Open channel junction’s flow field simulation and flowmetery accuracy in different discharge ratios and confluence angles

Junctions are used commonly in various hydraulic phenomena such as natural rivers, water and wastewater treatment equipment, irrigational networks, and crossing points for passing fish. The confluence zone faces high turbulence due to the confluence of two currents with various momentums that lead to developing the separation and contraction zones. In Separation Zone, speed of current is declined and there is the possibility of aggregation of sediment which follows obstruction of current. Therefore, studying on the dimensions of Separation Zone under various conditions is very important in hydraulic and environmental engineering.

Computational Fluid Dynamics (CFD) methods can used for various hydraulic engineering problems. The three-dimensional, numerical model employed to simulate the confluence flow in a rectangular cross section. This numerical study has been conducted in order to attain two main objectives as follows: (1) Studying effect of various angles of intersection on behavior of flow and separation zone, (2) Studying precision of measurement of flow meter under various conditions to ratio of various flow rates and various angles of intersection. The numerical models have been created with various angles of 30-, 45-, 60- and 90- degree intersections coupled with different discharge ratio in main and tributary channels.

It is observed that deviation of main flow is increased towards outer wall of main channel with increased angle of intersection in a way that main flow has occupied almost 40% of width in the beginning of downstream intersection in 30-degree mode, the rate of which is almost 20% in 90-degree mode. This deviation in near the free surface exceeds near bed. No effect of rotational current has been observed in the vicinity of inner wall in 30- and 45- degree angles. Separation of flow in 90- degree angle is occurred at the larger area in the vicinity of inner wall. Another point which is observed here is this that high velocity zone is started from downstream corner of tributary channel and nears gradually to the outer wall of channel main downstream. It was concluded that increased intersecting angular will lead to the increased deviation of main current towards external wall of main channel. With reduced intersection angle, area of separation zone is decreased, so that this area is disappeared completely in 30 and 45- degree angles. With studying performance of flow meter in measurement of flow rate, it can be concluded that increase of intersectional angle and decrease of discharge ratio will cause increased flow meter measurement error. So that, the flowmeter accuracy increased from 5.75% in 30̊ confluence angle and 0.75 discharge ratio to the 61.32% in 90̊ confluence angle and 0.25 discharge ratio.