Computational Insights into Unconfined Flow of a Non-Newtonian Power-Law Fluid past Counter-Rotating Circular Cylinders: Implications for Rotating Process Equipment

This study numerically examines the steady unconfined laminar flow of incompressible non-Newtonian power-law fluids past a pair of side-by-side counter-rotating circular cylinders using the finite element method. The cylinders simultaneously rotate at equal angular speeds in opposite directions, with the upper cylinder (UC) rotating clockwise and the lower cylinder (LC) counter-clockwise. The numerical simulations are performed over wide parameter ranges: power-law index (0.2 ≤ n ≤ 1.8), rotational rate (0 ≤ α ≤ 2), gap ratio (0.2 ≤ G ≤ 1), and Reynolds number (1 ≤ Re ≤ 40). The detailed influence of these flow parameters on key flow characteristics, including streamlines, surface pressure, centerline velocity, and individual as well as total drag and lift coefficients, is systematically analyzed. For stationary cylinders (α = 0), a twin-vortex structure forms in the wake, which progressively vanishes with increasing rotation (α = 2). The surface pressure coefficient attains its maximum values for shear-thickening fluids (n > 1) at higher rotational speeds, irrespective of the gap ratio. The total drag coefficient (CD) decreases with increasing α and n, whereas the lift coefficient (CL) exhibits a more intricate dependence on the flow parameters. The individual components of the drag and lift coefficients exhibit complex and coupled dependences on the governing parameters. The results provide fundamental insights into the interplay among rotation, non-Newtonian rheology, and geometric confinement in determining the hydrodynamic behavior of rotating cylinder systems. Relative to the stationary Newtonian configuration, cylinder rotation leads to substantial wake suppression and drag reduction under favorable combinations of rotational rate and rheology, while the least favorable conditions exhibit only marginal changes in drag. The lift coefficient shows a non-monotonic response due to rotation-induced pressure asymmetry.