The present aim is to reveal the flow past a pipe which is immersed parallel to the mainstream at high Reynolds numbers. In a wind tunnel, we carry out (1) base-pressure measurements, (2) velocity-fluctuation measurements using a hot-wire anemometer and (3) flow visualisations by a smoke-wire method with PIV analyses, where we take consecutive picutures using a high-speed camcorder to obtain quantitative flow-field information such as velocity vector and vorticity. The tested parameter ranges are as follows: Re = 2.0×10 3 -1.3×10
4, d/t = 4.0 -10.0 and l/t = 1.0 -10.0, where Re, d, t and l are the Reynolds number, mean diameter, thickness and length of the pipe, respectively. As a result, the Re effects are negligible. The base-suction coefficient C pb monotonically decreases with decreasing d/t, or with increasing l/t. We propose a unified formula to predict C pb , which are consistent with both a two-dimensional prism and a rod for l/t < 4.0 in addition to a ring. In contrast, the Strouhal number St almost coincides with that for a two-dimensional prism at any l/t, if we can detected any dominant frequencies. In addition, we conduct flow visualisations, and reveal the effects upon axisymmetry of wake. Finally, we classify the flow into three modes based on both periodicity and axisymmetry. Such a modal classification reveals that the enhancement of flow's irregularity corresponds to the decrease of C pb .Key Words:
IntroductionWhile the flow past a bluff body at higher Reynolds numbers often becomes important in many practical aspects, it is one of rather later topics in the long history of fluid mechanics. Among such bluff-body-flow problems, there have been less researches concerning three-dimensional bluff bodies, in comparison with well-researched two-dimensional ones such as a circular cylinder which has advantages in some compatibilities with analytical, experimental and numerical approaches.As basic three-dimensional bluff bodies, it seems natural to consider axisymmetrical bodies with simple geometries like a sphere and a disc, whose knowledge is useful in the analyses of flying or suspended objects. But, even the flow past such a simple axisymmetrical body has not been revealed enough, despite of the wide ranges of its engineering applicabilities, in comparison with a circular cylinder.The present study concerns the flow past another simple axisymmetrical bluff body, a pipe or a tube, which is immersed parallel to the mainstream at high Reynolds numbers. The researches on the flow have been still less active than a sphere or a disc, although the knowledge is useful in many industrial fields, such as the designs for combustors, ventilator (2) − (8)
