TWO STAGGERED SURFACE-MOUNTED FINITE-HEIGHT CYLINDERS IN A CROSS-FLOW

Two surface-mounted finite-height cylinders in a staggered configuration immersed in a cross-flow can be used to represent a broad variety of engineering applications. The centre-to-centre pitch ratio, P/D, incidence angle, α, and Reynolds number, Re, are known to strongly influence the aerodynamic loading on the cylinders and vortex shedding, however the effects of cylinder aspect ratio, AR (= height/diameter) and the boundary layer thickness on the ground plane, δ/D, are not yet well understood. Most studies of the flow around two surface-mounted finite-height cylinders have focused on two main configurations, tandem (α = 0°) and side-by-side (α = 90°). In the present research, the flow around two finite cylinders in a staggered configuration was systematically studied, to better understand the effects of the cylinder aspect ratio. Wind tunnel experiments were conducted at Re = 6.5×104 with a relative boundary layer thickness of δ/D = 1.4 for cylinders of AR = 9, 7, 5, and 3. The pitch ratio was varied from 1.125 ≤ P/D ≤ 4.5 in 12 discrete steps and the incidence angle was varied from 0° ≤ α ≤ 90° in small increments. Of specific interest were the effects of AR on the mean drag coefficient, CD, mean lift coefficient, CL, and Strouhal number, St, experienced by the upstream and downstream cylinders in each staggered configuration, and any differences between the behaviour of two finite cylinders compared to more extensively studied case of two infinite (two-dimensional) cylinders. The results for the two finite cylinders in tandem showed a relatively smooth transition from the reattachment flow pattern to the co-shedding flow pattern, in contrast to what is seen for two infinite cylinders. A reduced range of P/D was found where the downstream cylinder experienced a thrust force. These changes were attributed to downwash entering the gap between the cylinders and impinging on the downstream cylinder. For the side-by-side configuration, the biased flow pattern continued to be observed for two finite cylinders, but the proximity interference effects were strengthened compared to two infinite cylinders. For closely spaced finite cylinders (P/D ≤ 1.75) near α = 90°, hysteresis was found over a small range of incidence angle where CD and CL could attain different values depending on whether α was incremented positive or negative. At the upper and lower ends of this range of incidence angle, a sudden change in the wake state and the values of CD and CL occurred. Many aspects of the hysteresis were found to be complex functions of AR and P/D. For the staggered configuration, the behaviours of CD, CL, and St were found to be complex functions of P/D, α, and AR. In many cases, but not all, the general effect of reducing aspect ratio was to weaken the wake and proximity interference effects. The data for the two cylinders of AR = 3 behaved distinctly from those of AR = 9, 7, and 5, which showed that these cylinders were less than the critical aspect ratio