Flow around circular cylinders with very low aspect ratio

Abstract

Experiments on the flow around stationary circular cylinders with very low aspect ratio piercing the water free surface were carried out in a recirculating water channel. Eight different aspect ratios were tested, namely L/D=0.1, 0.2, 0.3, 0.5, 0.75, 1.0, 1.5 and 2.0; no end-plates were employed. Forces were measured using a six degree-of-freedom load cell and the Strouhal number was inferred through the transverse force fluctuation frequency. The range of Reynolds number covered 10 000<Re<50 000. PIV measurements were performed in some aspect ratio cases, namely L/D=0.3, 0.5, 1.0 and 2.0 for Reynolds number equal to 43 000. The results showed a decrease in drag force coefficients with decreasing aspect ratio, as well as a decrease in Strouhal number with decreasing aspect ratio. The PIV measurements and the PSD of forces showed different behavior for cylinders with L/D≤0.5, in which cases the free-end effects were predominant. Even without von Kármán street main characteristics around the majority length of the cylinder, in the range of 0.2<L/D≤0.5, the vortex shedding around it is capable of producing alternating forces in the transverse direction. Therefore, alternating forces were not observed in the transverse direction for cylinders with L/D≤0.2.