A numerical study of granular shear flows of rod-like particles using the discrete element method

2012 ◽  
Vol 713 ◽  
pp. 1-26 ◽  
Author(s):  
Y. Guo ◽  
C. Wassgren ◽  
W. Ketterhagen ◽  
B. Hancock ◽  
B. James ◽  
...  
Keyword(s):  
Aspect Ratio ◽  
Flow Direction ◽  
Particle Surface ◽  
Granular Flows ◽  
Spherical Particles ◽  
Surface Geometry ◽  
High Solid ◽  
Volume Fractions ◽  
Aspect Ratios ◽  
Particle Aspect Ratio

AbstractThe effect of particle aspect ratio and surface geometry on granular flows is assessed by performing numerical simulations of rod-like particles in simple shear flows using the discrete element method (DEM). The effect of particle surface geometry is explored by adopting two types of particles: glued-spheres particles and true cylindrical particles. The particle aspect ratio varies from one to six. Compared to frictionless spherical particles, smaller stresses are obtained for the glued-spheres and cylindrical particle systems in dilute and moderately dense flows due to the loss of translational energy, which is partially converted to rotational energy, for the non-spherical particles. For dilute granular flows of non-spherical particles, stresses are primarily affected by the particle aspect ratio rather than the surface geometry. As the particle aspect ratio increases, the effective particle projected area in the plane perpendicular to the flow direction increases, so that the probability of the occurrence of the particle collisions increases, leading to a reduction in particle velocity fluctuation and therefore a decrease in the stresses. Hence, a simple modification is made to the kinetic theory for granular flows to describe the stress tensors for dilute flows of non-spherical particles by incorporating a normalized effective particle projected area to account for the effect of particle collision probability. For dense granular flows, the stresses depend on both the particle aspect ratio and the surface geometry. Sharp stress increases at high solid volume fractions are observed for the glued-spheres particles with large aspect ratios due to the bumpy surfaces, which impede the flow. However, smaller stresses are obtained for the true cylindrical particles with large aspect ratios at high solid volume fractions. This trend is attributed to the combined effects of the smooth particle surfaces and the particle alignments such that the major/long axes of particles are aligned in the flow direction. In addition, the apparent friction coefficient, defined as the ratio of shear to normal stresses, is found to decrease as the particle aspect ratio increases and/or the particle surface becomes smoother at high solid volume fractions.

Simulation By Cellular Automata Of The (Re) Crystallization Of A Matrix Containing An Inert Second Phase

1993 ◽  
Vol 321 ◽  
Author(s):  
C. F. Pezzee ◽  
D. C. Dunand
Keyword(s):  
Aspect Ratio ◽  
Cellular Automata ◽  
Single Phase ◽  
Second Phase ◽  
Particle Fraction ◽  
Aspect Ratios ◽  
Inert Particles ◽  
Mean Size ◽  
Particle Area ◽  
Particle Aspect Ratio

ABSTRACTTwo-dimensional cellular automata simulations were carried out to study the case of the crystallization (or recrystallization) of a matrix containing an inert, immobile second phase. A range of particle area fractions and aspect ratios were investigated under continuous grain nucleation conditions, assuming that the effect of particles is limited to geometric impingement upon contact with the growing grains. Systematic deviations from the classical Johnson, Mehl, Avrami, Kolmogo-rov equation for single-phase materials are observed with increasing particle aspect ratio and particle fraction. Inert particles also influence both mean size and mean aspect ratio of the final grains.

Buoyancy Effects on Heat Transfer in Five Different Aspect-Ratio Rectangular Channels With Smooth Walls and 45-Degree Ribbed Walls

2005 ◽  
Author(s):  
Wen-Lung Fu ◽  
Lesley M. Wright ◽  
Je-Chin Han
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Aspect Ratio ◽  
Thermal Performance ◽  
Flow Direction ◽  
Aspect Ratios ◽  
Number Ratio ◽  
Buoyancy Parameter ◽  
Rectangular Channels ◽  
Channel Aspect Ratio

This paper experimentally studies the effects of the buoyancy force and channel aspect ratio on heat transfer in two-pass rotating rectangular channels with smooth walls and 45° ribbed walls. The channel aspect ratios include 4:1, 2:1, 1:1, 1:2 and 1:4. Four Reynolds numbers are studied: 5000, 10000, 25000 and 40000. The rotation speed is fixed at 550 rpm for all tests, and for each channel, two channel orientations are studied: 90° and 45° or 135°, with respect to the plane of rotation. Rib turbulators are placed on the leading and trailing walls of the channels at an angle of 45° to the flow direction. The ribs have a 1.59 by 1.59 mm square cross section, and the rib pitch-to-height ratio (P/e) is 10 for all tests. The effects of the local buoyancy parameter and channel aspect ratio on the regional Nusselt number ratio are presented. The results show that increasing the local buoyancy parameter increases the Nusselt number ratio on the trailing surface and decreases the Nusselt number ratio on the leading surface in the first pass for all channels. However, the trend of the Nusselt number ratio in the second pass is more complicated due to the strong effect of the 180° turn. Results are also presented for this critical turn region of the two-pass channels. In addition to these regions, the channel averaged heat transfer, friction factor, and thermal performance are determined for each channel. With the channels having comparable Nusselt number ratios, the 1:4 channel has the superior thermal performance because it incurs the least pressure penalty.

The effect of Brownian motion on the rheological properties of a suspension of non-spherical particles

1972 ◽  
Vol 52 (4) ◽  
pp. 683-712 ◽  
Author(s):  
E. J. Hinch ◽  
L. G. Leal
Keyword(s):  
Brownian Motion ◽  
Shear Flow ◽  
Spherical Particles ◽  
Aspect Ratios ◽  
Bulk Stress ◽  
Strong Shear ◽  
Steady Shear Flow ◽  
Dilute Suspension ◽  
Limiting Case ◽  
Particle Aspect Ratio

The effect of rotary Brownian motion on the rheology of a dilute suspension of rigid spheroids in shear flow is considered for various limiting cases of the particle aspect ratio r and dimensionless shear rate γ/D. As a preliminary the probability distribution function is calculated for the orientation of a single, axisymmetric particle in steady shear flow, assuming small particle Reynolds number. The result for the case of weak-shear flows, γ/D [Lt ] 1, has been known for many years. After briefly reviewing this limiting case, we present expressions for the case of strong shear where (r3 + r−3) [Lt ] γ/D, and for an intermediate regime relevant for extreme aspect ratios where 1 [Lt ] γ/D [Lt ] (r3 + r−3). The bulk stress is then calculated for these cases, as well as the case of nearly spherical particles r ∼ 1, which has not hitherto been discussed in detail. Various non-Newtonian features of the suspension rheology are discussed in terms of prior continuum mechanical and experimental results.

Effect of Temperature Dependent Properties and Particle Shape on Heat Transfer in Plasma Flow

2003 ◽  
Author(s):  
Yuemin Wen ◽  
Milind A. Jog
Keyword(s):  
Flow Field ◽  
Plasma Flow ◽  
Particle Shape ◽  
Flow Direction ◽  
Particle Surface ◽  
Spherical Particles ◽  
Effect Of Temperature ◽  
Far Field ◽  
Constant Property ◽  
Axis Ratio

In this paper, plasma flow over non-spherical particles has been investigated numerically. The conservation equations for mass, momentum, and energy are solved simultaneously using finite volume method. To body-fit the non-spherical particle surface, an adaptive orthogonal grid is generated. The flow field and the temperature distribution are calculated for oblate and prolate particle shapes. A number of particle surface temperatures and far field temperatures are considered and thermophysical property variation is fully accounted for in our model. The shapes are represented in terms of variations in the axis ratio which is defined as the ratio of axis along the flow direction to the axis perpendicular to the flow direction. For oblate shape, axis ratios from 0.4 (disk-like) to 1 (sphere) are used whereas for proate shape, axis ratios of 1 (sphere) to 1.6 (cylinder-like) are used. The computational model is first validated by comparison with results and correlations available in literature for constant property flow. Effects of flow Reynolds number, particle shape, surface and far field temperatures, and variable properties, on the flow field, temperature variations, drag coefficient, and Nusselt number are outlined.

Particle mesh Ewald Stokesian dynamics simulations for suspensions of non-spherical particles

2011 ◽  
Vol 675 ◽  
pp. 297-335 ◽  
Author(s):  
A. KUMAR ◽  
J. J. L. HIGDON
Keyword(s):  
Transport Properties ◽  
Numerical Solutions ◽  
Volume Fraction ◽  
Particle Surface ◽  
Spherical Particles ◽  
Stokesian Dynamics ◽  
Volume Fractions ◽  
Particle Mesh Ewald ◽  
The One ◽  
Many Body Interactions

A particle mesh Ewald (PME) Stokesian dynamics algorithm has been developed to model hydrodynamic interactions in suspensions of non-spherical dicolloidal particles. Dicolloids, which have recently been synthesized by a number of independent research groups (Johnson, van Kats & van Blaaderen (Langmuir, vol. 21, 2005, p. 11510), Mocket al. (Langmuir, vol. 22, 2006, p. 4037), Kim, Larsen & Weitz (J. Am. Chem. Soc., vol. 128, 2006, p. 14374)), consist of two intersecting spheres of varying radii and centre-to-centre separation. One-body resistance tensors and disturbance velocity fields are computed for general linear flows using a superposition of Stokes singularities along the symmetry axis of the dicolloid particles. The coefficients and the locations of the singularities are optimized to minimize the norm of the velocity error on the particle surface. The one-body solution provides all coefficients required for the far-field many-body interactions in the Stokesian dynamics algorithm. These generalize the analytical results for spheres employed in the classic algorithm. Modified lubrication interaction tensors are developed for dicolloids for the singular near-field lubrication interactions. Accuracy of the one-body solutions and two-body generalized Stokesian dynamics solutions are validated by comparison with high-precision numerical solutions computed with the spectral boundary element method of Muldowney & Higdon (J. Fluid Mech., vol. 298, 1995, p. 167). The newly developed PME Stokesian dynamics algorithm was used to study transport properties in dicolloidal suspensions over a range of volume fractions (φ ≤ 0.5). The effects of the degree of anisotropy on the properties of the suspension are discussed. For these mildly anisotropic particles, the transport properties remain close to those of spheres, however certain interesting trends emerge, with non-monotonic viscosity dependence as a function of increasing aspect ratio. The minimum viscosity in concentrated suspensions is lower than that for spheres with equal volume fraction over a range of volume fractions.

Experiments on Vortex Shedding From Reconfigured Flexible Filaments Vibrating in Flow

2019 ◽  
Author(s):  
Jorge Silva-Leon ◽  
Andrea Cioncolini
Keyword(s):  
Aspect Ratio ◽  
Vortex Shedding ◽  
Flow Direction ◽  
Axial Flow ◽  
Reynolds Numbers ◽  
Wake Flow ◽  
Aspect Ratios ◽  
Inclination Angles ◽  
Spanwise Vortex ◽  
Vortex Shedding Frequency

Abstract This paper describes an experimental study of the spanwise vortex shedding frequencies from cantilever flexible filaments which are bent (reconfigured) when exposed to air crossflow. At a reduced velocity of approximately U* = 1500 (based on filament diameter) the filaments started to vibrate in the inline direction. Hot-wire anemometry was thus employed to investigate the wake flow of filaments of three aspect ratios (L/D = 38, 80, and 113) at Reynolds numbers Re < 300. Despite the large relative inclination angles between the filament and the flow direction, the vortex shedding frequency measured along the span of the filaments remained close to those of a cylinder in pure crossflow. Moreover, it was found that as the aspect ratio (axial length) of the filaments was increased, vortex shedding lost coherence towards the free end of the filaments, whereas this was not the case for the shortest aspect ratio filament currently tested. This is thought to be due to the interaction between the crossflow vortex shedding and the axial flow component developing along the wake of the inclined filaments. Through comparisons with stiff inclined wires it was confirmed that the spanwise vortex shedding behaviors observed (frequency and coherence) were not modulated by the motions of the filaments.

DEPENDENCE OF ENDOCYTOSIS CAUSED BY DEPLETION EFFECTS ON THE ASPECT RATIO OF COLLOIDAL PARTICLE

2015 ◽  
Vol 23 (01) ◽  
pp. 49-56 ◽  
Author(s):  
YAN ZENG ◽  
YANHUI LIU ◽  
YINGBING CHEN ◽  
WEI MAO ◽  
LIN HU ◽  
...  
Keyword(s):  
Aspect Ratio ◽  
High Aspect Ratio ◽  
Continuum Model ◽  
Colloidal Particle ◽  
Colloidal Suspensions ◽  
Small Particles ◽  
Aspect Ratios ◽  
Large Particles ◽  
Spherical Colloidal Particle ◽  
Particle Aspect Ratio

In colloidal suspensions containing large and small particles, a peculiar attractive force caused by entropy appears, this force can cause aggregation of large particles. With the concentration of small particles increasing, the large particles can be endocytosed by vesicle. A continuum model is developed to investigate the equilibrium mechanics between a biomembrane and an enveloped colloidal particle with different aspect ratios. The results show that the endocytosis of colloidal particle depends on the aspect ratio of colloidal particle. For a spherical colloidal particle (aspect ratio is zero), the entropy provides sufficient favorable energy to drive its engulfment; however, at a high aspect ratio, the entropy is not sufficient to overcome the resistance from the biomembrane and causes endocytosis.

Heat Transfer in a Rotating Two-Pass Rectangular Channel Featuring Reduced Cross-Sectional Area After Tip Turn (AR=4:1 to 2:1) With Profiled 60 Deg Angled Ribs

2018 ◽  
Author(s):  
Andrew F. Chen ◽  
Chao-Cheng Shiau ◽  
Je-Chin Han ◽  
Robert Krewinkel
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Aspect Ratio ◽  
Cross Section ◽  
Flow Direction ◽  
Rectangular Channel ◽  
Secondary Flows ◽  
Internal Cooling ◽  
First Passage ◽  
Aspect Ratios

The internal cooling channels of an advanced gas turbine blade typically have varying aspect ratios from one pass to another due to the varying thickness of the blade profile. Most of the fundamental internal cooling studies found in the open literature used a fixed aspect ratio for multi-pass channels. Studies on a reduced cross-section and aspect ratio channel are scarce. The current study features a two-pass rectangular channel with an aspect ratio AR = 4:1 in the first pass and an AR = 2:1 in the second pass after a 180 deg tip turn. In addition to the smooth-wall case, ribs with a profiled cross-section are placed at 60 deg to the flow direction on both the leading and trailing surfaces in both passages (P/e = 10, e/Dh ≈ 0.11, parallel and inline). Regionally averaged heat transfer measurement method was used to obtain the heat transfer coefficients on all surfaces within the flow passages. The Reynolds number (Re) ranges from 10,000 to 70,000 in the first passage, and the rotational speed ranges from 0 to 400 rpm. Under pressurized condition (570 kPa), the highest rotation number achieved was Ro = 0.39 in the first passage and 0.16 in the second passage. Rotation effects on both heat transfer and pressure loss coefficient for the smooth and rib-roughened cases are presented. The results showed that the turn induced secondary flows are reduced in an accelerating flow. The effects of rotation on heat transfer are generally weakened in the ribbed case than the smooth case. Significant heat transfer reduction on the tip wall was seen in both the smooth and ribbed cases under rotating condition. A reduced overall pressure penalty was seen for the ribbed case under rotation. Reynolds number effect was found noticeable in the current study. The heat transfer and pressure drop characteristics are sensitive to the geometrical design of the channel and should be taken into account in the design process.

Rheology and shear jamming of frictional ellipses

2018 ◽  
Vol 849 ◽  
pp. 718-740 ◽  
Author(s):  
Martin Trulsson
Keyword(s):  
Aspect Ratio ◽  
Constitutive Relations ◽  
Granular Flows ◽  
Packing Fraction ◽  
Industrial Applications ◽  
Critical Parameters ◽  
Aspect Ratios ◽  
Nematic Ordering ◽  
Interparticle Friction ◽  
Shear Jamming

Understanding and predicting dense granular flows is of importance in geology and industrial applications. Still, most theoretical work has been limited to flows and packings composed of discs or spheres, a narrow subset of all possible packings. To advance our understanding of more realistic flows we here study the granular rheology of ellipses in steady-state flow with a focus on the effects of elongation and interparticle friction. We carry out novel numerical simulations of amorphous granular flows in a shear cell under confining pressure, at constant shear rate and at various aspect ratios. Both frictionless and frictional particles are considered. The various rheological curves follow the semi-empirical constitutive relations previously found for granular flows composed of discs or spheres. At the shear jamming point one finds well-defined packings, all characterised by their own set of critical parameters such as critical packing fraction, effective friction, etc. Packings composed of frictionless or almost frictionless particles are found to have a non-monotonic dependence of the macroscopic friction but a monotonic increase in packing fraction as the aspect ratio increases. For packings composed of particles with high interparticle friction the reverse is found. While frictionless packings are found to be hypostatic (except in the disc limit) frictional packings are remarkably close to the isostaticity point of having three contacts per particle. Both frictional and frictionless packings are found to have an increasing nematic ordering as the aspect ratio increases. The onset of a rolling, rather than sliding, motion between very frictional particles diminish this nematic ordering substantially. These findings put new and previously unknown bounds on the packing ratios and yield criteria for these amorphous packings at shear jamming.

scholarly journals Simulation and Visualization of Flows Laden with Cylindrical Nanoparticles in a Mixing Layer

2018 ◽  
Vol 2018 ◽  
pp. 1-6
Author(s):  
Wenqian Lin ◽  
Peijie Zhang
Keyword(s):  
Shear Rate ◽  
Aspect Ratio ◽  
Flow Direction ◽  
Mixing Layer ◽  
Pseudospectral Method ◽  
Orientation Distribution ◽  
Stokes Number ◽  
Particle Model ◽  
Cylindrical Particles ◽  
Particle Aspect Ratio

The motion of cylindrical particles in a mixing layer is studied using the pseudospectral method and discrete particle model. The effect of the Stokes number and particle aspect ratio on the mixing and orientation distribution of cylindrical particles is analyzed. The results show that the rollup of mixing layer drives the particles to the edge of the vortex by centrifugal force. The cylindrical particles with the small Stokes number almost follow fluid streamlines and are mixed thoroughly, while those with the large Stokes number, centrifugalized and accumulated at the edge of the vortex, are poorly mixed. The mixing degree of particles becomes worse as the particle aspect ratio increases. The cylindrical particles would change their orientation under two torques and rotate around their axis of revolution aligned to the vorticity direction when the shear rate is low, while aligning on the flow-gradient plane beyond a critical shear rate value. More particles are oriented with the flow direction, and this phenomenon becomes more obvious with the decrease of the Stokes number and particle aspect ratio.

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