Friction and confinement effects on a shallow recirculating flow

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.