Unconfined flow of a non-Newtonian power-law fluid past counter-rotating circular cylinders

TL;DR

This paper numerically investigates how non-Newtonian power-law fluids flow past counter-rotating cylinders, analyzing the effects of fluid rheology, rotation speed, and gap ratio on flow characteristics and forces.

Contribution

It provides a comprehensive numerical analysis of unconfined laminar flow of non-Newtonian fluids around rotating cylinders, exploring parameter effects on flow and force coefficients.

Findings

Twin-vortex wake structure for stationary cylinders

Surface pressure peaks for shear-thickening fluids at high rotation

Drag coefficient decreases with rotation and rheology index

Abstract

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) counterclockwise. The numerical simulations are performed over wide parameter ranges: power-law index ($0.2 \le n \le 1.8$), rotational rate ($0 \le \alpha \le 2$), gap ratio ($0.2 \le G \le 1$), and Reynolds number ($1 \le Re \le 40$). 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 ($\alpha = 0$), a twin-vortex structure forms in the wake, which progressively vanishes with increasing rotation ($\alpha = 2$). 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 ($C_D$) decreases with increasing $\alpha$ and $n$, whereas the lift coefficient ($C_L$) exhibits a more intricate dependence on the flow parameters. The results provide fundamental insights into the interplay between rotation, non-Newtonian rheology, and geometric confinement in determining the hydrodynamic behavior of rotating cylinder systems.