Long slender cylinders in axial and near-axial flow

Abstract

An experimental investigation of axial and near - axial flow over long slender cylinders, which involved both flow visualisation and hot - wire anemometry, is detailed. The investigation of this type of flow was instigated by the current interest in towed underwater sonar arrays. The need to discriminate between background noise of mechanical origin and the flow - induced noise generated on a moving underwater soundrecording device has produced a requirement for a greater understanding of the larger scale, lower frequency, turbulent flow processes in the wake and the boundary layer of a cylinder in both axial and near - axial flow. Of particular interest are any regular periodic fluid - dynamic processes. Thick axisymmetric boundary layers with the ratio of outer - layer length scale ( the boundary - layer thickness δ ) to cylinder radius a in the range 31 [approximately equal to or less than] δ / a [approximately equal to or less than] 38 and the corresponding ratio of cylinder radius to the inner - layer length scale ( the viscous length v / U [subscript τ] ) in the range 22 [approximately equal to or less than] aU [subscript τ] / v = a [superscript +] [approximately equal to or less than] 41 have been investigated. In accord with previous experimental results their mean - flow and turbulence properties are found to be strongly influenced by transverse curvature and to diverge significantly from those of flat - plate boundary layers. A characteristic feature of such thick axisymmetric layers is the occurrence of " spots " of low - speed fluid which are attributed to displacement of inner - layer fluid by large - scale turbulent cross - flows. A front of low - speed fluid which propagates radially across the boundary layer is identified as the primary large - scale, low - frequency, coherent structure within the boundary layer turbulence. A flow mechanism that describes the process by which these fronts are formulated on the basis of the experimental evidence formed from low - speed spots is obtained. The stripping of low - speed fluid from the cylinder surface by large - scale crossflows within the turbulent boundary layer is seen as an additional vorticity - and turbulence - generating mechanism, which cannot occur in a flat - plate layer. When the cylinder is yawed to the free - stream, an attached boundary layer persists over a small range of yaw angle, before flow separation occurs. In this range the boundary layer becomes extremely asymmetric, even at yaw angles less than 1 °. The asymmetry and mean - flow properties of such layers have been investigated for yaw angles of 0.25 ° and 0.5 ° at several Reynolds numbers in the range 300 [approximately equal to or less than] Re [subscript a] [approximately equal to or less than] 600. At somewhat larger yaw angles, a new regime of regular vortex - shedding in near - axial flow has been identified. From the experimental results, an empirical relation for the vortex - shedding frequency ( in terms of yaw angle, vortex - shedding angle, and a Reynolds number based on the component of free - stream velocity normal to the vortex axes ) has been derived as an extension of the Roshko formula for the frequency of vortex shedding from cylinders with their axes normal to the flow. The results presented advance the current understanding of the fundamental fluid mechanics of cylinders in axial and near - axial flow, and thereby have the potential to contribute to the advancement of the signal - processing techniques applied to towed underwater sonar arrays.