Two staggered circular cylinders of equal diameter in cross-flow

Abstract

Wind tunnel experiments were conducted to measure the mean aerodynamic forces and vortex shedding frequencies for two staggered circular cylinders of equal diameter in cross-flow. The Reynolds number based on cylinder diameter ranged from Re=3.2×10 4 to 7.4×10 4, the centre-to-centre pitch ratio was varied from P/D=1.125 to 4.0, and the incidence angle was incremented in small steps from α = 0 ° to 90°. The mean drag and lift force coefficients and the Strouhal numbers were obtained for both the upstream and downstream cylinders. From the behaviour of the experimental data, the staggered configuration could be broadly classified by the pitch ratio as closely spaced ( P/D<1.5), moderately spaced (1.5⩽ P/D⩽2.5), or widely spaced ( P/D>2.5). Closely spaced staggered cylinders are characterized by mean aerodynamic forces that undergo large changes in magnitude and direction with the incidence angle for both cylinders. The same Strouhal number is measured behind both cylinders, an indication of single-bluff-body behaviour. Moderately spaced staggered cylinders are characterized by small changes in the mean aerodynamic forces on the upstream cylinder, but a relatively complex behaviour of the mean aerodynamic forces on the downstream cylinder. Two Strouhal numbers are measured for most incidence angles. For both closely spaced and moderately spaced configurations, a critical incidence angle was found that is associated with a local maximum drag coefficient on the upstream cylinder, a maximum inward-directed lift coefficient for the downstream cylinder (known as the inner lift peak), a local minimum drag coefficient for the downstream cylinder, and a maximum value for the Strouhal number. Widely spaced staggered cylinders are characterized by mean aerodynamic forces on the upstream cylinder that are mostly unchanged from the single cylinder, and a single Strouhal number for both cylinders indicative of synchronized vortex shedding from both cylinders at all incidence angles. The mean aerodynamic forces on the downstream cylinder are marked by a minimum drag coefficient in the tandem configuration and the appearance of the outer lift peak. The outer lift peak for the downstream cylinder is associated with the proximity or impingement of shed Kármán vortices from the upstream cylinder.