ALBERT

Description: In flows around three-dimensional surface obstacles in laminar or turbulent streamsthere are a number of points where the shear stress or where two or more components of the mean velocity are zero. In the first part of this paper we summarize and extend the kinematical theory for the flow near these points, particularly by emphasizing the topological classification of these points as nodes or saddles. We show that the zero-shear-stress points on the surface and on the obstacle must be such that the sum of the nodes ΣN and the sum of the saddles Σs satisfy formula omitted If the obstacle has a hole through it, such as a passageway under a building, formula omitted If the surface is a junction between two pipes, formula omitted We also consider, in two-dimensional plane sections of the flow, the points where the components of the mean velocity parallel to the planes are zero, both in the flow and near surfaces cutting the sections. The latter points are half-nodes N′ or half-saddles S′. We find that formula omitted where n is the connectivity of the section of the flow considered. In the second part new flow-visualization studies of laminar and turbulent flows around cuboids and axisymmetric humps (i.e. model hills) are reported. A new method of obtaining a high resolution of the surface shear-stress lines was used. These studies show how enumerating the nodes and saddle points acts as a check on the inferred flow pattern. Two specific conclusions drawn from these studies are that: For all the flows we observed, there are no closed surfaces of mean streamlines around the separated flows behind three-dimensional surface obstacles, which con-tradicts most of the previous suggestions for such flows (e.g. Halitsky 1968); the separation streamline on the centre-line of a three-dimensional bluff obstacle does not, in general, reattach to the surface. © 1978, Cambridge University Press. All rights reserved.

Description: A pair of curved gauzes with non-uniform porosity was used to generate a strong constant-shear flow with low turbulence intensity. The complex features of this flow around a circular cylinder with its axis normal to the vorticity and mean velocity vectors were studied. Methods such as the use of end plates, inclusion of gaps at the junctions of the cylinder with the end plates, and fluid withdrawal were applied to minimize end effects. Exploratory studies were made to investigate shear effects on the vortex-wake formation region with Reynolds numbers between 800 and 1.4 Χ 104. Effects of the steepness factor on vortex formation were explained by regarding secondary flow in the base region of the cylinder as negative base bleeding which behaves as a wake interference element. © 1989, Cambridge University Press. All rights reserved.