Spherical Couette flow of a viscoelastic fluid Part I: Experimental study of 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 solutions are used as viscoelastic fluid, and for the sake of comparison glycerin-water solutions are used as the Newtonian fluid. In experiments, measurements of the rotational torque and the velocity profile in the meridional plane of the spherical gap are made. Various transition phenomena of flow modes that are unique to viscoelastic fluids are investigated by flow visualization for a wide range of rotational Reynolds numbers. Experimental results revealed that a roll-cell-like structure (banded radial structure) caused by elastic instability is generated in the polar region, and that it propagates toward the equatorial region when rotation of the inner sphere is increased, resulting in the formation of two distinct regions: the elastic dominant region and the inertial dominant region. Flow modes are classified and the critical Reynolds numbers are obtained for different gap widths. A correlation is obtained for the torque data in the regime before the onset of instability.