29,386 research outputs found
TCP/IP Protocol-Based Model for Increasing the Efficiency of Data Transfer in Computer Networks
Lagrangian Refined Kolmogorov Similarity Hypothesis for Gradient Time-evolution in Turbulent Flows
We study the time evolution of velocity and pressure gradients in isotropic
turbulence, by quantifying their decorrelation time scales as one follows fluid
particles in the flow. The Lagrangian analysis uses data in a public database
generated using direct numerical simulation of the Naiver-Stokes equations, at
a Reynolds number 430. It is confirmed that when averaging over the entire
domain, correlation functions decay on timescales on the order of the mean
Kolmogorov turnover time scale, computed from the globally averaged rate of
dissipation and viscosity. However, when performing the analysis in different
subregions of the flow, turbulence intermittency leads to large spatial
variability in the decay time scales. Remarkably, excellent collapse of the
auto-correlation functions is recovered when using the `local Kolmogorov
time-scale' defined using the locally averaged, rather than the global,
dissipation-rate. This provides new evidence for the validity of Kolmogorov's
Refined Similarity Hypothesis, but from a Lagrangian viewpoint that provides a
natural frame to describe the dynamical time evolution of turbulence.Comment: 4 Pages, 4 figure
Volume and surface propellant heating in an electrothermal radio-frequency plasma micro-thruster
The temporal evolution of neutral gas temperature over the first 5 min of operation for an electrothermal radio-frequency micro-thruster with nitrogen (N2) propellant was measured using rovibrational band matching of the second positive N2 system. Three distinct periods of gas heating were identified with time constants of τ 1 = 8 × 10⁻⁵ s, τ 2 = 8 s, and τ 3 = 100 s. The fast heating (τ 1) is attributed to volumetric heating processes within the discharge driven by ion-neutral collisions. The slow heating (τ 3) is from ion neutralization and vibrational de-excitation on the walls creating wall heating. The intermediate heating mechanism (τ 2) is yet to be fully identified although some theories are suggested.This research was partially funded by the Australian
Space Research Program (APT project) and the Australian
Research Council Discovery Project (No. DP140100571)
A TiO study of the black-hole binary GRO J0422+32 in a very low state
We present 53 simultaneous photometric (I band) and spectroscopic (6900-9500
Angstroms) observations of J0422+32, taken during December 1997. From these we
determine that J0422+32 was in its lowest state yet observed, at
I=20.44+/-0.08. Using relative spectrophotometry, we show that it is possible
to correct very accurately for telluric absorption. Following this, we use the
TiO bands at 7055 Angstroms and 7589 Angstroms for a radial velocity study and
thereby obtain a semi-amplitude of 378+/-16kms-1, which yields
f(M)=1.191+/-0.021M_solar and q=9.0+2.2-2.7, consistent with previous
observations. We further demonstrate that this little explored method is very
powerful for such systems. We also determine a new orbital ephemeris of
HJD=2450274.4156+/-0.0009 + 0.2121600+/-0.0000002E.
We see some evidence for an ellipsoidal modulation, from which we determine
the orbital inclination of J0422+32 to be less than 45 degrees. We therefore
calculate a minimum mass for the primary of 2.22M_solar, consistent with a
black hole, but not necessarily the super-massive one proposed by Beekman et al
(1997). We obtain an M4-5 spectral type for the secondary star and determine
that the secondary contributes 38+/-2% of the flux that we observe from
J0422+32 over the range 6950-8400 Angstroms. From this we calculate the
distance to the system to be 1.39+/-0.15kpc.Comment: (1) Department of Physics, Keele University, Keele, Staffordshire,
ST5 5BG (2) Department of Astrophysics, Nuclear Physics Laboratory, Keble
Road, Oxfo rd, OX1 3RH Accepted, to appear in MNRAS 8 pages, 5 figure
Effect of magnetic field and temperature on the ferroelectric loop in MnWO4
The ferroelectric properties of MnWO4 single crystal have been investigated.
Despite a relatively low remanent polarization, we show that the sample is
ferroelectric. The shape of the ferroelectric loop of MnWO4 strongly depends on
magnetic field and temperature. While its dependence does not directly
correlate with the magnetocapacitance effect before the paraelectric
transition, the effect of magnetic field on the ferroelectric polarization loop
supports magnetoelectric coupling.Comment: 3 pages, 4 figures, first report on ferroelectric loop in MnWO
Aquila X--1: a low inclination soft X-ray transient
We have obtained I-band photometry of the neutron star X-ray transient Aql
X--1 during quiescence. We find a periodicity at 2.487 cd-1, which we interpret
as twice the orbital frequency (19.30+/-0.05 h). Folding the data on the
orbital period, we model the light curve variations as the ellipsoidal
modulation of the secondary star. We determine the binary inclination to be
20--31 degrees (90 per cent confidence) and also 95 per cent upper limits to
the radial velocity semi-amplitude and rotational broadening of the secondary
star to be 117 kms-1 and 50 kms-1 respectively.Comment: 4 pages text, 3 figures, to appear in MNRA
Volume of the set of unistochastic matrices of order 3 and the mean Jarlskog invariant
A bistochastic matrix B of size N is called unistochastic if there exists a
unitary U such that B_ij=|U_{ij}|^{2} for i,j=1,...,N. The set U_3 of all
unistochastic matrices of order N=3 forms a proper subset of the Birkhoff
polytope, which contains all bistochastic (doubly stochastic) matrices. We
compute the volume of the set U_3 with respect to the flat (Lebesgue) measure
and analytically evaluate the mean entropy of an unistochastic matrix of this
order. We also analyze the Jarlskog invariant J, defined for any unitary matrix
of order three, and derive its probability distribution for the ensemble of
matrices distributed with respect to the Haar measure on U(3) and for the
ensemble which generates the flat measure on the set of unistochastic matrices.
For both measures the probability of finding |J| smaller than the value
observed for the CKM matrix, which describes the violation of the CP parity, is
shown to be small. Similar statistical reasoning may also be applied to the MNS
matrix, which plays role in describing the neutrino oscillations. Some
conjectures are made concerning analogous probability measures in the space of
unitary matrices in higher dimensions.Comment: 33 pages, 6 figures version 2 - misprints corrected, explicit
formulae for phases provide
A common distributed language approach to software integration
An important objective in software integration is the development of techniques to allow programs written in different languages to function together. Several approaches are discussed toward achieving this objective and the Common Distributed Language Approach is presented as the approach of choice
Blade-mounted trailing edge flap control for BVI noise reduction
Numerical procedures based on the 2-D and 3-D full potential equations and the 2-D Navier-Stokes equations were developed to study the effects of leading and trailing edge flap motions on the aerodynamics of parallel airfoil-vortex interactions and on the aerodynamics and acoustics of the more general self-generated rotor blade vortex interactions (BVI). For subcritical interactions, the 2-D results indicate that the trailing edge flap can be used to alleviate the impulsive loads experienced by the airfoil. For supercritical interactions, the results show the necessity of using a leading edge flap, rather than a trailing edge flap, to alleviate the interaction. Results for various time dependent flap motions and their effect on the predicted temporal sectional loads, differential pressures, and the free vortex trajectories are presented. For the OLS model rotor, contours of a BVI noise metric were used to quantify the effects of the trailing edge flap on the size and directivity of the high/low intensity noise region(s). Average reductions in the BVI noise levels on the order of 5 dB with moderate power penalties on the order of 18 pct. for a four bladed rotor and 58 pct. for a two bladed rotor were obtained
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