Friction and confinement effects on a shallow recirculating flow

Abstract

Experiments were conducted to study confinement and friction effects on a shallow recirculating flow that was formed in a lateral expansion of an open-channel flow on a smooth bed. Dye was introduced to the flow as tracer. The instantaneous dye concentration in the flow was captured by video simultaneously across the width and along the length of the flume. The mean and the root-mean-square values of the dye-concentration fluctuations at selected cross sections along the flow are analyzed and compared with a previous series of data that were collected using a probe to measure the concentration one point at a time. The previous experiment was conducted in a small flume on a roughen bed. The new flume in the present experiment was 2.35 times wider. The large width of the new flume facilitates the study of the same friction effect on a smooth bed over a greater range of the confinement effect. In comparison with the previous experiments, the dye concentration in the present experiments is higher for the same range of bed-friction numbers. The difference in the results is tentatively attributed to the bed-generated turbulence. Key words: recirculating flow, shallow open-channel flow, quasi-two-dimensional turbulent flow, horizontal turbulence, bed-generated turbulence, friction effects, tracer concentration measurements, video imaging method.

Coupling between flow and sediment deposition in rectangular shallow reservoirs

“…Increasing the bottom roughness up to k s = 0.005 m leads to hardly noticeable changes in the velocity profiles. Flow resistance remains indeed very weak, since the head loss across the reservoir is in any case of the order of 1 − 2 × 10 −4 m. These results are in agreement with Babarutsi et al (1989), Babarutsi andChu (1991) and Chu et al (2004) for unilateral expansions: since the bed friction number remains here lower than 0.05, the flow is classified as "non-frictional" and is thus not influenced by the roughness. In the L-L configuration (Fig.…”

Classification of flow patterns in rectangular shallow reservoirs

“…Several studies indicate that the length of the recirculation zone over unilateral expansions depends only on the lateral expansion for small values of the bed friction number; for large values, however, the reattachment length depends only on a friction length scale (Babarutsi et al 1989, Babarutsi and Chu 1991, Chu et al 2004). …”

“…To the knowledge of the authors, only the median position has been reported in the literature, using, for example, visualization of displacement of confetti on the water surface (Babarutsi et al 1989, Babarutsi and Chu 1991, Chu et al 2004. Only Abbott and Kline (1962) characterized fluctuations of the reattachment lengths by carrying out a large number of tests.…”

Reservoir sedimentation

“…In the experiments, the reservoir boundary conditions were chosen to obtain flows with different friction regimes (frictional and non-frictional, see Chu et al (2004)) and the flow dynamics was quantified by LSPIV. The flows were then modelled using WOLF2D, which solves the shallow water equations and uses a depth-averaged k- turbulence model.…”

“…The friction numbers are characteristic of two different friction regimes. Referring to the work of Chu et al (2004), the flow-case F belongs to the frictional regime (the turbulence scale is mainly driven by the water depth), while the flow-case NF belong to the nonfrictional regime (the turbulence scale is mainly driven by the horizontal length-scale). The Reynolds numbers in both cases confirm that the flows are turbulent, but they are hydrodynamically smooth.…”