HYDRODYNAMICS OF A VERTICALLY FALLING THIN CYLINDER IN NON-NEWTONIAN FLUIDS

The effect of the shear-thinning characteristics of non-Newtonian fluids on the hydrodynamics of a vertically falling thin cylinder in a circular container was investigated. The effects of the aspect ratio, the diameter ratio of the cylinder to the container, and the density of the cylinder were also studied. Glycerin was used as the Newtonian fluid. A neutralized Carbopol 934 (1200 ppm by wt.) solution was used as a purely viscous non-Newtonian fluid, and a polyacrylamide (Separan AP-273 1000 ppm by wt.) solution was used as a viscoelastic non-Newtonian fluid. The final orientation of the thin cylinder in the polyacrylamide solution was vertical, regardless of the initial launching orientation if the concentration was greater than 200 ppm. In contrast, the thin cylinder fell horizontally both in glycerin and in the Carbopol solution, regardless of the initial launch orientation of the cylinder. The dimensionless terminal velocity increased in all test fluids as the aspect ratio increased. In the polyacrylamide solution it increased more rapidly with increasing aspect ratio than in the Carbopol solution. The maximum value of the dimensionless terminal velocity in glycerin and in the Carbopol solution was two. The corresponding maximum value in the polyacrylamide solution was 6.7, which was attributed to the preferred orientation of the long-chain molecules of polyacrylamide along the direction of the falling cylinder. The predictions of the drag coefficient of the thin cylinders based on the inelastic modeling gave good agreement with experimental data in the Carbopol solution when the aspect ratio was greater than 50. However, in the polyacrylamide solution the inelastic modelling showed a relatively large percentage deviation even at a large aspect ratio.