Measurements of the flow field in a rotating annular flume

Abstract

Annular flumes are used for studying erosion and/or deposition of cohesive sediments in the laboratory. These flumes have advantages over straight recirculating flumes in that effects of inflow and outflow conditions are avoided and that there are no pumps which break down the suspended flocculated sediment. A disadvantage of annular flumes is that, because of the curvature secondary flow, velocities are generated in the flowing water, yielding a complex 3-D flow field that is not well known up to now. The present report describes experiments in a rotating annular flume, Le. an annular flume in which not only the top lid rotates but the flume itself as well. The investigation is aimed at the accurate optimization of the ratio of rotational speeds of top lid and flume. Two different optimizing criteria were considered. The first criterion (criterion I) is the minimal intensity of secondary flow (especially in the lower part of the flume) and the second criterion (criterion IT) a uniform distribution of tangential velocity and of near bottom shear stress over the flume width. The measurements were done in a rotating annular flume with a mean diameter of 3.7 m. Flow velocity measurements were done with a laser-Doppler velocimeter. The water depth, the rotational speeds of top lid and flume and the top lid width were varied. Minimal secondary flow circulations near the bottom of the flume are found to occur in conjunction with nearly uniform distributions, in the radial direction, of near bottom shear stresses. The investigation also shows that in situations where the near-bottom secondary flow circulations are minimal, the remaining secondary flow velocities are generally not small compared with cohesive-sediment fall velocities. This has implications for future studies of erosion, flocculation and deposition of cohesive sediments in rotating annular flumes. Another topic discussed is the generalization of the results obtained from the experiments. Conditions for (strict or only geometrical) similarity of flows in carousels are investigated. In case of similar flows, relations between optimum conditions can be deduced. An experimental verification of these relations is presented. A simplified analysis of the flow in carousels is given. This analysis provides a method to obtain several relations for carousel flow. With these relations it is possible to estimate the bottom shear stress and to compare non-similar carousel flows, for example. The relations obtained in this way are confronted with the experimental evidence.