Numerical Examination of the Relative Effect of the Channel Width in the Intakes on the Velocity Distribution Curves in the Flow Deviation Location

1. One of the ways of controlling floods and deviating a part of the flow in open channels is to use intakes. Identifying the flow model and calculating the passing discharge are amongst the crucial issues of hydraulics engineering. Using flow meters is one of the most popular methods of measuring the velocity or the discharge in open channels. Flowmeters rely on measuring the velocity and they calculate the discharge through the continuity equation as the multiplication of the mean velocity by the wet cross section (Q = A(h ×Umean). The A (h) cross section is computed through measuring the height of the free surface (h) and using precise geometrical information. Determining the mean velocity passing through the cross section requires special knowledge. It should be noted that due to the three-dimensional and complex nature of the flow near the intake location and the presence of strong secondary flows in the transverse cross section, the velocity measured by the sensors located on the flowmeter in the intended area is different from the actual mean velocity of the channel. However, despite this difference, the mean velocities measured by the flowmeter are fairly consistent with the actual mean velocity area of the channel. The velocity of the passing flows and the geometrical size of the main and branch channels affect the difference between the velocity measured by the flowmeter and the actual mean velocity of the flow [1-6].2. The experimental model [7] of a 90-degree intake with a rectangular cross section has been three dimensionally simulated by the ANSYS- CFX software in this study. The k-ω turbulence model has been used to solve the turbulence equations in this simulation. The results of the numerical model have been compared with that of the experimental model in order to examine the accuracy of the numerical model. The flowmeter measurement accuracy was examined in different width ratios wr= 1.4, 1.2, 1.0, 0.8, 0.6 (the branch channel width to the main channel width) through using the numerical model and the results of the experimental model after verification. With regard to the fact that the discharge passing through the branch channel is constant in all the wr states, the effective ratio for fluid movement increases in the branch channel as the width ratio increases as the result of an increase in the branch channel width from wr= 0.6 to wr= 1.4. This causes the width of the separation zone and the contraction degree of the compression zone to increase and the longitudinal velocity to get close to v*max equal to -0.6. Therefore wr= 1.4 width ratio is considered the critical width ratio among all the presented wrs regarding the vastness and density of the separation zone and the compression zone. An increase in the width ratio of the channels increase the difference between the read velocities and the mean velocity of the branch channel which means as the width ratio increases from wr= 0.6 to wr=1.0, the difference between the read velocity and the branch channel mean velocity increases from 18 percent to 139 percent at the peak point. The reason behind this error percentage increase is that as wr= 0.6 increases to wr= 1.0, the size of the separation zone increases and the density of the contraction zone increases as well however this increasing process continues to wr= 1.0, width ratio and after the width ratio of 1 as the width ratio of the channels increase from wr= 1.0 to wr= 1.4, the difference between the read velocities and the actual mean velocity decreases in the branch channel which means as the width ratio increases from wr= 1.0 to wr= 1.4, the difference between the read velocity and the branch channel mean velocity drops from 139 percent to 47 percent because as the width ratio increases from wr= 1.0 to wr= 1.4, the effective ratio of fluid movement increases in the channel and the difference between the read velocity and the branch channel mean velocity increases. This is because when the channels’ width ratios become excessively large, the branch channel’s cross section excessively increases and as a result the flow velocity significantly drops and this decreases in the velocity leads to lesser volume of flow entering the separation zone and the value of vmax decreases in the compression zone and so the difference between the read longitudinal velocity and the branch channel mean velocity decreases.4. With regard to the results, the flowmeter measurement accuracy is fairly desirable in the middle of the channel in specific longitudinal distances except for the rotating areas. In case the flowmeters are installed exactly in the complex rotation areas and the deviation location their measurement accuracy decreases and the maximum error is equal to 139 percent in some cross sections and this error percentage occurs in wr= 1.0 width ratio (Fig. 1). In other words when the branch channel width is equal to the main channel width, the difference between the velocity read by the flowmeter and the branch channel mean velocity reaches its maximum level.