摘要:

When a natural porous formation is viewed from an Eulerian perspective, incomplete characterization of the hydraulic conductivity leads to nonlocality in the constitutive theory, irrespective of whether the medium has evolving heterogeneity with velocity fluctuations over all scales. Within this framework two nonlocal constitutive models are developed for natural systems, one for conservative tracers and the other for reactive chemicals undergoing linear nonequilibrium sorption with deterministic rate constants. Exact solutions for the mean concentrations are obtained and used to determine the mean values of the spatial moments up to the third. A Lagrangian model (Dagan and Cvetkovic, 1993) for a similar problem is reviewed and comparisons are made between the expected Lagrangian and Eulerian moments. If the local-scale dispersive process is neglected in the Eulerian analysis, then the Lagrangian moments obtain. However, if the local-scale dispersive process is included in the Eulerian model, then the second transverse moment disagrees with that obtained through the Lagrangian analysis. It is shown that this disagreement is especially acute in the asymptotic limits.

ISSN:0098-6445    

DOI:10.1080/00986449608936505

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

This article discusses the validity of describing single-breath, gas or aerosol bolus transport processes in terms of effective Gaussian characteristics. This is done in the context of a comparison between the predictions of a recent macrotransport model or pulmonary transport phenomena and predictions based upon the numerical solution of a so-called ‘exact’ microtransport problem, in addition, comparison is made with the predictions of a former network model of convective dispersion in the conducting airways of the lung. For conditions of physiological interest, the macrotransport model is shown to predict to within reasonable accuracy the mode (i.e., time of exit of the peak of an inspired bolus) as well as the dispersion of gas or aerosol for the case of an impulsive introduction of the gas/aerosol bolus (varying the shape and size of the bolus). The relative merits of micro- and macroscopic descriptions of lung transport problems are discussed. Comparisons with experimental data, as well as a discussion of the implications of the present study on the future development of transport models of lung dispersion, are provided.

ISSN:0098-6445    

DOI:10.1080/00986449608936506

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Generalized Taylor dispersion theory (GTDT) is extended with the goal of obtaining a macroscale description of the transport processes in cases of time-periodic zero-mean convection in a homogeneous shear flow under the action of a spatially-uniform force field. The resulting scheme is uniformly valid throughout the frequency spectrum provided that particle hydrodynamic properties may be obtained from the solution of appropriate steady Stokes problems.

ISSN:0098-6445    

DOI:10.1080/00986449608936507

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

This paper reports the results of simulations of the motion of dense suspensions of magnetotactic bacteria, bacteria that contain small magnetic dipoles. These simulations build on a previously described mechanism in which pair-wise, far-field, hydrodynamic interactions between swimming bacteria are proposed to be responsible for the band structure that appears when such bacteria are oriented in a given direction by an external magnetic field. Previous work has shown that these hydrodynamic forces act in a direction and with a magnitude that are fully consistent with the observed behavior. Applying this pair-wise, hydrodynamic force mechanism to hundreds of bacteria in a numerical simulation shows that band formation and stability is predicted by this simple model, while the precise band shape appears to depend on effects not included in the simulation.

ISSN:0098-6445    

DOI:10.1080/00986449608936508

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The fluctuations of continuous elastic bodies are studied, applying the equipartition principle of energy to each normal mode of vibration. The mean square displacement of the particles from their equilibrium configurations is expressed through an infinite series, which is always converging, provided that the elasticity condition is satisfied even at high modes of vibration.

ISSN:0098-6445    

DOI:10.1080/00986449608936509

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The affect of the Reynolds number on the characteristics of the coherent structures in a flume was studied for Reynolds numbers Reθ= 300,470,690 and ratio of channel width to water depth from 6.4 to 3.4.

ISSN:0098-6445    

DOI:10.1080/00986449608936510

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

We have measured the hindered mobility of microscopic spheres {7 to 15 μm diameter) suspended in aqueous solutions very near a wall (0-500 nm) using the technique of total internal reflection microscopy. The elevation of the sphere above the plate is monitored by measuring the intensity of light that is scattered by the sphere when illuminated by an evanescent wave which penetrates about 500 nm into the solution. The mobility is determined by using a laser beam to apply a known radiation force to the same Brownian particle several hundred times and measuring the resulting ensemble-average change in intensity as a function of time due to that force. The intensity of scattered light can be translated into an absolute separation distance, thus enabling simultaneous, independent measurements of the mobility and elevation of the sphere. When the sphere comes within a few radii of the wall, its mobility is expected to be significantly hindered owing to hydrodynamic interactions with the wall. Results show excellent agreement with Brenner's analysis for the hindered mobility of a rigid sphere near a planar surface: for separation distances from 2% to 10% of the sphere's radius, the measured mobilities ranged from 2% to 10% of the Stokes value far from the wall.

ISSN:0098-6445    

DOI:10.1080/00986449608936511

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Relations between interparticle effective interactions, structure formation, stability and sedimentation for a colloidal system are presented in this paper. For a binary mixture of large and small particles, the potential of the mean forces between large particles is obtained from the Ornstein-Zernike equation. We incorporated the small particles in our numerical simulations by using this potential of the mean force as the interparticle effective interaction. Our numerical results reveal the phenomenon of strong particle aggregation due to the attractive depletion force exerted by small particles. In the absence of the effect of gravity, this aggregation can result in flocculation and the formation of particle clusters thereby forming “void” structures, while under the influence of gravity, the aggregation can greatly affect the sedimentation rates. An analytical expression relating the aggregation number to the sedimentation velocity is presented. Our sedimentation experiments with a bidisperse latex suspension as well as clay particle dispersions show both the destabilizing and stabilizing effects of small particles, which are in qualitative agreement with our theoretical predictions.

ISSN:0098-6445    

DOI:10.1080/00986449608936512

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Axisymmetric viscous flow in a conical channel is considered for geometrical systems in which the cross section is multiply-connected. The volumetric flux rates are compared for a 2-flow region and a 1 -flow region of equal cross sectional area, and same mean radial pressure gradient. It is found that for various configurations of the 2-flow system there can be both flux rate enhancement and reduction. As part of an introduction some similar results are presented for Poiseuille flow, and flow between non parallel plane walls the former of which satisfies the exact Navier-Stokes equations.

ISSN:0098-6445    

DOI:10.1080/00986449608936513

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Few exact solutions of the Stokes equations are known, even for steady or quasi-steady flows, involving finite sized bodies, and numerical techniques generally have to be resorted to for finding solutions. However, quite effective modelling of flows involving complicated boundary geometries is possible using the three-dimensional Stokeslet and rotlet point singularities. Two problems are studied in detail. In the first example, exact solutions for the three-dimensional Stokeslet and rotlet placed axisymmetrically along the axis of a circular disc are found and combined with Brenner's first order interaction formulae to determine the effect of the presence of the disc on the force and torque acting on a particle whose dimensions are small compared with its distance from the disc. The results are compared with those of a full numerical integration of the Stokes equations for a sphere translating towards a disc. In the second example, Brenner's first order wall correction theory is applied to the motion of a particle in a circular cylinder using the exact solutions for a torus translating or rotating in isolation. The theoretical predictions for the drag on a torus settling symmetrically in a circular cylinder are compared with those determined experimentally.

ISSN:0098-6445    

DOI:10.1080/00986449608936514

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor