Flow over a Rectangular Cylinder in Proximity to the Plane Wall: Effect of Cylinder Aspect Ratios and Power-Law Index

Abstract

Newtonian (power-law index n = 1) and pseudoplastic (n = 0.8 and 0.5) fluid flow past a rectangular cylinder placed close to the plane wall (the gap ratio G/a varying from 0.25 to 1) is studied numerically using Ansys Fluent when the body shape of the cylinder changes from a bluff body (the aspect ratio AR = 0.5, 1, and 2) to an elongated body (AR = 3 and 4). The 2-D unsteady, incompressible Navier–Stokes equations with varying viscosity are solved at a constant Reynolds number, namely, Re = 100, using a second-order upwind momentum and least square cell-based pressure solver. The instantaneous and time-averaged flow fields are analyzed and compared in 45 cases concerning AR, n and G/a. It has been observed that the body shape of the cylinder and pseudoplasticity play pivotal roles in changing the flow field variables along with aerodynamic forces for a relatively intermediate and high gap ratio (G/a = 0.5 and 1, respectively). The quantity \({{\bar {C}}_{L}}\) increases and \({{\bar {C}}_{D}}\) reduces with increase in the AR value of the cylinder. In the presence of high pseudoplasticity in fluid flow (n = 0.5), this trend becomes more prominent. Also for n = 0.5, multiple values of Strouhal number (St) are obtained for a cylinder with low AR (AR = 0.5 and 1) at a large gap ratio (G/a = 1) owing to differences in the celerity of the vortices produced from outer and inner shear layers of the cylinder after the interactions. When the rectangular cylinder is kept very close to the plane wall (G/a = 0.25), vortex suppression occurs irrespective of the cylinder size or flow nature which negates the changes in the flow field and the aerodynamic forces.