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1.
实验研究了竖直振动颗粒床中,倍周期运动对尺寸分离的影响.实验中,当振动加速度足够大时,系统中出现稳定的对称对流,进一步增大振动加速度到某个临界值时,还会出现倍周期运动.观察表明,背景颗粒的对流运动对分离过程起主导作用,对流速度决定着分离过程的快慢,而在2倍周期和4倍周期分岔之后,分离时间有所减慢.对引起对流运动的起因进行了分析,以此为基础分析了倍周期运动产生影响的物理机理,并对分离时间进行了定量计算,结果与实验值符合很好.
关键词:
颗粒物质
“巴西果”效应
倍周期分岔
对流 相似文献
2.
Effect of Thermal Convection on Density Segregation in Binary Granular Gases with Dissipative Lateral Walls
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《中国物理快报》2017,(11)
Molecular dynamics simulations are employed to investigate the effect of thermal convection induced only by dissipative lateral walls on density segregation of the strongly driven binary granular gases under low gravity conditions. It is found that the thermal convection due to dissipative lateral walls has significant influence on the segregation intensity of the system. The dominant factor in determining the degree of segregation achieved by the system is found to be the relative convection rate between differing species. Moreover, a qualitative explanation is proposed for the relationship between the thermal convection due to dissipative lateral walls and the observed segregation intensity profiles. 相似文献
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本文通过实验和理论研究了影响"巴西果"分离的因素及其物理机理. 分析了振动加速度、大小颗粒尺寸和密度对分离时间的影响, 并利用流体模型对分离时间作估算, 对实验结果进行定性解释. 结果表明在振动频率固定时, 调节振动加速度是控制"巴西果"分离的一个主要手段. 振动加速度存在一个临界值, 当高于此临界值时, "巴西果"分离的主要物理机理由对流机理转变为几何填空机理, 且振动加速度对分离影响变小, 大颗粒尺寸对分离的影响增大. 可通过调节大颗粒的尺寸来改变分离效果. 当大、 小颗粒密度比为1时, 仍会出现"巴西果"分离现象. 增大小颗粒尺寸或密度可以促进"巴西果"分离. 相似文献
5.
A molecular dynamics simulation of segregation behaviours of horizontally vibrated binary granular mixture
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This paper performs the two-dimensional, soft-sphere
molecular dynamics simulations to study the granular segregation in
a binary granular mixture with the same size but different density
in the container with the sawtooth base under horizontal vibration.
The segregation phase diagram is presented in the
acceleration-frequency space. When the acceleration is high enough
to result in relative motions of the particles, the system can be in
various states (mixed state, vertical and horizontal segregation
state), which depend on both acceleration and frequency. Due to the
sawtooth base there is stratified flow effect besides density
effect. The density effect raises the light particles. The
stratified flow drives the particles in the upper levels to the
right and the particles in the lower particles to the left, those
fact results in the appearance of the left segregation state. The
left segregation state can be changed to the right segregation by
changing the shape of the sawtooth. As the vibration frequency
increases, the stratified flow effect becomes weaker and weaker, so
at high vibration frequencies the vertical segregation state appears
instead of the left segregation state. 相似文献
6.
King PJ Lopez-Alcaraz P Pacheco-Martinez HA Clement CP Smith AJ Swift MR 《The European physical journal. E, Soft matter》2007,22(3):219-226
When a bed of fluid-immersed fine grains is exposed to vertical vibration a wealth of
phenomena may be observed. At low frequencies a horizontal bed geometry is
generally unstable and the bed breaks spatial symmetry, acquiring a tilt. At the same
time it undergoes asymmetric granular convection. Fine binary mixtures may
separate completely into layers or patterns of stripes. The separated regions may
exhibit instabilities in which they undergo wave-like motion or exhibit quasi-periodic
oscillations. We briefly review these and a number of related behaviours, identifying
the physical mechanisms behind each. Finally, we discuss the magneto-vibratory
separation of binary mixtures which results from exposing each component to
a different effective gravity and describe the influence of a background fluid on this
process. 相似文献
7.
Convective heat transfer coefficients were measured experimentally for a tube immersed vertically in a circulating fluidized bed. Circulating fluidized beds operate in the dilute transport regime of two-phase (solid/gas) flow. The dominant mechanism for heat transfer to surfaces is particle-induced convection. In this study, experiments were carried out in a circulating fluidized bed of15 cm diameter and 11 m height. An instrumented tube of 9.5 mm diameter and 1.3 m length was placed vertically at the centeriine of the fluidized bed to measure convective heat transfer coefficients at several different elevations in the bed. Three types of particles, with mean diameters ranging from 68 to 2S1 urn, were used in the experiments at superficial gas velocities in the range of 1.3 to 8.2 m/s. Results showed that the convective heat transfer coefficients with solid/gas two-phase circulation were two to three times greater than those for single-phase gas convection at the same velocity. For a given gas velocity, the coefficients increased with increasing solid mass flux, but decreased with elevation. It was demonstrated that the heat transfer coefficients for the immersed tube and for the bed wall could be correlated with different functional dependence on the two-phase suspension density. 相似文献
8.
MohammadG. Rasul 《Particle & Particle Systems Characterization》2003,20(6):398-407
The mechanism causing mixing and segregation of multi‐component fluidized systems consisting of both density and size variant particle mixtures is presented. The validity of the model is demonstrated with the experimental results of three‐component (ternary) liquid fluidized systems. Ternary bed fluidized systems separated into different mixed layers and a pure layer of excess component. Location of the stratified mixed layers and pure layer can be predicted from the mixture bulk density evaluation. The volume fractions of solids and fluid corresponding to each mixed layer can be theoretically predicted from the force balance applied to the particles in that layer knowing the systems physical properties. 相似文献
9.
Size segregation and convection of granular mixtures almost completely packed in a thin rotating Box
Awazu A 《Physical review letters》2000,84(20):4585-4588
We simulate size segregation in granular mixtures which are almost completely packed in a rotating drum. Instead of a 3D drum, we simulate a 2D thin rotating box which is almost completely packed with granular mixtures. The phase inversion of a radially segregated pattern which was found in a 3D experiment is qualitatively reproduced with this simulation. A global convection appears after a radial segregation pattern is formed, and this convection induces an axially segregated pattern. 相似文献
10.
Mohammad G. Rasul 《Particle & Particle Systems Characterization》1999,16(6):284-289
The literature represents the buoyancy force on particles in a fluidized bed in two different ways: one is based on the density of the fluid alone and the other is based on the density of the suspension comprised of the fluid and solid. To clarify this problem is especially important for mixing/segregation phenomena in fluidized beds of mixed particles. This paper, presents a brief review of the literature and provides experimental evidence to conclude for the correct expression of the buoyancy force in fluidized beds. 相似文献
11.
The effects of time-periodic forcing in a few-mode model for zero-Prandtl-number convection with rigid body rotation is investigated.
The time-periodic modulation of the rotation rate about the vertical axis and gravity modulation are considered separately.
In the presence of periodic variation of the rotation rate, the model shows modulated waves with a band of frequencies. The
increase in the external forcing amplitude widens the frequency band of the modulated waves, which ultimately leads to temporally
chaotic waves. The gravity modulation, on the other hand, with small frequencies, destroys the quasiperiodic waves at the
onset and leads to chaos through intermittency. The spectral power density shows more power to a band of frequencies in the
case of periodic modulation of the rotation rate. In the case of externally imposed vertical vibration, the spectral density
has more power at lower frequencies. The two types of forcing show different routes to chaos.
相似文献
12.
研究了强磁场复合不同强度和不同频率交变电流对Zn-30wt%Bi合金凝固组织的影响规律.结果表明:在25℃/min的冷却速率下,只施加交变电流无法抑制该合金的分层比重偏析;而单独施加纵向强磁场对合金的偏析有一定的改善作用;如果同时施加纵向强磁场和工频交变电流,在产生的交变洛仑兹力的作用下,合金的分层比重偏析基本得到抑制;增加电流密度、磁场强度和交流电流频率均有利于合金凝固组织的改善.但对上述三个参数而言,均存在一个最佳值,偏离该最佳值时,均难以获得均质的偏晶合金组织.从电磁场动力学角度探讨了复合场影响偏晶
关键词:
强磁场
偏晶合金
重力偏析
低冷却速度 相似文献
13.
J. N. Koster R. Derebail A. Grötzbach 《Applied Physics A: Materials Science & Processing》1996,64(1):45-54
A vertical layer of an eutectic gallium indium mixture was subjected to a horizontal temperature gradient. The well stirred
melt was first solidified and then remelted in a slow, quasi-steady fashion. Real-time visualization of the processing is
provided by radioscopy. It was observed that gravitational segregation leads to 1) a break down of convection, and 2) an early
solidification of β-indium, followed by an eutectic solidification of most of the remaining melt. The amount of observed concentrational
segregation is higher than predicted by theory. The experimental findings have never been predicted in modeling efforts of
similar situations including convection; possible reasons are discussed.
Received: 13 March 1996/Accepted: 28 May 1996 相似文献
14.
We study spontaneously forming convection in a container that is almost completely filled with a bidisperse granular mixture. The container with an aspect ratio close to 1 rotates slowly about a horizontal axis. In this geometry, single vortex rolls are observed in the cell plane, after a spontaneous symmetry breaking. The circulation of grains produces nonuniform segregation patterns of the mixture that in turn interact with the convective flow. We describe oscillatory modulations of the convection velocity, cessations and spontaneous reversals of the circulation. All these features are absent in multiroll granular convection. 相似文献
15.
Natural convection in an internally heated porous bed of height and diameter of 450 mm and 500 mm, respectively, and superposed with the fluid layer has been experimentally investigated. The onset of natural convection in the bed is indicated by change in the rate of temperature rise within the bed. An empirical model based on local Nusselt number and local Rayleigh number has been developed. A comparison of the present model with the models in literature is made to draw out the differences between the local heat transfer of large multidimensional beds and the average heat transfer of small beds. 相似文献
16.
Granulates are ubiquitous in nature and technology, but, despite their great importance, their dynamics are by far less well understood than those of liquids. We demonstrate in an almost compactly filled flat (Hele-Shaw) cell, where slow horizontal rotation simulates a variable gravitational force, that unexpected dynamic structures may arise under geometrical restrictions. The cell motion drives regular flow in the compact interior, and convection rolls combine with segregation. The container fill level is crucial for the dynamic regime. A transition from chute flow at lower fill levels to convection in densely packed containers is found. These observations suggest the existence of comparable phenomena in situations where so far no systematic search for dynamic patterns has been performed. 相似文献
17.
Daniel R. Ohlsen S. Y. Yamamoto C. M. Surko Paul Kolodner 《Journal of statistical physics》1991,64(5-6):903-912
We study nonlinear traveling-wave (TW) and stationary states of convection in experiments in ethanol-water mixtures. While the TW phase velocity as a function of Rayleigh number has been recently shown to be in agreement with the predictions of theory and numerical calculations, we find that this velocity is temporally modulated at frequencies corresponding to the travel time of a single convection roll and of a roll pair past a point stationary in the convection cell. This modulation could be due to the pinning of the convection pattern by experimental inhomogeneities. For large Rayleigh numbers where stationary overturning convection is expected, we sometimes observe extremely slow unidirectional TW states. For larger Rayleigh numbers, this slow TW state starts and stops intermittently on a characteristic time scale of several days. The possible origin of these phenomena and their potential utility are discussed. 相似文献
18.
We present a granular-hydrodynamic model that captures the essence of convection in a fully vibrofluidized granular system. The steady temperature distribution is solved analytically. Numerical simulation shows that the convection always develops through a supercritical bifurcation, with its energy about
of the random (heat) one. A comparison calculation is performed for a normal fluid. The convection roll, or an active roll as we call it, has an angular velocity gradient from its interior to exterior. We conclude that active rolls are universal.Received: 25 March 2004, Published online: 9 September 2004PACS:
45.70.Mg Granular flow: mixing, segregation and stratification - 47.20.Bp Buoyancy-driven instability - 47.27.Te Convection and heat transfer 相似文献
19.
We report an experimental study of a binary sand bed under an oscillating water flow. The formation and evolution of ripples is observed. The appearance of a granular segregation is shown to strongly depend on the sand bed preparation. The initial wavelength of the mixture is measured. In the final steady state, a segregation in volume is observed instead of a segregation at the surface as reported before. The correlation between this phenomenon and the fluid flow is emphasised. Finally, different exotic patterns and their geophysical implications are presented.Received: 25 July 2003, Published online: 25 March 2004PACS:
45.70.Qj Pattern formation - 45.70.Mg Granular flow: mixing, segregation and stratification - 47.20.Ma Interfacial instability 相似文献