554,493 research outputs found
Particle Resuspension in Turbulent Boundary Layers and the Influence of Non-Gaussian Removal Forces
The work described is concerned with the way micron-size particles attached
to a surface are resuspended when exposed to a turbulent flow. An improved
version of the Rock'n'Roll model (Reeks and Hall, 2001) is developed where this
model employs a stochastic approach to resuspension involving the rocking and
rolling of a particle about surface asperities arising from the moments of the
fluctuating drag forces acting on the particle close to the surface. In this
work, the model is improved by using values of both the streamwise fluid
velocity andacceleration close to the wall obtained from Direct Numerical
Simulation (DNS) of turbulentchannel flow. Using analysis and numerical
calculations of the drag force on a sphere near a wall in shear flow (O'Neill
(1968) and Lee and Balachandar (2010)) these values are used to obtain the
joint distribution of the moments of the fluctuating drag force and its time
derivative acting on a particle attached to a surface. In so doing the
influence of highly non-Gaussian forces (associated with the sweeping and
ejection events in a turbulent boundary layer) on short and long term
resuspension rates is examined for a sparse monolayer coverage of particles,
along with the dependence of the resuspension upon the timescale of the
particle motion attached to the surface, the ratio of the rms/ mean of the
removal force and the distribution of adhesive forces. Model predictions of the
fraction resuspended are compared with experimental results.Comment: 31 pages 21 figure
Mass neutrino flavor evolution in spacetime with torsion
In the framework of the spacetime with torsion, we obtain the flavor
evolution equation of the mass neutrino oscillation in vacuum. A comparison
with the result of general relativity case, it shows that the flavor
evolutionary equations in Riemann spacetime and Weitzenb\"ock spacetimes are
equivalent in the spherical symmetric Schwarzschild spacetime, but turns out to
be different in the case of the axial symmetry.Comment: 8 papes, no fiur
Helicity Observation of Weak and Strong Fields
We report in this letter our analysis of a large sample of photospheric
vector magnetic field measurements. Our sample consists of 17200 vector
magnetograms obtained from January 1997 to August 2004 by Huairou Solar
Observing Station of the Chinese National Astronomical Observatory. Two
physical quantities, and current helicity, are calculated and their
signs and amplitudes are studied in a search for solar cycle variations.
Different from other studies of the same type, we calculate these quantities
for weak () fields separately. For
weak fields, we find that the signs of both and current helicity are
consistent with the established hemispheric rule during most years of the solar
cycle and their magnitudes show a rough tendency of decreasing with the
development of solar cycle. Analysis of strong fields gives an interesting
result: Both and current helicity present a sign opposite to that of
weak fields. Implications of these observations on dynamo theory and helicity
production are also briefly discussed.Comment: accepted for publication in ApJ Lette
- …