Effect of Mouth Shape on the Energy and Momentum Coefficients and Flow Separation Dimensions

The study of flow diversion in open channels which has been, since long, under consideration by hydraulic engineers, is much used to divert flow from a main channel or from a river into an irrigation or hydropower channel. When a water intake with an angle is installed at one side of the channel, the streamlines of the flow deflect towards the intake. As a result, a separation zone is produced in the lateral channel. The separation zone develops in the lateral channel and reduces the discharge capacity and efficiency of water intake by delimiting the channel width available for the flow. Therefore, determination of water intake geometry and flow conditions to produce minimum separation zone is very important and they are the focus of this study. The majority of previous studies was conducted on sharp edged water intake entrances. Therefore, in this study, to find the optimum radius for a round edged entrance water intake, a comprehensive experimental program was carried out in a laboratory flume and the separation zone dimensions and Alpha and Beta coefficients were measured.
The experimental model was built in hydraulics laboratory. The water intake was installed at 55 degrees to the main channel. The main channel consisted of a rectangular cross-section with a base width of 0.5 m, height of 0.4 m and a length of 15.80 m. The lateral diversion channel was 0.25 m wide, 0.40 m high. According to previous experiments that performed by Keshavarzi and Habibi (2005), radii of 10, 15 and 20 cm were selected for the edges of the intakes, upstream of the 55 degree water intake. The velocities of the flow in transverse and flow directions were measured using an electromagnetic velocity meter at three distances Z= 3 cm, 6 cm and 12 cm, in which Z is the distance from the bed. Then the size of the separation zone, Alpha and Beta coefficients were determined.
To find a relationship between the radius of the round edge entrance in the 55 degree water intake and the size of separation, the geometry of the separation zone must be determined. To find the geometry and pattern of separation zone for different flow conditions, the particle traces technique was employed using Tec plot Software version 8.0. In open end flow condition, for discharge ratios of 0.2, 0.4, 0.6 and 0.8, and for the radii of 10, 15 and 20 cm, flow separation occurs at 3 cm and 12 cm distance and only upstream of the intake inlet. The separation size in r=20 cm is less than for other radii. Also, the separation size for Qr = 0.8 is minimized and for Qr =0.2 is the maximum and for r/Wb=0.8, the length and width of separation are minimum. In close end flow condition and for radii of 10, 15 and 20 cm, the size of separation zone at upstream of water intake is much larger than that in downstream. Comparing with the separation length downstream of the intake it can be concluded that with increasing the inlet radius, the separation length upstream of the intake inlet decreases. Therefore, in close end conditions, rounding of the intake inlet is effective to decrease separation length at upstream side of water intake. Also, in close end conditions, flow separation occurs at downstream side of water intake. Furthermore, the separation size for r=20 cm is less than for other radii, therefore, r/Wb=0.8 is the optimum radius ratio with a minimum separation size at the 55 degree water intake.
When a water intake with an angle is installed at one side of the channel, the streamlines of the flow deflect towards the intake. As a result, a separation zone is produced in the lateral channel. The separation zone development in the lateral channel and reduces the discharge capacity and efficiency of water intake by delimiting the channel width available for the flow. In this study, to find the optimum round inlet radius, the experimental tests were carried out at a water intake installed in a rectangular channel with rounded edge with 10, 15 and 20 cm inlet radius. Then separation zone dimensions and alpha and beta coefficients determined. These experiments were carried out in close end and open end flow conditions for diversion flow ratio 0.2, 0.4, 0.6 and 0.8. Using particle trace plot for different flow pattern, the values of length and width of flow separation upstream and downstream of the intake were determined. The result showed that the separation size for Qr = 0.8 is minimized, whereas it is maximum for Qr =0.2. Furthermore, the separation size for r=20 cm is less than for other radius, therefore, r/Wb=0.8 with a minimum separation size was selected as the optimum radius ratio.