بررسی عددی تغییرات الگوی جریان و پروفیل های سرعت در اثر اختلاف رقوم بستر کانال ها در آبگیرهای جانبی

Diverting flow from rivers and channels is one of the methods of transfer water for agricultural and industrial purposes. Dividing channels are also used in urban wastewater in collecting system. Intakes are amongst hydraulic structures which are used for the purposes of controlling and deviating flow in agricultural irrigation and drainage networks and also in sewer systems. In some cases, energy is produced through intakes. Determining the model of the flow, which is entering the branch channels from the main channel, is of utmost important in these hydraulic structures.
A part of flow in the main channel is deviated and enters the branch channel when impounding the river due to the suction force applied to it; as a result, flow separation zone and flow compression zone are formed near the entrance of the intake channel. The flow deviated into the intake has complex properties. The flow particles rotate in the separation zone, near the entrance wall of the intake channel, and the longitudinal velocity of the flow decreases and it increases in the opposite direction of the flow. The longitudinal velocity intensely increases in the compression zone, due to the density of the flow lines, and reaches its maximum levels. Therefore measuring the flow velocity in the flow separation zone and the flow compression zone is difficult and accompanied with error. Normally, flowmeters are installed in intakes in order to compute the flow discharge. A very complex flow is formed as the flow diverts from the main channel into the branch. As the flow approaches the branch channel, it undergoes lateral acceleration and is divided into two sections due to the suction pressure from the diversion. Entering the intake, the flow loses balance because of lateral pressure gradient and shear force and the centripetal force and this leads to the formation of secondary flow. A large number of experimental and numerical studies have been conducted on the flow within the intakes. Simulating the flow field, using numerical methods, reduces experimental costs and saves time therefore numerous studies have been conducted on the flow hydraulics in the diversion channels. The flow hydraulics in diversion channels have been three- dimensionally simulated in liquid and gas phases. In these studies finite volume method was widely used. Ramamurthy et al. (2007) conducted a numerical research in an intake with a 90 degree diversion and calculated the velocity of the flow within the channel through numerical calculations and compared the results with experimental data. They studied the separation zone in the intake channel and came to the conclusion that increase in dewatering ratio leads to an increase in the vortex’s strength. Mignot et al. (2012) used 3D numerical model to simulate flow patterns through junctions. In their study, in order to evaluate errors flow rate measurements were done by installing sensors in various locations, at the downstream of the junction. They showed that the measured discharge performed by sensors can produce the error more than 60% from real discharge. When fluid is flowing within a main channel with an intake, it enters the branch channel due to the suction force from the branch channel which causes lateral acceleration in the flow. When the flow enters the branch channel, separation area is formed on one side of the branch channel and flow contraction forms at the other side, due to the development of the flow. These phenomena lead to the flow having a thoroughly three- dimensional behavior.
The flow’s behavior has been three- dimensionally modeled in the deviation location through using ANSYS-CFX software, in this study. The results of the numerical model with a 90- degree contact angle have been compared to the experimental results, for the purposes of verification and then effects of the bed elevation difference of the meeting channels on the flow pattern structure were studied. The obtained results indicated that increase in the bed level of the main channel (0.05m higher than the branch channel bed) leads to: (1) moving high-speed lines of the flow upwards, from the bottom to the water surface; (2) disappearing of the separation area in the entrance of the branch channel; (3) increasing the intake ratio in the branch channel; (4) transferring the secondary flow from the bottom to the middle and therefore reducing of erosion intensity in the bed and floor of the branch channel.