2,709 research outputs found
The effect of polydispersity in a turbulent channel flow laden with finite-size particles
We study turbulent channel flows of monodisperse and polydisperse suspensions
of finite-size spheres by means of Direct Numerical Simulations using an
immersed boundary method to account for the dispersed phase. Suspensions with 3
different Gaussian distributions of particle radii are considered (i.e. 3
different standard deviations). The distributions are centered on the reference
particle radius of the monodisperse suspension. In the most extreme case, the
radius of the largest particles is 4 times that of the smaller particles. We
consider two different solid volume fractions, 2% and 10%. We find that for all
polydisperse cases, both fluid and particles statistics are not substantially
altered with respect to those of the monodisperse case. Mean streamwise fluid
and particle velocity profiles are almost perfectly overlapping. Slightly
larger differences are found for particle velocity fluctuations. These increase
close to the wall and decrease towards the centerline as the standard deviation
of the distribution is increased. Hence, the behavior of the suspension is
mostly governed by excluded volume effects regardless of particle size
distribution (at least for the radii here studied). Due to turbulent mixing,
particles are uniformly distributed across the channel. However, smaller
particles can penetrate more into the viscous and buffer layer and velocity
fluctuations are therein altered. Non trivial results are presented for
particle-pair statistics.Comment: Under review in the European Journal of Mechanics/B - Fluid
Unsolvability Cores in Classification Problems
Classification problems have been introduced by M. Ziegler as a
generalization of promise problems. In this paper we are concerned with
solvability and unsolvability questions with respect to a given set or language
family, especially with cores of unsolvability. We generalize the results about
unsolvability cores in promise problems to classification problems. Our main
results are a characterization of unsolvability cores via cohesiveness and
existence theorems for such cores in unsolvable classification problems. In
contrast to promise problems we have to strengthen the conditions to assert the
existence of such cores. In general unsolvable classification problems with
more than two components exist, which possess no cores, even if the set family
under consideration satisfies the assumptions which are necessary to prove the
existence of cores in unsolvable promise problems. But, if one of the
components is fixed we can use the results on unsolvability cores in promise
problems, to assert the existence of such cores in general. In this case we
speak of conditional classification problems and conditional cores. The
existence of conditional cores can be related to complexity cores. Using this
connection we can prove for language families, that conditional cores with
recursive components exist, provided that this family admits an uniform
solution for the word problem
Sedimentation of finite-size spheres in quiescent and turbulent environments
Sedimentation of a dispersed solid phase is widely encountered in
applications and environmental flows, yet little is known about the behavior of
finite-size particles in homogeneous isotropic turbulence. To fill this gap, we
perform Direct Numerical Simulations of sedimentation in quiescent and
turbulent environments using an Immersed Boundary Method to account for the
dispersed rigid spherical particles. The solid volume fractions considered are
0.5-1%, while the solid to fluid density ratio 1.02. The particle radius is
chosen to be approximately 6 Komlogorov lengthscales. The results show that the
mean settling velocity is lower in an already turbulent flow than in a
quiescent fluid. The reduction with respect to a single particle in quiescent
fluid is about 12\% and 14\% for the two volume fractions investigated. The
probability density function of the particle velocity is almost Gaussian in a
turbulent flow, whereas it displays large positive tails in quiescent fluid.
These tails are associated to the intermittent fast sedimentation of particle
pairs in drafting-kissing-tumbling motions. The particle lateral dispersion is
higher in a turbulent flow, whereas the vertical one is, surprisingly, of
comparable magnitude as a consequence of the highly intermittent behavior
observed in the quiescent fluid. Using the concept of mean relative velocity we
estimate the mean drag coefficient from empirical formulas and show that non
stationary effects, related to vortex shedding, explain the increased reduction
in mean settling velocity in a turbulent environment.Comment: In press on Journal of Fluid Mechanic
Clustering and increased settling speed of oblate particles at finite Reynolds number
We study the settling of rigid oblates in quiescent fluid using
interface-resolved Direct Numerical Simulations. In particular, an immersed
boundary method is used to account for the dispersed solid phase together with
lubrication correction and collision models to account for short-range
particle-particle interactions. We consider semi-dilute suspensions of oblate
particles with aspect ratio AR=1/3 and solid volume fractions
. The solid-to-fluid density ratio and the Galileo
number (i.e. the ratio between buoyancy and viscous forces) based on the
diameter of a sphere with equivalent volume . With this choice of
parameters, an isolated oblate falls vertically with a steady wake with its
broad side perpendicular to the gravity direction. At this , the mean
settling speed of spheres is a decreasing function of the volume and is
always smaller than the terminal velocity of the isolated particle, . On
the contrary, we show here that the mean settling speed of oblate particles
increases with in dilute conditions and is larger than . At
higher concentrations, the mean settling speed decreases becoming smaller than
the terminal velocity between and . The increase of the
mean settling speed is due to the formation of particle clusters that for
appear as columnar-like structures. From the pair-distribution
function we observe that it is most probable to find particle-pairs almost
vertically aligned. However, the pair-distribution function is non-negligible
all around the reference particle indicating that there is a substantial amount
of clustering at radial distances between 2 and (with the polar radius
of the oblate).Comment: Submitted to Journal of Fluid Mechanic
Reduced particle settling speed in turbulence
We study the settling of finite-size rigid spheres in sustained homogeneous
isotropic turbulence (HIT) by direct numerical simulations using an immersed
boundary method to account for the dispersed solid phase. We study semi-dilute
suspensions at different Galileo numbers, Ga. The Galileo number is the ratio
between buoyancy and viscous forces, and is here varied via the solid-to-fluid
density ratio. The focus is on particles that are slightly heavier than the
fluid. We find that in HIT, the mean settling speed is less than that in
quiescent fluid; in particular, it reduces by 6%-60% with respect to the
terminal velocity of an isolated sphere in quiescent fluid as the ratio between
the latter and the turbulent velocity fluctuations is decreased. Analysing the
fluid-particle relative motion, we find that the mean settling speed is
progressively reduced while reducing the density ratio due to the increase of
the vertical drag induced by the particle cross-flow velocity. Unsteady effects
contribute to the mean overall drag by about 6%-10%. The probability density
functions of particle velocities and accelerations reveal that these are
closely related to the features of the turbulent flow. The particle mean-square
displacement in the settling direction is found to be similar for all Ga if
time is scaled by (2a)/u' (where 2a is the particle diameter and u' is the
turbulence velocity root mean square).Comment: Accepted for publication in Journal of Fluid Mechanic
Suspensions of finite-size neutrally-buoyant spheres in turbulent duct flow
We study the turbulent square duct flow of dense suspensions of
neutrally-buoyant spherical particles. Direct numerical simulations (DNS) are
performed in the range of volume fractions , using the immersed
boundary method (IBM) to account for the dispersed phase. Based on the
hydraulic diameter a Reynolds number of is considered. We report flow
features and particle statistics specific to this geometry, and compare the
results to the case of two-dimensional channel flows. In particular, we observe
that for and , particles preferentially accumulate on the
corner bisectors, close to the duct corners as also observed for laminar square
duct flows of same duct-to-particle size ratios. At the highest volume
fraction, particles preferentially accumulate in the core region. For channel
flows, in the absence of lateral confinement particles are found instead to be
uniformily distributed across the channel. We also observe that the intensity
of the cross-stream secondary flows increases (with respect to the unladen
case) with the volume fraction up to , as a consequence of the high
concentration of particles along the corner bisector. For the
turbulence activity is strongly reduced and the intensity of the secondary
flows reduces below that of the unladen case. The friction Reynolds number
increases with in dilute conditions, as observed for channel flows.
However, for the mean friction Reynolds number decreases below the
value for .Comment: Submitted to Journal of Fluid Mechanic
ROSAT Results on Narrow-Line Seyfert 1 Galaxies
The excellent soft X-ray sensitivity of the PSPC detector onboard the ROSAT
satellite provided the first chance to study precisely the spectral and timing
properties of Narrow-Line Seyfert 1 galaxies. ROSAT observations of Narrow-Line
Seyfert 1 galaxies have revealed (1) the existence of a giant soft X-ray
excess, (2) a striking, clear correlation between the strength of the soft
X-ray excess emission and the FWHM of the H-beta line, (3) the general absence
of significant soft X-ray absorption by neutral hydrogen above the Galactic
column, (4) short doubling time scales down to about 1000 seconds, (5) the
existence of persistent giant (above a factor of 10), and rapid (less than 1
day) X-ray variability in extragalactic sources. The soft X-ray results on
Narrow-Line Seyfert 1 galaxies indicate that their black hole regions are
directly visible, further supporting the Seyfert 1 nature of these objects. The
extreme X-ray properties of Narrow-Line Seyfert 1 galaxies make them ideal
objects for understanding many of the problems raised generally by the Seyfert
phenomenon.Comment: Invited talk presented at the Joint MPE,AIP,ESO workshop on NLS1s,
Bad Honnef, Dec. 1999, to appear in New Astronomy Reviews; also available at
http://wave.xray.mpe.mpg.de/conferences/nls1-worksho
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