Hydrodynamic Forces on a Pair of Cylinders

Abstract

ABSTRACT Measurements are presented of forces on a pair of identical parallel circular cylinders with a scotch surface under a steady uniform flow. The drag coefficient of the cylinders was found to be a continuous function of the cylinders' spacing. With increasing the stream wise separation, whereas the transverse extent of the force field on the downstream cylinder indicated an increase, that on the upstream one showed a decrease. In the near wake, the lift forces attained peak values while the drag forces remained lower than the single cylinder value. The forces on the upstream body differed from those on an isolated body only if the two cylinders were as close as two diameters. Mutual interference effects were observed to be most significant at small separations. INTRODUCTION The resistance to fluid flow around a body is strongly affected by the presence of other nearby bodies. When two cylinders are in close proximity, the flow around the downstream body can affect that about the stream one and vice versa. Adjacent structural members subjected to fluid flow may generate an interaction which can cause the force on individual members to increase or decrease over the single cylinder value. Examples include members of the jacket type drilling platforms, semi-submersibles, floating tubular structures, risers, and other tubular structures in offshore engineering; cooling towers and transmission lines in civil engineering and so on. Characteristics of the flow around two interfering circular cylinders are a research topic much more complex than that concerning isolated bluff bodies. When two parallel cylinders are far apart and the rear (downstream) body is well outside the wake of the front (upstream) one there is no interference between them. The flows around both cylinders are the same as that around a single cylinder. The interference between the two bodies will start either when they are sufficiently close to each other or when the downstream cylinder is adjacent to or within the wake of the upstream one. A careful review of flow interference between two parallel circular cylinders in various arrangements in a cross-flow has been presented by Zdravkovich12. The quantification of the interference effects in terms of static forces on individual members, and vortex shedding frequency in terms of the governing flow parameters constitute the essence of the problem. This paper describes a series of experiments aimed at determining the lift and drag forces on two parallel and closely spaced circular cylinders with a smooth surface positioned at right angle to approaching flow direction. EXPERIMENTAL ARRANGEMENT AND PROCEDURE The investigation was conducted in a long rectangular horizontal flume, 0.3 m wide, 0.3 m high. The working section was situated at a distance of about 1 m from the upstream end of the flume.