Experimental study of Separation Zone characteristics of flow in 90 Degrees Four-branch Open Channel Junction with Subcritical Flow

Introduction: In open channel junctions development of separation zone at the downstream channels is one of the engineering challenges. Development of separation zone has important role on determination of position of hydraulic structures and also increase of its dimensions caused decreasing of flow area so that leads to more erosion of the bed. As the flow is entered to the channel, flow separated from the wall and a zone with low pressure and vortex is created that is called separation zone. As regards of destructive effects of development of the separation zone, researchers always have focused on this case. The aim of this study is to investigate the effects of factors such as inlet discharge ratio, height of outlets weirs and bed elevation of inlet channels on dimensions of the separation zone in 90 degrees four-branch junction with subcritical flow.
Materials and methods: Experiments were carried out in hydraulic Lab of Tabriz University. The setup consists of an intersection of four identical channels which intersect at right angles. Each channel is made from glass with a width 0.4 m and height 0.5 m. Junction was right-angled and bed of the channels was considered horizontal. In this study effects of different geometric and hydraulic parameters such as height of the outlet weirs, bed elevation of the lateral channel and inlet discharge ratio on characteristics of the separation zone in outlet channels in 90 degrees four-branch junction with two inlet flows and two outlet flows was investigated experimentally. In order to investigate the flow pattern were used pigment and sawdust and dimensions of the separation zone were measured by ruler and meter.
Results and discussion: The results revealed that decreasing height of outlet weirs increases dimensions of the separation zone so that minimum variations of (ratio of length of the separation zone in main outlet channel to channel width) was about 45 percents. As the bed elevation of lateral channels increases dimensions of the separation zone at the main outlet channel increase which variations of was between 0.8 to 2.75 but dimensions of the separation zone at the lateral outlet channel decrease and variations of (ratio of length of the separation zone in lateral outlet channel to channel width) was at the range of 0.2-2 . The results also indicated that as the ratio of outlet weir height of the lateral channel to outlet weir height of the main channel increases, dimensions of the separation zone in both lateral and main outlet channels increase so that variations of for both main and lateral channels were at 0.2-1.4 and 0.2-2, respectively. Although the dimensions of the separation zone change with respect to different conditions but width to length ratios in lateral and main outlet channels were 0.16 and 0.25, respectively. Finally in order to calculate the dimension of the separation zone, regression equations were presented so that experimental and analytical results had good agreement.
Conclusion: In this research effect of the inlet discharge ratio, height of the outlet weirs and bed elevation of the lateral channels on dimensions of the separation zone in 90 degrees open channel junctions was investigated. Results showed that as increase of the inlet discharge ratio and height of the outlet weirs, dimensions of the separation zone increased. Increase of the bed elevation of the lateral channel increased dimensions of the separation zone in main outlet channel while decreased dimensions of the separation zone in lateral outlet channel. As increased of dimensions of the separation zone in both outlet channels increased.