SPHERICAL COUETTE-FLOW OF A VISCOELASTIC FLUID .1. EXPERIMENTAL-STUDYOF THE INNER-SPHERE ROTATION

Flow of viscoelastic fluid contained between a stationary outer sphere and a rotating inner sphere is studied experimentally. In the present investigation, relatively low-concentration polyacrylamide-water solu tions are used as viscoelastic fluid, and for the sake of comparison g lycerin-water solutions are used as the Newtonian fluid. In experiment s, measurements of the rotational torque and the velocity profile in t he meridional plane of the spherical gap are made. Various transition phenomena of flow modes that are unique to viscoelastic fluids are inv estigated by flow visualization for a wide range of rotational Reynold s numbers. Experimental results revealed that a roll-cell-like structu re (banded radial structure) caused by elastic instability is generate d in the polar region, and that it propagates toward the equatorial re gion when rotation of the inner sphere is increased,resulting in the f ormation of two distinct regions: the elastic dominant region and the inertial dominant region. Flow modes are classified and the critical R eynolds numbers are obtained for different gap widths. A correlation i s obtained for the torque data in the regime before the onset of insta bility. (C) 1997 Elsevier Science B.V.