Dye‐Concentration Distribution in Shallow Recirculating Flows

Dye‐concentration measurements are conducted in the recirculating region of a sudden open‐channel‐flow expansion, where the water depth is shallow compared with the horizontal length scale of the channel expansion. The turbulent motion is anisotropic, and is subjected to a stabilizing bed‐friction influence, in the confinement between the free surface and the channel bed. Two opposing mechanisms affect the transverse mixing processes in the shallow recirculating flow. On the one hand, the transverse mixing is reduced by the stabilizing bed‐friction effect. On the other hand, it is enhanced by a confinement effect, which becomes effective when the width‐to‐depth ratio of the channel expansion exceeds a value of about 9∼12. These opposing influences are equally significant in the range of the bed‐friction number from 0.08–0.10. The concentration data of the present investigation are complementary to the published velocity data obtained in the same recirculating flows. Both sets of data are useful for model calibration of shallow open‐channel flows.