Abstract
The present paper aims to gain deeper insight into the evolution of secondary flows in open channel bend. A U-shaped open channel with long straight inflow/outflow reaches was used for experiments. Efforts were made to precisely specify flow conditions and to achieve high precision measurement of quasi-three-dimensional velocities with a multi-pass, two-dimensional PIV (Particle Image Velocimetry) method. The experimental results show that the flow begins to redistribute before entering the bend and it takes a long distance to re-establish to uniform conditions after exiting the bend. Complex secondary flow patterns were found to be present in the bend, as well as in the straight inflow and outflow reaches. A “self-breaking” (process was identified, which correlates stream-wise velocity with the intensity of flow circulation.
Highlights
Complex flow patterns exist in open channel bends and they play important roles in alluvial and ecological processes
A PIV system was employed to measure velocity distributions at 13 cross-sections, which are located at the straight inflow reach (−4.5 m and −0.9 m), the bend (10◦, 37◦, 60◦, 90◦, 120◦, and 169◦ ), and the straight outflow reach (+1.0 m, +3.3 m, +5.0 m, +7.0 m, and +8.5 m)
90° bend with a short exit, Gholami et al [21] reported a higher degree of surface tilting closer to the inner bank inconsistent characteristics. In their experimental and numerical study of a bend with a short exit, Gholami et al [21] reported a higher degree of surface tilting closer to the inner rather than the outer bank, a finding that is contradictory to the present result
Summary
Complex flow patterns exist in open channel bends and they play important roles in alluvial and ecological processes. The experimental design in most curved flow studies is insufficient for a closer observation of the evolution of the flow along the entire channel, including the bend and the straight inflow/outflow reaches. The influence of the inflow/outflow reaches still remains unclear, and as a result, the evolution of secondary flow in the bend involves much ambiguity. To further investigate the evolution of secondary flow in open channel bend, a U-shaped bend with long straight inflow/outflow reaches has been designed and experiments have been conducted. To set up a well-designed experimental system that facilitates accurate specification of flow conditions and detailed observation of the three components of flow velocity in a U-shaped open channel flume with long straight inflow/outflow reaches. To present high-quality data on the secondary flows that may be used for validation and calibration of numerical schemes
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