find a different delocalized wetting regime for nanoconfined water between surfaces with high spatial frequency charge densities, where water is organized into delocalized hydration layers instead of localized hydration shells, and are strongly resistant to `freezing' down to molecular distances (<6 Å)
Feelings about Math and Science: Reciprocal Determinism and Catholic School Education
Applying Bandura’s reciprocal determinism model, differences in math and science experiences influenced by individual, gender, and school variables were investigated within 1,368 elementary students who attended 21 Catholic schools. Math and Science were evaluated positively and favored more than other academic subjects. However, advantages were found for boys by lowered math anxiety levels and favoring of math, and for large schools by lowered math anxiety levels and higher student ratings of science. No advantages were found for small schools. However, school poverty rate appeared to have a confounding effect on school size. Discussion is presented pertaining to the specific need to study Catholic school systems regarding student perceptions in light of distinguishing Catholic school factors
State-to-state rotational transitions in H+H collisions at low temperatures
We present quantum mechanical close-coupling calculations of collisions
between two hydrogen molecules over a wide range of energies, extending from
the ultracold limit to the super-thermal region. The two most recently
published potential energy surfaces for the H-H complex, the so-called
DJ (Diep and Johnson, 2000) and BMKP (Boothroyd et al., 2002) surfaces, are
quantitatively evaluated and compared through the investigation of rotational
transitions in H+H collisions within rigid rotor approximation. The
BMKP surface is expected to be an improvement, approaching chemical accuracy,
over all conformations of the potential energy surface compared to previous
calculations of H-H interaction. We found significant differences in
rotational excitation/de-excitation cross sections computed on the two surfaces
in collisions between two para-H molecules. The discrepancy persists over a
large range of energies from the ultracold regime to thermal energies and
occurs for several low-lying initial rotational levels. Good agreement is found
with experiment (Mat\'e et al., 2005) for the lowest rotational excitation
process, but only with the use of the DJ potential. Rate coefficients computed
with the BMKP potential are an order of magnitude smaller.Comment: Accepted by J. Chem. Phy
Charge Transfer in Slow Collisions of O⁸⁺ and Ar⁸⁺ Ions with H(1s) Below 2 KeV/Amu
We calculated the charge-transfer cross sections for O⁸⁺ + H collisions for energies from 1eV/amu to 2keV/amu, using the recently developed hyperspherical close-coupling method. In particular, the discrepancy for electron capture to the n = 6 states of O⁷⁺ from the previous theoretical calculations is further analyzed. Our results indicate that at low energies (below 100eV∕amu) electron capture to the n=6 manifold of O7+ becomes dominant. The present results are used to resolve the long-standing discrepancies from the different elaborate semiclassical calculations near 100eV/amu. We have also performed the semiclassical atomic orbital close-coupling calculations with straight-line trajectories. We found the semiclassical calculations agree with the quantal approach at energy above 100eV/amu, where the collision occurs at large impact parameters. Calculations for Ar⁸⁺ + H collisions in the same energy range have also been carried out to analyze the effect of the ionic core on the subshell cross sections. By using diabatic molecular basis functions, we show that converged results can be obtained with small numbers of channels
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Isotope Effects in Complex Scattering Lengths for He Collisions With Molecular Hydrogen
We examine the effect of theoretically varying the collision-system reduced mass in collisions of He with vibrationally excited molecular hydrogen and observe zero-energy resonances for select atomic “hydrogen” masses less than 1 u or a “helium” mass of 1.95 u. Complex scattering lengths, state-to-state vibrational quenching cross sections, and a low-energy elastic scattering resonance are all studied as a function of collision-system reduced mass. Experimental observations of these phenomena in the cold and ultracold regimes for collisions of He and He with H, HD, HT, and DT should be feasible in the near future.Astronom
Close-coupling calculations of low-energy inelastic and elastic processes in He collisions with H: A comparative study of two potential energy surfaces
The two most recently published potential energy surfaces (PESs) for the
HeH complex, the so-called MR (Muchnick and Russek) and BMP (Boothroyd,
Martin, and Peterson) surfaces, are quantitatively evaluated and compared
through the investigation of atom-diatom collision processes. The BMP surface
is expected to be an improvement, approaching chemical accuracy, over all
conformations of the PES compared to that of the MR surface. We found
significant differences in inelastic rovibrational cross sections computed on
the two surfaces for processes dominated by large changes in target rotational
angular momentum. In particular, the H() total quenching cross
section computed using the BMP potential was found to be a factor of 1000
larger than that obtained with the MR surface.Comment: 26 pages, 11 figure
A large-scale R-matrix calculation for electron-impact excitation of the Ne O-like ion
The five J levels within a or ground state complex provide
an excellent testing ground for the comparison of theoretical line ratios with
astrophysically observed values, in addition to providing valuable electron
temperature and density diagnostics. The low temperature nature of the line
ratios ensure that the theoretically derived values are sensitive to the
underlying atomic structure and electron-impact excitation rates. Previous
R-matrix calculations for the Ne O-like ion exhibit large spurious
structure in the cross sections at higher electron energies, which may affect
Maxwellian averaged rates even at low temperatures. Furthermore, there is an
absence of comprehensive excitation data between the excited states that may
provide newer diagnostics to compliment the more established lines discussed in
this paper. To resolve these issues, we present both a small scale 56-level
Breit-Pauli (BP) calculation and a large-scale 554 levels R-matrix Intermediate
Coupling Frame Transformation (ICFT) calculation that extends the scope and
validity of earlier JAJOM calculations both in terms of the atomic structure
and scattering cross sections. Our results provide a comprehensive
electron-impact excitation data set for all transitions to higher shells.
The fundamental atomic data for this O-like ion is subsequently used within a
collisional radiative framework to provide the line ratios across a range of
electron temperatures and densities of interest in astrophysical observations.Comment: 17 pages, 8 figure
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