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1.
Betat A Kruelle CA Frette V Rehberg I 《The European physical journal. E, Soft matter》2002,8(5):465-476
The formation of sand ripples under water shear flow in a narrow annular channel and the approach of the ripple pattern towards a steady state were studied experimentally. Four results are obtained: i) The mean amplitude, the average drift velocity and the mean sediment transport rate of the evolving bed shape are strongly related. A quantitative characterization of this relation is given. ii) The ripple pattern reaches a stationary state with a finite ripple amplitude and wavelength. The time needed to reach the state depends on the shear stress and may be several days. iii) The onset of ripple formation is determined by the bed shear stress, but it seems neither to depend on the grain diameter nor on the depth of the water layer. iv) The ripple amplitude, drift velocity and sediment transport in this stationary state depend on the grain size. This dependency is neither captured by the particle Reynolds number nor by the Shields parameter: an empirical scaling law is presented instead. 相似文献
2.
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
3.
Grain segregation mechanism in aeolian sand ripples 总被引:2,自引:0,他引:2
H.A. Makse 《The European physical journal. E, Soft matter》2000,1(2-3):127-135
Many sedimentary rocks are formed by migration of sand ripples. Thin layers of coarse and fine sand are present in these rocks,
and understanding how layers in sandstone are created has been a longstanding question. Here, we propose a mechanism for the
origin of the most common layered sedimentary structures such as inverse graded climbing ripple lamination and cross-stratification
patterns. The mechanism involves a competition between three segregation processes: (i) size-segregation and (ii) shape-segregation
during transport and rolling, and (iii) size segregation due to different hopping lengths of the small and large grains. We
develop a discrete model of grain dynamics which incorporates the coupling between moving grains and the static sand surface,
as well as the different properties of grains, such as size and roughness, in order to test the plausibility of this physical
mechanism.
Received 19 July 1999 and Received in final form 4 August 1999 相似文献
4.
ZHENG Renrong CHEN Zhiqian & ZHU Shunquan . Department of Physics Shanghai Normal University Shanghai China . Department of Physics Southwest Normal University Chongqing China . Department of Computer Electric Technology Shanghai Business School Shanghai China 《中国科学G辑(英文版)》2005,48(4):431-441
1 IntroductionThe superconductivity of small metallic grains has been attracting a lot of attentions. On one hand, Anderson[1] predicted that the superconductivity would disappear if a me- tallic grain was so small that the spaces between the nearest neighbor energy levels in the system became larger than the energy gap of the bulk metallic superconductor; On the other hand, it was found in experiments made in the 1960s[2] that the critical tem- perature of superconductivity of small metallic … 相似文献
5.
Vandewalle N Lumay G Gerasimov O Ludewig F 《The European physical journal. E, Soft matter》2007,22(3):241-248
This article is a review of our recent and new experimental works on granular compaction. The effects of various microscopic
parameters on the
compaction dynamics are addressed, in particular the influence
of the grain shape, the friction and the cohesion between the grains. Two
dimensionnal and three dimensionnal systems are analysed. And the role of
dimensionality will be emphasized. Theoretical and numerical investigations provide additional informations about that phenomenon.
Indeed numerical models permit us to study the influence of some parameters not easily accessible experimentally. Our results
show that the above mentioned parameters have a deep impact on the compaction dynamics. Anisotropic grains lead to two different
compaction regimes separated by a “burst" of the packing fraction. Friction is observed to modify how the grains are arranged
in the pile. This is confirmed by numerical simulations. Cohesive forces between particles inhibit compaction and lead to
extremely low values of the packing fraction. 相似文献
6.
The silo discharge process is studied by molecular
dynamics simulations. The development of the velocity profile and
the probability density function for the displacements in the
horizontal and vertical axis are obtained. The PDFs obtained at
the beginning of the discharge reveal non-Gaussian statistics and
superdiffusive behaviors. When the stationary flow is developed,
the PDFs at shorter temporal scales are non-Gaussian too. For big
orifices a well-defined transition between ballistic and diffusive
regime is observed. In the case of a small outlet orifice, no
well-defined transition is observed. We use a nonlinear diffusion
equation introduced in the framework of non-extensive
thermodynamics in order to describe the movements of the grains.
The solution of this equation gives a well-defined relationship
(γ= 2/(3-q)) between the anomalous diffusion exponent
γ and the entropic parameter q introduced by the
non-extensive formalism to fit the PDF of the fluctuations. 相似文献
7.
A series of large-eddy simulations of a turbulent asymptotic suction boundary layer (TASBL) was performed in a periodic domain, on which uniform suction was applied over a flat plate. Three Reynolds numbers (defined as ratio of free-stream and suction velocity) of Re = 333, 400 and 500 and a variety of domain sizes were considered in temporal simulations in order to investigate the turbulence statistics, the importance of the computational domain size, the arising flow structures as well as temporal development length required to achieve the asymptotic state. The effect of these two important parameters was assessed in terms of their influence on integral quantities, mean velocity, Reynolds stresses, higher order statistics, amplitude modulation and spectral maps. While the near-wall region up to the buffer region appears to scale irrespective of Re and domain size, the parameters of the logarithmic law (i.e. von Kármán and additive coefficient) decrease with increasing Re, while the wake strength decreases with increasing spanwise domain size and vanishes entirely once the spanwise domain size exceeds approximately two boundary-layer thicknesses irrespective of Re. The wake strength also reduces with increasing simulation time. The asymptotic state of the TASBL is characterised by surprisingly large friction Reynolds numbers and inherits features of wall turbulence at numerically high Re. Compared to a turbulent boundary layer (TBL) or a channel flow without suction, the components of the Reynolds-stress tensor are overall reduced, but exhibit a logarithmic increase with decreasing suction rates, i.e. increasing Re. At the same time, the anisotropy is increased compared to canonical wall-bounded flows without suction. The reduced amplitudes in turbulence quantities are discussed in light of the amplitude modulation due to the weakened larger outer structures. The inner peak in the spectral maps is shifted to higher wavelength and the strength of the outer peak is much less than for TBLs. An additional spatial simulation was performed, in order to relate the simulation results to wind tunnel experiments, which – in accordance with the results from the temporal simulation – indicate that a truly TASBL is practically impossible to realise in a wind tunnel. Our unique data set agrees qualitatively with existing literature results for both numerical and experimental studies, and at the same time sheds light on the fact why the asymptotic state could not be established in a wind tunnel experiment, viz. because experimental studies resemble our simulation results from too small simulation boxes or insufficient development times. 相似文献
8.
Ribière P Richard P Bideau D Delannay R 《The European physical journal. E, Soft matter》2005,16(4):415-420
We report on experiments to measure the temporal and spatial evolution of packing arrangements of anisotropic and weakly confined granular material, using high-resolution -ray adsorption. In these experiments, the particle configurations start from an initially disordered, low-packing-fraction state and under vertical solicitations evolve to a dense state. We find that the packing fraction evolution is slowed by the grain anisotropy but, as for spherically shaped grains, can be well fitted by a stretched exponential. For a given type of grains, the characteristic times of relaxation and of convection are found to be of the same order of magnitude. On the contrary, compaction mechanisms in the media strongly depend on the grain anisotropy. 相似文献
9.
We study how the aggregate statistical properties for density fluctuations in granular aggregates scale with the sample size and how such a scaling is associated with the correlations between grains. Correlations are studied both between grain positions and between Vorono? cell volumes, showing distinct behaviors and properties. A non-linear scaling in the aggregate volume fluctuations as function of the sample size is discovered and the connection between such anomalous scaling and correlations is explained. It emerges that volume fluctuations might be described by means of a single universal equation for all samples at all cluster sizes. 相似文献
10.
We investigate the segregation effect of binary granular mixtures with the same size but different densities under vibration at different air pressures. Our experiments show that the segregation state is seriously dependent on the air pressure and there is a new type of partially segregated state at high air pressure, which has the characteristic that the lighter grains tend to stay at the bottom and form a pure layer, while heavier grains and remained lighter ones tend to rise and to form a mixed layer on the top of the system. We redefine the order parameter to study the variation of the segregation effect with the air pressure and vibration parameter in detail. Finally, the mechanism of the air-driven segregation is illustrated by the faster acceleration due to the airflow through the granular bed for lighter particles. 相似文献
11.
B.K. Nayak M.C. Gupta K.W. Kolasinski 《Applied Physics A: Materials Science & Processing》2008,90(3):399-402
We report for the first time that a regular array of sharp nano-textured conical microstructures are formed on the titanium
metal surface by irradiation with ultrafast laser pulses of 130 fs duration, 800 nm wavelength in vacuum (∼1 mbar) or in 100 mbar
He. The microstructures are up to 25 μm tall, and taper to about 500 nm diameters at the tip. Irradiation in the presence
of SF6, air or HCl creates a textured surface but does not create sharp conical microstructures. The surfaces of these microstructures
exhibit periodic nano-texture of feature size comparable to the wavelength of light consistent with ripple formation. Contrary
to pillar formation by femtosecond laser irradiation of silicon where the initial ripples evolve into the pillars and the
ripples disappear, the ripples on titanium pillars have a much smaller periodicity than the pillars and remain on the surface
of the pillars. The textured surface is pitch black compared to its original silver-grayish color, i.e, it exhibits greatly
reduced reflectivity throughout the measured visible spectrum.
PACS 52.38.Mf; 79.20.Ds; 81.07.-b; 81.16.-c; 82.53.-k 相似文献
12.
13.
R.M. Thaokar V. Shankar V. Kumaran 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,23(4):533-550
The stability of linear shear flow of a Newtonian fluid past a flexible membrane is analysed in the limit of low Reynolds
number as well as in the intermediate Reynolds number regime for two different membrane models. The objective of this paper
is to demonstrate the importance of tangential motion in the membrane on the stability characteristics of the shear flow.
The first model assumes the wall to be a “spring-backed” plate membrane, and the displacement of the wall is phenomenologically
related in a linear manner to the change in the fluid stresses at the wall. In the second model, the membrane is assumed to
be a two-dimensional compressible viscoelastic sheet of infinitesimal thickness, in which the constitutive relation for the
shear stress contains an elastic part that depends on the local displacement field and a viscous component that depends on
the local velocity in the membrane. The stability characteristics of the laminar flow in the limit of low are crucially dependent on the tangential motion in the membrane wall. In both cases, the flow is stable in the low Reynolds number limit in the absence of tangential motion in the membrane. However, the presence of tangential
motion in the membrane destabilises the shear flow even in the absence of fluid inertia. In this case, the non-dimensional velocity (Λt) required for unstable fluctuations is proportional to the wavenumber k ( Λ
t∼k) in the plate membrane type of wall while it scales as k2 in the viscoelastic membrane type of wall ( Λ
t∼k
2) in the limit k→ 0. The results of the low Reynolds number analysis are extended numerically to the intermediate Reynolds number regime for
the case of a viscoelastic membrane. The numerical results show that for a given set of wall parameters, the flow is unstable
only in a finite range of Reynolds number, and it is stable in the limit of large Reynolds number.
Received 8 November 2000 and Received in final form 20 March 2001 相似文献
14.
Goujon C Dalloz-Dubrujeaud B Thomas N 《The European physical journal. E, Soft matter》2007,23(2):199-215
Experiments were performed to provide insight into the
flow behavior and structure of bimodal mixtures of grains in
gravity-driven, free-surface flows. Unsteady unconfined flows
were produced by releasing instantaneously a dry granular mass,
composed of two particle sizes, over a rough inclined plane. As a
result of size segregation, the small particles are found at the
bottom of the flow and final deposit, the large particles are
found at the free surface, but also on the lateral borders and at
the front of the flow. The lateral and vertical inhomogeneous
repartitions of particles lead to two main effects that are
completely absent in monodispersed flows. The outline effect
results from the accumulation of large beads on the periphery of
the flow depending on the value of the relative friction of each
particle species on the plane. This effect in turn causes a
narrowing of the flow and/or an increase of length of the final
deposit. The interface effect results of the interaction between
layers of different size particles and causes the modification of
the thickness of the deposit. These effects occur simultaneously
and their combination leads to a great variety of behaviors. In
this investigation, evidence of the diversity of behaviors is
presented as the size ratio, relative friction and concentration
of each particle species are varied. 相似文献
15.
T. Li Q. Lou J. Dong Y. Wei J. Liu 《Applied Physics A: Materials Science & Processing》2001,73(3):391-397
Surface ablation of cobalt-cemented tungsten carbide hard metal has been carried out in this work using a 308 nm, 20 ns XeCl
excimer laser. Surface microphotography and XRD, as well as an electron probe have been used to investigate the transformation
of phase and microstructure as a function of the pulse-number of laser shots at a laser fluence of 2.5 J/cm2. The experimental results show that the microstructure of cemented tungsten carbide is transformed from the original polygonal
grains of size 3 μm to interlaced large, long grains with an increase in the number of laser shots up to 300, and finally
to gross grains of size 10 μm with clear grain boundaries after 700 shots of laser irradiation. The crystalline structure
of the irradiated area is partly transformed from the original WC to βWC1-x, then to αW2C and CW3, and finally to W crystal. It is suggested that the undulating ‘hill–valley’ morphology may be the result of selective removal
of cobalt binder from the surface layer of the hard metal. The formation of non-stoichiometric tungsten carbide may result
from the escape of elemental carbon due to accumulated heating of the surface by pulsed laser irradiation.
Received: 13 July 2000 / Accepted: 27 October 2000 / Published online: 10 January 2001 相似文献
16.
Shinbrot T 《The European physical journal. E, Soft matter》2007,22(3):209-217
Analysis of granular flows has been a significant theoretical challenge over
the past several decades. These flows are difficult to analyze largely
because they exhibit both solid-like and fluid-like behaviors side-by-side
in single experiments. In this paper, we examine two experiments in which
the co-existence between these states is especially marked and leads to
unique patterns that may serve as signatures for underlying granular
dynamics deserving of further scrutiny. In these experiments, we find that
when fluidization of grains is prolonged — as can be expected to occur for
example under reduced gravity environments or under conditions of strong
kinetic forcing (e.g. during earthquakes) — grains can produce residual
depositional patterns that are difficult to distinguish from fluvial
deposits. This suggests that geological landforms under low gravity (for
example on Mars) or influenced by strong forcing (for example during
earthquakes) may behave in a fluid-like manner despite being entirely dry. 相似文献
17.
Stability limit of a granular monolayer 总被引:3,自引:0,他引:3
Dorbolo S 《The European physical journal. E, Soft matter》2005,17(1):77-81
A granular monolayer is composed by spherical grains on a horizontal plate. The plate is then tilted until the monolayer breaks down. This critical angle has been measured for different widths and heights of the rectangular monolayer. The highest critical angles are found when one of these two characteristic lengths is less than about 30 bead diameters. When the polydispersity is less than one percent, the monolayer may be stable till angles close to 90°. Arches induce large critical angles. On the other hand, for a large and high monolayer, the critical angle saturates towards a lower value. This angle is related to the static friction of a grain on the plate. A model based on the block dynamics is proposed to describe the behavior of the avalanche angle as a function of the size of the monolayer and the polydispersity of the beads.-1 相似文献
18.
采用Potts模型Monte Carlo方法研究了晶粒棱长、尺寸与拓扑学特征之间的统计关系.结果表明,晶粒棱长与晶粒面数之间呈线性统计关系,并且平均N面体晶粒模型和Poisson-Voronoi组织均支持该结论.不同时刻的晶粒长大仿真数据表明,在准稳态晶粒长大阶段晶粒棱长的分布具有自相似性.个体晶粒的平均棱长随晶粒面数(或晶粒尺寸)的增加而逐渐增大,这说明一些理论模型中采用的“不同面数的晶粒平均棱长均相等”的假设具有局限性.仿真数据和纯铁实验数据均表明,晶粒尺寸与晶粒面数之间的统计关系表现为一条单调递增的凸曲线.
关键词:
晶粒棱长
晶粒尺寸
拓扑学
Monte Carlo仿真 相似文献
19.
Simon Dagois-Bohy Sandrine Ngo Sylvain Courrech du Pont Stéphane Douady 《Ultrasonics》2010,50(2):127-132
Some desert sand dunes have the peculiar ability to emit a loud sound up to 110 dB, with a well-defined frequency: this phenomenon, known since early travelers (Darwin, Marco Polo, etc.), has been called the song of dunes. But only in late 19th century scientific observations were made, showing three important characteristics of singing dunes: first, not all dunes sing, but all the singing dunes are composed of dry and well-sorted sand; second, this sound occurs spontaneously during avalanches on a slip face; third this is not the only way to produce sound with this sand.More recent field observations have shown that during avalanches, the sound frequency does not depend on the dune size or shape, but on the grain diameter only, and scales as the square root of g/d - with g the gravity and d the diameter of the grains - explaining why all the singing dunes in the same vicinity sing at the same frequency.We have been able to reproduce these singing avalanches in laboratory on a hard plate, which made possible to study them more accurately than on the field. Signals of accelerometers at the flowing surface of the avalanche are compared to signals of microphones placed above, and it evidences a very strong vibration of the flowing layer at the same frequency as on the field, responsible for the emission of sound.Moreover, other characteristics of the booming dunes are reproduced and analyzed, such as a threshold under which no sound is produced, or beats in the sound that appears when the flow is too large. Finally, the size of the coherence zones emitting sound has been measured and discussed. 相似文献
20.
M. Soetratmo H. Natte R. Hempelmann O. Hartmann R. Wäppling M. Ekström 《Hyperfine Interactions》1997,105(1-4):245-252
A systematic μSR study on nano‐Cu has demonstrated that the diffusion of μ+ in nanocrystalline metals is influenced by both features of the nanostructure, i.e., by the very small grain size and by
the comparatively large fraction of grain boundaries. The former feature yields a size effect of the phonon‐assisted muon
tunneling, but only at particle diameters below 20 nm. The latter feature, in samples with crystallite sizes above 20 nm diameter,
i.e., with bulk diffusional behaviour, establishes a connection between μ+ diffusion coefficient and particle size: if one of these quantities is known, the other could be evaluated.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献