Experimental Studies of Flow Structure at Confluent Main Bend Channel

Abstract

In this paper experimental researches of secondary flow in cross section of confluent bend channel are carried out in a conceptual physical model channel with U type bend intersected by a straight tributary at the bend apex. Flow velocities in cross sections are measured by means of three-dimensional acoustics doppler velocimeter (ADV). For quite a few of cross sections with different central angle, figures of secondary flow velocity vector are drawn and velocity circulation distributions of cross section along flow direction are calculated. According to the experimental results the structure of the secondary flow are discussed in details. The results show that the existence of the separation zone of confluent meander bend downstream depends upon the size of junction angle. A large junction angle and discharge ratios correspond to high streamwise velocity near the inner bank and low streamwise velocity close to the outer bank. The direction of transverse velocity near the riverbed is affected by junction angle, and positive transverse velocity is near the riverbed at downstream at junction angle of 90°. The junction angle plays more important roles in changing flow structures of bend confluence. For big junction angle, the rotation direction of cross-section secondary flow becomes counter-clockwise, and a separation flow zone occurs near outer bank of junction downstream. There exist twin counter-rotating secondary cells for small junction angle (α = 30°, 60°) at bend downstream. The increases of discharge ratio together with junction angle have changed the intensity of velocity circulation of cross-section secondary flow along bend channel, and enlarged the magnitude of flow separation zone scope. The velocity circulation reaches its maximum at junction cross-section.