THE MOTION OF 2 SPHERES FALLING ALONG THEIR LINE OF CENTERS IN A BOGER FLUID

In this experimental investigation the motion of two identical spheres , both accelerating from rest at the same time along the centre Line o f a cylindrical tube filled with a Boger fluid, is determined as a fun ction of time. It is shown that if two spheres are far apart, the wake of the lower sphere induces a higher velocity for the upper sphere. F or not too large distances between the spheres, however, it is found t hat the velocity of the lower sphere increases considerably for decrea sing distance between the spheres. The result of these two effects is that the spheres attract for large distances but separate for small di stances. This behaviour eventually results in a stable distance which is found to be independent of the initial distance. This stable distan ce increases when the Deborah number increases, whereas the presence o f the cylindrical tube wall reduces the stable distance. The observed interactions between the spheres are in sharp contrast to the results of Riddle et al. [J. Non-Newtonian Fluid Mech. 2 (1977) 23-35], who fo und a critical value for the initial distance in several shear-thinnin g fluids. (C) 1998 Elsevier Science B.V. All rights reserved.