Experimental Investigation Of Flow In A U-shaped Open Channel By Using PIV

Ubiquitous in natural rivers and engineering applications,bend flow plays an important role in flood control,navigation improvement,port construction,project operation,and long-term planning of catchments.Although a lot of achievements have been obtained in analytical,numerical and experimental investigations,it still remains unclear as how the flow evolves along the entire bend when both the upstream and downstream sections exert important influence to flow characteristics due to difficulty in instrumentation.An experimental system with PIV in a U-shaped open channel flume was set up in the present study.Experiments under five different flow conditions were conducted.Three-component velocity of the flow was measured at 13 cross-sections and flow depth was monitored at 25 cross-sections.Based on analysis of the measured data,the following conclusions have been drawn:?1?PIV is suitable for measurement of open channel bend flow,as it performs better than commonly used ADV in terms of precision and reliability.?2?The water depth topography is mainly shaped by the bend when the flow maintains uniform in the upstream/downstream,the transverse slope grows greater at higher water depth;however,it becomes complex when the flow condition varies,i.e.,the cross-section-averaged depth changes along the whole channel.?3?The velocity distribution becomes complex due to the occurrence of secondary flow.The flow is symmetrical in the straight inflow reach,deflects to the inner bank before transferring to the outer bank.The turning cross-section is closely related to the width of the bend?B?,the radius of the bend centerline?R_c?,and the flow depth?H?.The most asymmetry occurs at the downstream of the bend rather than within the bend.?4?The maximum stream-wise velocity(U_max)occurs at the water surface in the straight section,then sinks because of gravity and the centrifugal force,resulting in a submersion of U_max well below the water surface.In addition,the submersion of U_max becomes more evident at higher water depth.?5?The influence to turbulence parameters caused by the bend needs a long distance to decay.The flow in both the upstream and downstream sections is influenced by the bend.?6?The evolution of secondary flows is similar in different cases.In general,the strength shows a gradual increase at the first half of the bend,then turns to decreases a little,and finally goes down rapidly after the bend.The findings of the present study may help to enrich experimental data,calibrate numerical schemes,and promot engineering practice.