No abstract

The steady separated flow past a circular cylinder was investigated experimentally. By artificially stabilizing the steady wake, this system was studied up to Reynolds numbers R considerably larger than any previously attained, thus providing a much clearer insight into the asymptotic character of such flows at high Reynolds numbers. Some of the experimental results were unexpected. It was found that the pressure coefficient at the rear of the cylinder remained unchanged for 25 [les ] R [les ] 177, that the circulation velocity within the wake approached a non-zero limit as the Reynolds number increased, and that the wake length increased in direct proportion to the Reynolds number.

The ratio of the effective to the normal diffusivity of a material diffusing within porous solids is less than unity. In the simple theory the porosity and tortuosity, or labyrinth, factors are used to explain the magnitude of this ratio and to account respectively for the reduced cross-sectional area and the increased diffusion distance. However, abnormally large values of the tortuosity factor are obtained from experimentally measured effective diffusivities within pelleted or extruded porous solids. This work is concerned with the quantitative effect of periodic pore constrictions on the effective diffusivity. The pore model assumed for this study is a hyperbola of revolution giving a pore constriction at the vertex of the hyperbola. Solutions to the steady state diffusion equation in a pore of this shape were obtained at various values of p, the ratio of the maximum to the minimum cross-section in the pore. Comparison of the rate of diffusive transport in this pore and an equivalent Cylindrical pore indicates that 6, the ratio of the effective to the normal diffusivity, is about 0.33 at p = 25 for large pores. At the same value of p, 6 would be smaller for dsusion in the Knudsen region.The effective diffusivity D. of a gas being transported within a porous solid by a diffusive mechanism is smaller than the normal diffusion coefficient Db. Efforts to relate the ratio D,/Db quantitatively to the variables characterizing the porous medium can be broadly classified into two types. The more fundamental method begins with the derivation of an expression for the value of D,/Db in dilute suspension of particles of simple geometric shapes. Maxwell (5) considered uniform spheres and Rayleigh (7) infinite cylinders normal to the direction of flow, leading to the following equations, respectively:(1) D, Equation (4) is essentially a defining equation for T . From a geometrical point of view, this form appears reasonable because B alloms for the reduced area for diffusive flow and T accounts for the fact that the actual diffusion path between two points within a porous medium is generally greater than the distance between points. For loose powders and randomly packed glass spheres the labyrinth factors range from 1.42 t o 1.58

The equations describing the flow of a power-law non-Newtonian fluid on a rotating disk have been solved in general form. This makes it possible to calculate how the shape of an initial surface contour will vary with time and to investigate the possibility of producing uniform films by applying the materials to a rapidly spinning disk. It is shown that the latter process, which has potential industrial applications, has a much better chance of succeeding if the fluid is Newtonian than if it is not, in the sense that whereas for a Newtonian substance centrifugation will smooth out irregularities in the surface contour, for a non-Newtonian fluid even an initially uniform film thickness will be deformed by rotating the plate.

A method is presented for the analysis of a reaction between a porous solid and a gaseous reactant where the kinetic expression is linear in the concentration and where appreciable concentration gradients are established in the pore system as a result of diffusive transport rate. Two cases are treated mathematically: a single cylindrical pore initially of uniform diameter and a porous solid initially containing uniform cylindrical pores with random intersections. The mathematical solutions to the latter case are used to interpret the experimental results reported in the literature on the gasification of graphite rods with carbon dioxide. Values of the computed effective diffusivity are an order of magnitude smaller than the bulk diffusivity at the same temperature and pressure.