EFFECTS OF DISCHARGE RATIOS ON FLOW STRUCTURE AT A 90-DEGREE FLOW CONFLUENCE WITH DEGRADED CHANNEL BEDS

摘要

This paper describes the results of an experimental investigation taken up to study the flow structure through bed profile of a laboratory confluence channel that was degraded by detachment of its bed material consisting of sand-gravel mixtures. The water in the channel can be recycled that flow is pumped from downstream tank to two upstream tanks throw two pipes, of which the discharge can be controlled accurately by valves with the help of ultrasonic flowmeter. The measurements of velocity over the degraded bed profile were made at five cross sections at the post-confluence zone using Acoustic Doppler Velocimetry. Two discharge ratios of tributary channel and main channel, 3:2 and 2:3 were conducted to observe the different bed morphologies and their influences on flow structure. For the case with the larger tributary discharge, the deep hole scoured by flow was farther away from the tributary than the case with small tributary discharge, while the thalweg shifted about 7cm. The deepest place was 2.6cm deep in the case with large tributary discharge while it was 2cm deep in the case with small tributary discharge. Observing the vector description (V, W) of each cross section, we found that there are secondary currents at the post-confluence zone. In order to further study the effects of discharge ratios on flow structure, contour lines of vorticity in each cross section were drawn. They were compared among the different cross sections of the same ratio and between the two ratios of the same cross section.