摘要:

Buoyancy-driven motion of viscous drops and air bubbles through a vertical capillary with periodic constrictions is studied. Experimental measurements of the average rise velocity of buoyant drops are reported for a range of drop sizes in a variety of two-phase systems. The instantaneous drop shapes at various axial positions within the capillary are also quantitatively characterized using digital image analysis. Periodic corrugations of the capillary wall are found to have a substantial retarding effect on the mobility of drops in comparison with previous experimental results in a straight cylindrical capillary. For systems characterized by small Bond numbers, drop deformations are found to be periodic. In large Bond number systems, however, drop breakup eventually occurs as the drop size is increased beyond a critical limit. The observed mode of breakup is a tail-pinching process similar to that observed by Oibricht and Leal (1983) for pressure-driven motion of low viscosity ratio drops through a sinusoidally constricted capillary. In contrast to their results, however, the same mode of breakup was also observed for systems withO(1) viscosity ratios,

ISSN:0098-6445    

DOI:10.1080/00986449608936525

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The propagator of the unsteady Stokes equation is shown to be dominated by the solution of a purely diffusive equation, whose dispersion coefficient is the viscosity. Pressure plays an indirect role only, by creating instantaneously a steady velocity field which decays slowly in space. Viscosity appears to measure the temporal growth of the second moment of the unsteady Stokes propagator.

ISSN:0098-6445    

DOI:10.1080/00986449608936526

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A concise review is provided of fundamentals of interfacial transport and rheology, which includes simple proofs of the surface divergence theorem and the surface Reynolds transport theorem, needed in the derivations. The results are applied to the prototype example of a dilute emulsion of spherical drops suspended in an immiscible continuous fluid, in the circumstance where the interface separating the two phases possesses its own Theological properties (i.e., surface shear and dilatational viscosities). The low-Reynolds-number hydrodynamics of the emulsion droplets in a general linear flow is analyzed using invariant methods, providing the velocity, pressure and stress fields everywhere and allowing the leading order deformation of the drops to be calculated. The average stress in the dilute emulsion is also obtained to first order in the volume fraction of the dispersed phase. The results suggest that simultaneous measurements of the effective viscosity of the emulsion and the deformation of the drops in a linear shear flow can provide an indirect experimental determination of the surface shear and dilatational viscosities.

ISSN:0098-6445    

DOI:10.1080/00986449608936527

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The linear elastic constants for a dry Weaire-Phelan foam with bidisperse cell-size distribution are computed. This highly ordered structure has eight polyhedral bubbles in the unit cell: two pentagonal dodecahedra and six tetrakaidecahedra with twelve pentagonal faces and two hexagonal faces. Both dodecahedra have equal volume and all of the tetrakaidecahedra have equal volume, but these volumes are different for the bidisperse structures considered here. These volume constraints preserve the cubic symmetry of the Weaire-Phelan structure so that the elastic response is completely characterized by a bulk moduluskfor volume compression and two shear moduli μ1and μ2. The shear moduli can be combined to obtain an effective isotropic shear modulus μ¯, which represents the shear response averaged over all orientations of the foam. For the monodisperse case, μ¯ = 0.8684TV¯,-1/3where TTis surface tension and V is the average bubble volume. For bidisperse structures, p. never differs from the monodisperse case by more than 0.5% and the at always lie within 4% of ft. The pressure inside the dodecahedra is always greater than the pressure inside the tetrakaidecahedra even when the dodecahedra are larger.

ISSN:0098-6445    

DOI:10.1080/00986449608936528

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The relative viscosity of concentrated suspensions of mixtures of rodlike and spherical particles are measured by falling-ball rheometry. The suspensions are well mixed and homogeneous in the sense that the particles are well dispersed and the rods are randomly oriented. For a constant total volume fraction of solids, the addition of spheres to suspensions of rods results in large decrease in the relative viscosity of the suspension. In these experiments the length of the suspended rods is approximately 10 times the diameter of the suspended spheres. Due to this difference in the characteristic sizes of the two types of particles, the spheres may be considered as part of the suspending homogeneous continuum. A simple model based on this physical picture, after Farris [1968], is very successful in predicting the relative viscosity of the mixed suspensions.

ISSN:0098-6445    

DOI:10.1080/00986449608936529

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Fractures are relatively planar discontinuities in rocks induced by the huge internal stresses which are created by the slow but constant motions of the underground masses. Deposition of a single solute in a single fracture is addressed in the limit where the geometrical changes are very slow compared to the average fluid velocity. The deposition fluxes are calculated by means of a finite-difference scheme which is much more efficient than random walks. Examples of deterministic fractures, random but uncorreiated fractures, Gaussian and self-affine fractures are studied for four different values of the Peclet and the Peclet-Damkohler numbers. Some general trends are discussed.

ISSN:0098-6445    

DOI:10.1080/00986449608936530

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A model was developed to describe the elutriation of fines from a spouted bed of pulp fibers, which is a new method to separate fines from fibers. During spouting the pulp fibers are aggregated in fiber floes, which we model as coarse porous spheres. In general, we can distinguish three liquid regions inside and around a coarse particle in a shear flow: bulk liquid, liquid passing through the sphere, and liquid circulating inside it. A system of three ordinary differential equations describing the balance of fines in these regions was solved numerically. A source term was included to account for the release of fines during the dynamic equilibrium between formation and break-up of fiber floes. Comparison between theory and experiment showed that a simplified two-region model, which contains two adjustable parameters, is adequate to describe the elutriation data.

ISSN:0098-6445    

DOI:10.1080/00986449608936531

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The intensity of light transmitted through an evolving suspension of two-dimensional deformable liquid drops in shear-driven or pressure-driven flow within a channel with parallel-sided walls is studied using a numerical method. The rays enter the suspension normal to upper wall, undergo reflections and refractions at the drop interfaces, and either return to the upper wall or leave from the lower wall. The motion and deformation of the drops are computed using a boundary integral method, and the optics is studied using a standard ray-tracing method. Evidence is presented that, in the case of shear-driven flow, a consistent correlation exists between the intensity of light transmitted through the channel, the geometry of the microstructure, and the global rheological properties of the emulsion. In the case of pressure-driven flow, a correlation between the transmitted intensity and the effective viscosity is also observed, but its features are not entirely consistent. These differences are attributed to the profound effect of variable flow rate or presence of a mean pressure drop on the state of the microstructure and accompanying dynamical features of an emulsion.

ISSN:0098-6445    

DOI:10.1080/00986449608936532

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A review is presented of the motion of an isolated, nonspherical particle of general shape settling at small Reynolds numbers through an unbounded quiescent fluid—with a view towards establishing whether or not all particles ultimately attain a unique, time-independent terminal state, independently of their initial orientation and state of motion. Effects of inhomogeneities in internal mass distribution are incorporated into the analysis. Differences are pointed out between gravity and centrifugal settling rates for nonspherical particles. These arise from the tendency of such particles to adopt preferential orientations in a centrifugal field of force owing to variations in field strength over the length of the particle, ft is pointed out that Coriolis forces acting on both the fluid and particle in a centrifuge cause the particles to settle more slowly. Moreover, in the case of spherical particles, the particle path deviates from a purely radial trajectory. Effects of both translational and rotational Brownian motions on the mean settling velocities of submicron particles is discussed, again for generally-shaped particle. A detailed summary of the contents of this paper is provided at its conclusion.

ISSN:0098-6445    

DOI:10.1080/00986449608936533

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The symmetrical sedimentation of a slender solid torus in a quiescent fluid contained in an infinitely long vertical circular cylinder (so that the torus's symmetry axis lies along the cylinder axis) is examined theoretically by making an expansion in terms of a slenderness ratio k. The results for the drag force on the torus, obtained up to terms of orderk2, are compared with the results obtained from the theories of Johnson and Wu (1979) and of O' Neill (1984) and also with the experimental results obtained by Amakaroonet al(1982)

ISSN:0098-6445    

DOI:10.1080/00986449608936534

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor