Motion and wake structure of spherical particles

This paper presents results from a flow visualization study of the wake structures behind solid spheres rising or falling freely in liquids under the action of gravity. These show remarkable differences to the wake structures observed behind spheres held fixed. The two parameters controlling the rise or fall velocity (i.e., the Reynolds number) are the density ratio between sphere and liquid and the Galileo number.Comment: 9 pages, 8 figures. Higher resolution on demand. To appear in Nonlinearity January 200

An experimental study of the regimes of motion of spheres falling or ascending freely in a Newtonian fluid

This paper presents the results of an experimental investigation aimed at verifying some of the interesting conclusions of the numerical study by Jenny et al. concerning the instability and the transition of the motion of solid spheres falling or ascending freely in a Newtonian fluid. The phenomenon is governed by two dimensionsless parameters: the Galileo number G, and the ratio of the density of the spheres to that of the surrounding fluid ρs/ρ. Jenny et al. showed that the (G, ρs/ρ) parameter space may be divided into regions with distinct features of the trajectories followed eventually by the spheres after their release from rest. The characteristics of these ‘regimes of motion’ as described by Jenny et al., agree well with what was observed in our experiments. However, flow visualizations of the wakes of the spheres using a Schlieren optics technique raise doubts about another conclusion of Jenny et al., namely the absence of a bifid wake structure

Freely rising light solid spheres

This paper examines the behavior of spheres rising freely in a Newtonian fluid when the ratio between the density of the spheres and that of the surrounding fluid is about 0.02. High-speed imaging is used to reconstruct three-dimensional trajectories of the rising spheres. From the analysis of the trajectories the magnitudes of the drag and lift forces exerted by the surrounding fluid are deduced. It is argued that the two main contributions to the drag force are (i) a viscous drag that may be estimated from the standard drag curve by evaluating the Reynolds number using the actual value of the velocity, and (ii) an inertial drag that arises essentially by the same mechanisms that cause the lift-induced drag familiar from wing theory. Estimates of both contributions, the latter using visualizations of the wakes of the spheres, give a favorable agreement with the measured drag forces. These findings are closely related to recent numerical results of in the literature on the forces experienced by oblate ellipsoidal bubbles rising in quiescent water