共查询到20条相似文献,搜索用时 15 毫秒
1.
Wierschem A Groh C Rehberg I Aksel N Kruelle CA 《The European physical journal. E, Soft matter》2008,25(2):213-221
The formation of granular ripples under liquid shear flow in an annular channel is studied experimentally. The erodible granular
bed is subject to weakly turbulent flows without a defined sharp boundary layer close to the granular bed. The flow field
and the degree of turbulence is characterized quantitatively by using a particle image velocimeter and a laser-Doppler velocimeter,
respectively. A new range of particle Reynolds numbers at the lower limit of the Shields diagram were explored. Quantitative
measurements of the granular flow on the surface reveal that the threshold for particle motion coincides within the order
of one percent with the threshold for ripple formation. In fully developed ripples it was found that on the leeward side of
the ripples regions of low-velocity gradients exist where granular motion is scarce, indicating that the coupling between
the ripples is mainly caused by the flow field of the liquid. 相似文献
2.
Saikat Chakraborty Prabhu R. Nott J. Ravi Prakash 《The European physical journal. E, Soft matter》2000,1(4):265-273
This paper considers the segregation of a granular mixture in a rotating drum. Extending a recent kinematic model for grain
transport on sandpile surfaces to the case of rotating drums, an analysis is presented for radial segregation in the rolling
regime, where a thin layer is avalanching down while the rest of the material follows rigid body rotation. We argue that segregation
is driven not just by differences in the angle of repose of the species, as has been assumed in earlier investigations, but
also by differences in the size and surface properties of the grains. The cases of grains differing only in size (slightly
or widely) and only in surface properties are considered, and the predictions are in qualitative agreement with observations.
The model yields results inconsistent with the assumptions for more general cases, and we speculate on how this may be corrected.
Received 4 June 1999 and Received in final form 28 September 1999 相似文献
3.
J.P. Vishwakarma 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,16(2):369-372
The variation of flow-variables with distance, in the flow-field behind a shock wave propagating in a dusty gas with exponentially
varying density, are obtained at different times. The equilibrium flow conditions are assumed to be maintained, and the results
are compared with those obtained for a perfect gas. It is found that the presence of small solid particles in the medium has
significant effects on the variation of density and pressure.
Received 20 October 1999 and Received in final form 9 March 2000 相似文献
4.
We investigate the formation and dynamics of sand ripples under a turbulent water flow. Our experiments were conducted in
an open flume with spherical glass beads between 100 and 500μm in diameter. The flow Reynolds number is of the order of 10 000
and the particle Reynolds number of the
order of 1 to 10. We study the development of ripples by measuring their
wavelength and amplitude in course of time and investigate the influence of the grain size and the flow properties.
In particular, we demonstrate two different regimes according to the grain size.
For fine grains, a slow coarsening process (i.e., a logarithmic increase of the wavelength and amplitude) takes place,
while for coarser grains, this process occurs at a much faster rate (i.e., with a linear growth) and stops after a finite
time. In the later case, a stable pattern is eventually observed. Besides, we carefully analyze the wavelength of ripples
in the first stages of the instability as a function of the grain size and the shear velocity of the flow, and compare
our results with other available experimental
data and with theoretical predictions based on linear stability analyses. 相似文献
5.
T. Boutreux 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(3):419-424
We present the generalization of a theoretical model for segregation of granular mixtures due to surface flows, published
in J. Phys. I France 6, 1295 (1996). Our generalized model is valid for grains differing by their size and/or their surface properties; in the present
paper, we describe the case of two species with the same surface properties but two different sizes. The rolling stream is
assumed to be homogeneous. Exchanges between the grains at rest and the rolling stream are modelized via binary collisions.
The model predicts that during the filling of a two-dimensional silo, continuous segregation appears inside the static phase: small (respectively large) grains tend to stop uphill (respectively downhill), although
both species remain present everywhere. This fits the observations when the size difference between the species is small.
When the size difference is large, a different regime is observed. We argue that in this case, segregation occurs directly inside the rolling stream.
Received: 25 February 1998 / Received in final form and Accepted: 6 July 1998 相似文献
6.
Cerisier P Porterie B Kaiss A Cordonnier J 《The European physical journal. E, Soft matter》2005,18(1):85-93
The motion of particles moving under gravity in the velocity field of a liquid in a Bénard hexagonal cell is studied experimentally
and numerically for Stokes flow conditions. We then explain the settlement of particles in the centers of cells to form a
regular quincunx. It is found that sedimentation also occurs preferentially along the lines connecting the centers of adjacent
cells to form a triangle deposition tessellation. Finally, it is explained why particles occupy the central part of each convective
cell while the peripheral part of the cell quickly becomes limpid. Numerical results are in agreement with the experimental
observations of Bénard and those of the present study. 相似文献
7.
T. Boutreux H.A. Makse P.-G. de Gennes 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,9(1):105-115
We present the generalization of the minimal model for surface flows of granular mixtures, proposed by Boutreux and de Gennes
[J. Phys. I France 6, 1295 (1996)]. The minimal model was valid for grains differing only in their surface properties. The present model also
takes into account differences in the size of the grains. We apply the model to study segregation in two-dimensional silos
of mixtures of grains differing in size and/or surface properties. When the difference in size is small, the model predicts
that a continuous segregation appears in the static phase during the filling of a silo. When the difference in size is wide,
we take into account the segregation of the grains in the rolling phase, and the model predicts complete segregation and stratification
in agreement with experimental observations.
Received 9 September 1998 and Received in final form 4 November 1998 相似文献
8.
H.A. Makse 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,7(2):271-276
We study the segregation of granular mixtures in two-dimensional silos using a recently proposed set of coupled equations
for surface flows of grains. We study the thick flow regime, where the grains are segregated in the rolling phase. We incorporate
this dynamical segregation process, called kinematic sieving, free-surface segregation or percolation, into the theoretical
formalism and calculate the profiles of the rolling species and the concentration of grains in the bulk in the steady state.
Our solution shows the segregation of the mixture with the large grains being found at the bottom of the pile in qualitative
agreement with experiments.
Received: 6 July 1998 / Revised and Accepted: 13 August 1998 相似文献
9.
A. Martin P. Odier J.-F. Pinton S. Fauve 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,18(2):337-341
Magnetohydrodynamics studies in laboratory experiments have long been restricted to low magnetic Reynolds number flows, mainly
as a result of the very high magnetic diffusivity λ = 1/μσ of common conducting fluids (μ is the fluid's magnetic permeability
and σ its electrical conductivity). The best conductivities are found in liquid metals which have a unit magnetic permeability,
relative to vacuum. We show experimentally that a suspension of solid particles with a high magnetic permeability in a liquid
metal yields an effective medium that has a high electrical conductivity and an enhanced magnetic permeability. The dispersion
of the beads results from the turbulent fluid motion. The range of accessible magnetic Reynolds number can be increased by
a factor of as much as 4 in our experimental setup.
Received 6 March 2000 and Received in final form 13 July 2000 相似文献
10.
11.
V. Pot T. Karapiperis 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,13(2):387-393
We developed a numerical model based on a multi-species lattice gas cellular automaton to study passive and reactive tracer
migration in saturated geological media. The model was made of multiple lattice gases interacting via a two-species collision rule. For a binary mixture, the model displayed a negative deviation from Raoult's law and therefore
behaved as a real solution. By biasing the initial two-species collision rule, our model was made to obey the tracer assumption
which requires that the tracer species does not affect the velocity of the vehicle fluid. In a 2D fracture, we checked the
Taylor-Aris relation. An irreversible adsorption between the tracer and the solid phase was numerically added to perform filtration
of the colloids. A good agreement was found with the solution of the filtration equation. An attachment efficiency was defined
and was found to bear a linear relationship to the filtration coefficient. We added a third species to study the potential
role of colloids in the transport of contaminants. Contaminant migration was enhanced when contaminants were bound to colloids
and was slightly reduced when colloids were allowed to adsorb on the solid phase.
Received 14 January 1999 and Received in final form 8 June 1999 相似文献
12.
A. Lange M. Schröter M.A. Scherer A. Engel I. Rehberg 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(4):475-484
The temporal evolution of a water-sand interface driven by gravity is experimentally investigated. By means of a Fourier analysis
of the evolving interface the growth rates are determined for the different modes appearing in the developing front. To model
the observed behavior we apply the idea of the Rayleigh-Taylor instability for two stratified fluids. Carrying out a linear
stability analysis we calculate the growth rates from the corresponding dispersion relations for finite and infinite cell
sizes. Based on the theoretical results the viscosity of the suspension is estimated to be approximately 100 times higher
than that of pure water, in agreement with other experimental findings.
Received: 12 December 1997 / Revised: 23 March 1998 / Accepted: 28
April 1998 相似文献
13.
14.
The airborne transport of particles on a granular surface by the
saltation mechanism is studied through numerical simulation of
particles dragged by turbulent air flow. We calculate the
saturated flux qs and show that its dependence on the wind
strength u* is consistent with several empirical relations
obtained from experimental measurements. We propose and explain
a new relation for fluxes close to the threshold velocity ut,
namely, qs=a(u*-ut)α with α≈2. We
also obtain the distortion of the velocity profile of the wind due to the
drag of the particles and find a novel dynamical
scaling relation. We also obtain a new expression for the dependence of
the height of the saltation layer as function of the strength of the wind. 相似文献
15.
Long-duration experiments with clouds of microparticles are planned for the ICAPS facility on board the International Space Station ISS. The scientific objectives of such experiments are widespread and are ranging from the simulation of aerosol behaviour in Earths atmosphere to the formation of planets in the early solar system. It is, however, even under microgravity conditions, impossible to sustain a cloud of free-floating, microscopic particles for an extended period of time, due to thermal diffusion and due to unavoidable external accelerations. Therefore, a trap for dust clouds is required which prevents the diffusion of the particles, which provides a source of relative velocities between the dust grains and which can also concentrate the dust to higher number densities that are otherwise not achievable. We are planning to use the photophoretic effect for such a particle trap. First short-duration microgravity experiments on the photophoretic motion of microscopic particles show that such an optical particle-cloud trap is feasible. First tests of a two-dimensional trap were performed in the Bremen drop tower. 相似文献
16.
T. Döppner J. P. Müller A. Przystawik J. Tiggesbäumker K.-H. Meiwes-Broer 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,43(1-3):261-266
Silver clusters embedded in helium nanodroplets are
exposed to intense femtosecond laser pulses (1013 - 1016 W/cm2).
The signal of highly charged (q≤11) atomic fragments is maximized by
delayed plasmon enhanced ionization using stretched laser pulses. Further
details with respect to the dynamics of the charging process can be obtained,
when the intensity distribution within the laser focus is taken into
account. For the first time, the z-scan method is applied to clusters
which offers a route to investigate the explicit dependence of the ion signals
with respect to the laser intensity. By taking advantage of the volumetric
weighting effect ionization thresholds are determined, yielding values well
below 1014 W/cm2 for Agq+ ions with q≤11. 相似文献
17.
M. N. Kuperman S. Risau-Gusman 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,62(2):233-238
In this work we present an analysis of a spatially non homogeneous
ultimatum game. By considering different underlying topologies as
substrates on top of which the game takes place we obtain
nontrivial behaviors for the evolution of the strategies of the
players. We analyze separately the effect of the size of the
neighborhood and the spatial structure. Whereas this last effect
is the most significant one, we show that even for disordered
networks and provided the neighborhood of each site is small, the
results can be significantly different from those obtained in the
case of fully connected networks. 相似文献
18.
J. Gu W. Li X. Cai 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,62(2):247-255
We introduce a new mechanism—the forget-remember
mechanism into the spreading process. Equipped with such a mechanism
an individual is prone to forget the “message" received and
remember the one forgotten, namely switching his state between
active (with message) and inactive (without message). The
probability of state switch is governed by linear or exponential
forget-remember functions of history time which is measured by the
time elapsed since the most recent state change. Our extensive
simulations reveal that the forget-remember mechanism has
significant effects on the saturation of message spreading, and may
even lead to a termination of spreading under certain conditions.
This finding may shed some light on how to control the spreading of
epidemics. It is found that percolation-like phase transitions can
occur. By investigating the properties of clusters, formed by
connected, active individuals, we may be able to justify the
existence of such phase transitions. 相似文献
19.
20.
Shihong Wang Junzhong Yang 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,56(1):81-92
Generalized synchronization in a drive-response Chua circuits is investigated. A cascade of transitions to GS is observed
with increasing the interaction strength. The mechanism on the
transitions to GS is given based on the asymptotic behaviors of response dynamics. 相似文献