Staggered circular cylinders immersed in a uniform planar shear flow

Abstract

Two circular cylinders of equal diameter, arranged in staggered configurations with dimensionless centre-to-centre pitch ratios of P/ D=1.125 and 1.25, were immersed in a uniform planar shear flow with a dimensionless shear parameter of K=0.05 and a Reynolds number of Re=5.0×10 4. The mean aerodynamic forces and the vortex shedding frequencies were measured for the upstream and downstream cylinders at each pitch ratio. Under uniform flow conditions, where there is no shear, the cylinders behave as a single bluff body with a single vortex shedding process. The aerodynamic force coefficients on both cylinders, and the Strouhal number, undergo complex changes as the incidence angle is varied from α=0° to 90°. At several critical incidence angles there are discontinuous changes in, or extreme values of, the aerodynamic force coefficients. These are related to the shear layer dynamics between the upstream and downstream cylinders at lower incidence angles, and the changes to the gap flow between the cylinders and the effect of base bleed at higher incidence angles. The effects of the uniform planar shear flow were mostly seen in the aerodynamic forces, particularly for the mean drag coefficient, where there was a reduction in drag for the upstream cylinder at all incidence angles, a reduction in drag for the downstream cylinder at higher incidence angles, small changes in the lift coefficient on the upstream cylinder, and shifts in the critical incidence angles. In contrast, the behaviour of the Strouhal number was generally unchanged, although a small reduction in Strouhal number was observed. Planar shear flow more strongly influenced the staggered cylinders at higher incidence angles, where it had an influence on the direction and strength of the gap flow, seen in the movement of two critical incidence angles, which was accompanied by a weakening of the vortex shedding process.