scholarly journals Columnar structure formation of a dilute suspension of settling spherical particles in a quiescent fluid

2016 ◽  
Vol 1 (7) ◽  
Author(s):  
Sander G. Huisman ◽  
Thomas Barois ◽  
Mickaël Bourgoin ◽  
Agathe Chouippe ◽  
Todor Doychev ◽  
...  
Keyword(s):  
Structure Formation ◽  
Columnar Structure ◽  
Spherical Particles ◽  
Dilute Suspension ◽  
Quiescent Fluid

scholarly journals Sedimentation of finite-size spheres in quiescent and turbulent environments

2016 ◽  
Vol 788 ◽  
pp. 640-669 ◽  
Author(s):  
Walter Fornari ◽  
Francesco Picano ◽  
Luca Brandt
Keyword(s):  
Turbulent Flow ◽  
Settling Velocity ◽  
Solid Phase ◽  
Density Ratio ◽  
Finite Size ◽  
Spherical Particles ◽  
Volume Fractions ◽  
Turbulent Environments ◽  
The Mean ◽  
Quiescent Fluid

Sedimentation of a dispersed solid phase is widely encountered in applications and environmental flows, yet little is known about the behaviour 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 ${\it\phi}=0.5{-}1\,\%$, while the solid to fluid density ratio ${\it\rho}_{p}/{\it\rho}_{f}=1.02$. The particle radius is chosen to be approximately six Kolmogorov length scales. The results show that the mean settling velocity is lower in an already turbulent flow than in a quiescent fluid. The reductions with respect to a single particle in quiescent fluid are approximately 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 with 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 behaviour observed in the quiescent fluid. Using the concept of mean relative velocity we estimate the mean drag coefficient from empirical formulae and show that non-stationary effects, related to vortex shedding, explain the increased reduction in mean settling velocity in a turbulent environment.

The effect of Brownian motion on the bulk stress in a suspension of spherical particles

1977 ◽  
Vol 83 (1) ◽  
pp. 97-117 ◽  
Author(s):  
G. K. Batchelor
Keyword(s):  
Brownian Motion ◽  
Probability Density ◽  
Volume Fraction ◽  
Joint Probability ◽  
Small Degree ◽  
Spherical Particles ◽  
Stress System ◽  
Bulk Stress ◽  
Dilute Suspension ◽  
Thermodynamic Forces

The effect of Brownian motion of particles in a statistically homogeneous suspension is to tend to make uniform the joint probability density functions for the relative positions of particles, in opposition to the tendency of a deforming motion of the suspension to make some particle configurations more common. This smoothing process of Brownian motion can be represented by the action of coupled or interactive steady ‘thermodynamic’ forces on the particles, which have two effects relevant to the bulk stress in the suspension. Firstly, the system of thermodynamic forces on particles makes a direct contribution to the bulk stress; and, secondly, thermodynamic forces change the statistical properties of the relative positions of particles and so affect the bulk stress indirectly. These two effects are analysed for a suspension of rigid spherical particles. In the case of a dilute suspension both the direct and indirect contributions to the bulk stress due to Brownian motion are of order ø2, where ø([Lt ] 1) is the volume fraction of the particles, and an explicit expression for this leading approximation is constructed in terms of hydrodynamic interactions between pairs of particles. The differential equation representing the effects of the bulk deforming motion and the Brownian motion on the probability density of the separation vector of particle pairs in a dilute suspension is also investigated, and is solved numerically for the case of relatively strong Brownian motion. The suspension has approximately isotropic structure in this case, regardless of the nature of the bulk flow, and the effective viscosity representing the stress system to order ϕ2 is found to be \[ \mu^{*} = \mu(1+2.5\phi + 6.2\phi^2). \] The value of the coefficient of ø2 for steady pure straining motion in the case of weak Brownian motion is known to be 7[sdot ]6, which indicates a small degree of ‘strain thickening’ in the ø2-term.

Transverse shear-induced gradient diffusion in a dilute suspension of spheres

1998 ◽  
Vol 357 ◽  
pp. 279-287 ◽  
Author(s):  
Y. WANG ◽  
R. MAURI ◽  
A. ACRIVOS
Keyword(s):  
Three Dimensional ◽  
Spherical Particles ◽  
Particle Interactions ◽  
Simple Shearing ◽  
Ambient Flow ◽  
Gradient Diffusion ◽  
Areal Fraction ◽  
Dilute Suspension ◽  
Neutrally Buoyant ◽  
Shearing Motion

We study the shear-induced gradient diffusion of particles in an inhomogeneous dilute suspension of neutrally buoyant spherical particles undergoing a simple shearing motion, with all inertia and Brownian motion effects assumed negligible. An expansion is derived for the flux of particles due to a concentration gradient along the directions perpendicular to the ambient flow. This expression involves the average velocity of the particles, which in turn is expressed as an integral over contributions from all possible configurations. The integral is divergent when expressed in terms of three-particle interactions and must be renormalized. For the monolayer case, such a renormalization is achieved by imposing the condition of zero total macroscopic flux in the transverse direction whereas, for the three-dimensional case, the additional constraint of zero total macroscopic pressure gradient is required. Following the scheme of Wang, Mauri & Acrivos (1996), the renormalized integral is evaluated numerically for the case of a monolayer of particles, giving for the gradient diffusion coefficient 0.077γa2c¯2, where is the applied shear rate, a the radius of the spheres and c¯ their areal fraction.

The Plane Parallel Flow of a Density-Stratified Fluid With and Without a Suspension of Particles

1973 ◽  
Vol 40 (2) ◽  
pp. 326-330 ◽  
Author(s):  
J. Val. Healy
Keyword(s):  
Particle Density ◽  
Infinite Plate ◽  
Axial Flow ◽  
Wall Stress ◽  
Spherical Particles ◽  
Uniform Density ◽  
Asymptotic Results ◽  
Infinite Region ◽  
Dilute Suspension ◽  
Long Cylinder

The response of a viscous incompressible nondiffusive fluid containing a dilute suspension of small spherical particles and occupying a semi-infinite region bounded by an infinite plate, when the latter is slowly moved parallel to itself in a prescribed manner normal to the direction of stratification, is studied. In particular, when the system density decays exponentially with distance from the plate, the problem is equivalent to the axial flow of the same system in a long tube. When the particle density vanishes, the problem reduces to the unsteady diffusion of vorticity in a long cylinder, with the “diffusivity” a function of the stratification length, whose solution is well known. A degeneracy occurs in the solution as the density approaches uniformity and this is examined using asymptotic expansions and some exact and asymptotic results for an impulsively moved plate are given. It is found that, for a given stratification, the ratio of the wall stress with particles to that without, increases first and then, after the non-equilibrium overshoot, it continues to increase with time; this is in contrast to the case of uniform density, when the ratio goes asymptotically to a constant value. When the particle density vanishes and the kinematic viscosity of the fluid goes to infinity at very large distances from the plate, the fluid there acquires, in a finite time, a finite velocity that strongly depends on the stratification length.

On the Interaction of Insoluble Spherical Particles with a Solidifying Interface in the Presence of Morphological Instabilities

1998 ◽  
Vol 529 ◽  
Author(s):  
L. Hadji ◽  
A.M.J. Davis
Keyword(s):  
Stability Analysis ◽  
Linear Stability ◽  
Binary Alloy ◽  
Linear Stability Analysis ◽  
Control Parameter ◽  
Critical Value ◽  
Planar Interface ◽  
Unidirectional Solidification ◽  
Spherical Particles ◽  
Dilute Suspension

AbstractThe interaction of a dilute suspension of spherical particles with a moving solidifying front is investigated analytically. The front emerges during the unidirectional solidification of a dilute binary alloy. A linear stability analysis of the planar interface reveals that the presence of the particles has a destabilizing effect, i.e. the critical value of the control parameter, taken here to be the concentration faraway from the interface, for the onset of the Mullins-Sekerka instability is lowered due to the particles' presence.

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