Uniform flow past one (or two in tandem) finite length circular cylinder(s)

Abstract

The problem of uniform flow past one (or two in tandem formation) finite length circular cylinder(s) is investigated experimentally in the present paper. For the case of flow past a single finite length cylinder, it is found that the flow that separates from the free end interacts strongly with those that separate from the sides and results in a three-dimensional flow. This separated flow from the free end delays the interaction between the separated flows from the sides, resulting in a less negative wake pressure and a lower drag when compared with flow past an infinitely long cylinder. Spanwise effects were found to be stronger over the longest (h/d = 8) cylinder investigated, with the spanwise drag coefficient increasing towards the free end. Vortex shedding with some regularity can be detected only at y/h = 0.95 and y/h ≤ 0.5 (with weak spectral peak) of the h/d = 8 cylinder, with corresponding Strouhal numbers equal to 0.08 and 0.191, respectively. For flow past two tandem finite length circular cylinders, although the transition from reattached flow to co-shedding flow is still observed, due to the highly three-dimensional nature of the flow, the change no longer takes place over the entire span simultaneously. When the dimensionless spacing between the cylinders increases from one (cylinders touching), the co-shedding flow structure was first detected near the free end and its extent later expands towards the mounted end with further increase in cylinders spacing, at the expense of the reattached flow region. At very small spacing between the cylinders, a third flow structure which is characterised by having lower surface pressure over the windward part of the downstream cylinder was also observed. Spanwise variation of the drag that acts on both cylinders was observed. The magnitude of the drag coefficient was found to be strongly dependent on the flow structure involved and normally varies as a direct consequence of the variation in the wake pressure. The overall trend is that of higher local drag towards the free end.