ISSN:0899-8213    

DOI:10.1063/1.3480472

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The motion of a spherical particle (with radius 1 to 6 mm) in a viscous fluid is measured using laser interferometry. The typical sensitivity on the measured displacement of the sphere is of the order of 50 nm. The particle is moving on the axis of a closed cylinder. The hydrodynamic interactions between the particle and the walls of the cylinder are compared with the theoretical result of Sano [J. Phys. Soc. Jpn.56, 2713 (1987)] valid for a very small sphere. The agreement is excellent for the smallest sphere used in the experiment. The experiment also agrees with the result from the theory of lubrication when the sphere is close to a plane end wall. The effect of the particle roughness appears at small distances. Laser interferometry appears as a useful tool to study particle–wall hydrodynamic interactions when the geometry is cumbersome.

ISSN:0899-8213    

DOI:10.1063/1.858787

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The translational and rotational velocities of a sphere rolling down an inclined plane at low Reynolds number were measured as a function of the angle of inclination of the plane. Both 140 &mgr;m glass and 6350 &mgr;m diam acrylic particles were used. A theoretical model including the effects of particle surface roughness was developed, and is in quantitative agreement with the measurements. In addition, statistically significant velocity fluctuations were measured and are explained by variation in the coefficient of friction of the sphere. The disagreement between the measurements of Carty (B.S. thesis, Massachusetts Institute of Technology, 1957) and the theory of Goldmanetal. [Chem. Eng. Sci.22, 637 (1967)] is explained.

ISSN:0899-8213    

DOI:10.1063/1.858799

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Experimental results are presented for the Stokes velocity of a circular cylinder of radiusaand lengthLmoving axially through a viscous fluid contained within a coaxial closed cylindrical tube of radiusband lengthH. The results cover the intermediate range 0.21 ≤a/b≤0.70 and are found to be consistent with theoretical results for the narrow gap case (a&bartil;b) and with previous experimental results for the wide gap case (a≪b). The fractional difference between the observed drag and the theoretical drag is represented well by 1.67x−0.017, wherex=(b−a)(a/b)1/2/L. The negative term is interpreted as representing an effective cylinder length of 0.983L, and the 1.67xterm is interpreted qualitatively as a measure of the degree to which the fluid is displaced radially by the falling cylinder. The limitations of our expression for end effects are explored experimentally. Additional experimental results are presented for two cases in which the ends of the cylindrical tube are not closed.

ISSN:0899-8213    

DOI:10.1063/1.858781

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The dynamics of a one‐dimensional granular medium has a finite time singularity if the number of particles in the medium is greater than a certain critical value. The singularity (‘‘inelastic collapse’’) occurs when a group of particles collides infinitely often in a finite time so that the separations and relative velocities vanish. To avoid the finite time singularity, a double limit in which the coefficient of restitutionrapproaches 1 and the number of particlesNbecomes large, but is always below the critical number needed to trigger collapse, is considered. Specifically,r→1 withN∼(1−r)−1. This procedure is called the ‘‘quasielastic’’ limit. Using a combination of direct simulation and kinetic theory, it is shown that a bimodal velocity distribution develops from random initial conditions. The bimodal distribution is the basis for a ‘‘two‐stream’’ continuum model in which each stream represents one of the velocity modes. This two‐stream model qualitatively explains some of the unusual phenomena seen in the simulations, such as the growth of large‐scale instabilities in a medium that is excited with statistically homogeneous initial conditions. These instabilities can be either direct or oscillatory, depending on the domain size, and their finite‐amplitude development results in the formation of clusters of particles.

ISSN:0899-8213    

DOI:10.1063/1.858896

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Tracer dispersion in double porosity packings of porous grains obtained by grinding sintered glass samples of various porosities and permeabilities is studied experimentally. The dispersivitylDincreases faster with velocity than for single porosity packings: this variation is predicted by modeling the medium as a one‐dimensional (1‐D) sequence of identical cells with one slow and one fast path in parallel. The grain size and the permeability contrast between the inside and the outside of the grains can also be determined. At low grain porosities, the transit time &tgr;iinsideindividualgrains becomes of the order of the meantransittimeacross thewholesample and non‐Gaussian dispersion curves are observed.

ISSN:0899-8213    

DOI:10.1063/1.858788

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The spreading of a localized distribution of surfactant on a thin viscous film is considered, in the situation in which the surfactant is soluble in the bulk layer and the boundary beneath the fluid is impermeable to surfactant. The surfactant distribution is controlled by advection and diffusion both at the surface of the film, where the surfactant forms a monolayer, and in the bulk. The bulk and surface surfactant concentrations are related by linearized sorption kinetics. The surfactant diffuses rapidly across the thin fluid layer, and lubrication theory is used to derive evolution equations for the film height and the surface and cross‐sectionally averaged bulk surfactant concentrations. A special case of the governing equations describes the Marangoni flow induced by a locally hot region of the layer. It is shown that in comparison to the spreading of insoluble surfactant, transient desorption of surfactant from the monolayer to the bulk causes the spreading rate to diminish, although once the bulk and surface concentrations are locally in equilibrium, film deformations are more severe, with a sharp pulse in the film height created just upstream of the leading edge of the surfactant distribution.

ISSN:0899-8213    

DOI:10.1063/1.858789

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The effect of dilute, insoluble surfactant on the deformation and breakup of a viscous drop is examined. Two cases are considered: the deformation and stretching of a drop in a uniaxial extensional flow and the surface‐tension‐driven motion of an elongated drop in a quiescent fluid. Aside from rescaling the mean capillary force through an average decrease in the interfacial tension, surfactants alter the motion of a viscous drop through gradients in interfacial tension. The effects of surfactants are found to be most pronounced for small viscosity ratios, where Marangoni stresses substantially retard the interfacial velocity and cause the drop to behave as though it were more viscous. Surfactants are found to facilitate the formation of pointed ends during drop stretching, and this may explain the observation of tip streaming in experiments with viscoelastic drops. Surfactant gradients also allow drops to be elongated to a larger degree without producing end pinching.

ISSN:0899-8213    

DOI:10.1063/1.858790

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Droplet patterns have been generated in a vacuum environment by capillary stream breakup initiated by a new form of applied disturbances. The method of producing droplet streams with flexibly controllable patterns is presented. The patterns are periodic sequences of droplets, characterized by specific drop‐to‐drop separations and diameters. The pattern repeats until the disturbance to the capillary stream is removed or altered. An important feature of this work is that the patterns are deterministic given knowledge of the characteristics of the disturbance waveform. A model has been developed that is based on conservation of momentum and linear theory, and predicts droplet stream configurations with excellent agreement with experiment. Use of spectral analysis demonstrates long‐term periodicity and stability of the droplet patterns as well as the excellent agreement between the predicted droplet patterns and those experimentally obtained.

ISSN:0899-8213    

DOI:10.1063/1.858791

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The capillary instability of a thin liquid layer on a cylinder is studied. Using an integral approach and lubrication approximation, transport equations governing the spatiotemporal evolution of a film thickness and the temperature along the film are obtained. Evolution of the system under both isothermal and nonisothermal conditions is studied numerically. It is shown that nonlinear interaction of the linearly unstable modes begets an additional mode with a wavelength equal to that of the fastest growing wave. This, in turn, causes the formation of satellite drops along with the main ones. Application of these results in a possible continuous technology of high‐temperature superconductor wire fabrication is discussed.

ISSN:0899-8213    

DOI:10.1063/1.858792

出版商:AIP    

年代:1993

数据来源: AIP