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The flow of an idealized granular material consisting of uniform smooth, but nelastic, spherical particles is studied using statistical methods analogous to those used in the kinetic theory of gases. Two theories are developed: one for the Couette flow of particles having arbitrary coefficients of restitution (inelastic particles) and a second for the general flow of particles with coefficients of restitution near 1 (slightly inelastic particles). The study of inelastic particles in Couette flow follows the method of Savage & Jeffrey (1981) and uses an ad hoc distribution function to describe the collisions between particles. The results of this first analysis are compared with other theories of granular flow, with the Chapman-Enskog dense-gas theory, and with experiments. The theory agrees moderately well with experimental data and it is found that the asymptotic analysis of Jenkins & Savage (1983), which was developed for slightly inelastic particles, surprisingly gives results similar to the first theory even for highly inelastic particles. Therefore the ‘nearly elastic’ approximation is pursued as a second theory using an approach that is closer to the established methods of Chapman-Enskog gas theory. The new approach which determines the collisional distribution functions by a rational approximation scheme, is applicable to general flowfields, not just simple shear. It incorporates kinetic as well as collisional contributions to the constitutive equations for stress and energy flux and is thus appropriate for dilute as well as dense concentrations of solids. When the collisional contributions are dominant, it predicts stresses similar to the first analysis for the simple shear case.

Two unequal rigid spheres are immersed in unbounded fluid and are acted on by externally applied forces and couples. The Reynolds number of the flow around them is assumed to be small, with the consequence that the hydrodynamic interactions between the spheres can be described by a set of linear relations between, on the one hand, the forces and couples exerted by the spheres on the fluid and, on the other, the translational and rotational velocities of the spheres. These relations may be represented completely by either a set of 10 resistance functions or a set of 10 mobility functions. When non-dimensionalized, each function depends on two variables, the non-dimensionalized centre-to-centre separation s and the ratio of the spheres’ radii λ. Two expressions are given for each function, one a power series in s−1 and the other an asymptotic expression valid when the spheres are close to touching.

The stress tensor in a granular shear flow is calculated by supposing that binary collisions between the particles comprising the granular mass are responsible for most of the momentum transport. We assume that the particles are smooth, hard, elastic spheres and express the stress as an integral containing probability distribution functions for the velocities of the particles and for their spatial arrangement. By assuming that the single-particle velocity distribution function is Maxwellian and that the spatial pair distribution function is given by a formula due to Carnahan & Starling, we reduce this integral to one depending upon a single non-dimensional parameter R: the ratio of the characteristic mean shear velocity to the root mean square of the precollisional particle-velocity perturbation. The integral is evaluated asymptotically for R [Gt ] 1 and R [Lt ] 1 and numerically for intermediate values. Good agreement is found between the stresses measured in experiments on dry granular materials and the theoretical predictions when R is given the value 1·7. This case is probably the one for which the present analysis is most appropriate. For moderate and large values of R, the theory predicts both shear and normal stresses that are proportional to the square of the particle diameter and the square of the shear rate, and depend strongly on the solids volume fraction. A provisional comparison is made between the stresses predicted in the limit R → ∞ and the experimental results of Bagnold for shear flow of neutrally buoyant wax spheres suspended in water. The predicted stresses are of the correct order of magnitude and yield the proper variation of stress with concentration. When R [Lt ] 1, the shear stress is linear in the shear rate, and the analysis can be applied to shear flow in a fluidized bed, although such an application is not developed further here.

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Two standard physics problems are solved in terms of the Lambert ¡ function, in order to show the applicability of this recently defined function to physics. Other applications of the function are cited, but not described. The problems solved concern Wien's displacement law and the fringing fields of a capacitor, the latter problem being representative of some problems solved using conformal transformations. The physical content of the solutions remains unchanged, but they gain a new elegance and convenience.

Experimental and theoretical work on the rheological properties of suspensions are reviewed. Attention is focused on systems consisting of rigid, neutrally buoyant particles suspended in Newtonian fluids; no restrictions, however, are placed on the concentration of the particles or on the forces acting in the suspension. The assumption that an effective viscosity depending solely on the volume fraction of the particles suffices to describe the rheology of suspensions is examined and shown to be inadequate. Indeed, the experimental evidence strongly supports the view that suspensions behave macroscopically as non-Newtonian fluids whose rheological properties are influenced by a large number of factors; these factors are listed. The various theories that have been put forward to explain the flow of suspensions are discussed, with particular emphasis being placed on the nature of the approximations made, so that purely empirical formulas can be clearly separated from those having a theoretical basis. Suggestions for future work, both theoretical and experimental, are provided. D. J. JEFFREY and ANDREAS ACRIVOS Department of Chemical EngineeringStanford University Stanford, California 94305 SCOPEA well-known and long-standing problem in fluid mechanics has been the calculation of the effective viscosity of a suspension. In recent years it has become clear that many of the complex phenomena associated with a flowing suspension cannot be explained by using a classical Newtonian description of a fluid with an effective viscosity. Thus, suspensions have to be treated as nowNewtonian fluids whose rheological (flow) properties are influenced by a large number of variables. This review presents some of the reasons why such non-Newtonian behavior occurs and describes the variables that must be included in any proposed theory for such behavior. The discussion is restricted to suspensions of rigid, neutrally buoyant particles in Newtonian fluids and thereby excludes emulsions, reinforced plastics, etc., but otherwise no restrictions are placed on the scope of the review. CONCLUSIONS AND SIGNIFICANCEThe volume fraction of the particles in a suspension (volume occupied by particles per unit volume of suspension) has often been assumed to be the only variable that influences the observed rheological properties of the suspension. Experimental evidence is presented to show that this is incorrect and that other factors, such as the shape and size distribution of the particles, the presence of electrical charges, and the type of flow being experienced must be considered. It readily follows then that no formula can exist that will give the effective viscosity or other flow properties solely as a function of the volume fraction of the particles. Of the many expressions that have been advanced for predicting the dependence of the flow properties on the factors referred to above, some are empirical while others rest on sound theoretical foundations. Successful empirical formulas generally contain one or more adjustable parameters which m...

The conduction of heat (or electricity) through a stationary random suspension of spheres is studied for a volume fraction of the spheres (c) which is small. The work of Maxwell (1873) is extended to calculate the flux of heat exactly to order c 2 by using the method of Batchelor (1972), which reduces the problem to a consideration of interactions between pairs of spheres while avoiding the usual convergence difficulties. The result depends on the way in which pairs of spheres are distributed with respect to each other; for the case of all possible pair configurations being equally probable the coefficient of c 2 is found explicitly for all values of the ratio of conductivities of the two phases. The results also apply to permittivities and permeabilities of suspensions.