Numerical Examination of the Effect Of The Location Of Flowmeters In Intakes On Flow-Velocity Measurement Accuracy

Intakes are used to control water and to deviate a part of the flow in agricultural networks and water transfer systems. The flow velocity is measured by flowmeters in order to calculate the discharge passing through the channels. The threedimensional and complex nature of the flow in the intakes and the presence of strong secondary flows in the cross section cause a difference between the velocity measured by the flowmeter and the flow velocity in the channel. The manner in which the flowmeter is located in the intake, the velocity of the passing flows, and etc. affect the difference between the flowmeter and the actual mean flow velocity. The objectives of this study include examining the following issues in different width ratios: (a) simulating the model of the intake flow through the ANSYSCFX software (b) examining the effect of flow deviation in intakes on flow hydraulics and velocity distribution (c) examining the effect of the location of the flowmeter on the velocity measurement accuracy in the intake channel. The results obtained in different width ratios indicate that the measurement accuracy decreases in the flow separation zone and flow compression zone when the velocity is measured by a flowmeter and a numerical model due to the velocity fluctuations and flow hydraulics complexity. Therefore the flowmeters must be installed in the middle areas and the sides of the channel in order to increase the measurement accuracy so that the results will be more consistent with the actual velocity of the channel. Key terms: intakes, flow deviation, flow velocity, location of the flowmeter, ANSYS-CFX. INTRODUCTION Intakes are amongst hydraulic structures which are used for the purposes of controlling and deviating flow in agricultural and irrigation networks, sewer systems. 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 the 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 mouth 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 has decreased and it will increase in the opposite direction of the flow. The longitudinal velocities intensely increase in the compression zone due to the density of the flow lines and it 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 [1-7]. They install flowmeters in intakes in order to compute the flow discharge and measure the flow velocity. Flowmeters rely on measuring the velocity and calculate the discharge using the continuity equation as the multiplication of the mean velocity by the wet cross section ) mean U × ) h ( A = Q ( . The A(h) cross section is calculated 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 [12]. Velocity sensors present data holding the assumption that the data represents the entire cross section it must be noted that the measured velocity is often different from the mean velocity of the cross section [13]. The relationship between the measured value and the actual mean velocity of the flow depends on the volume of the sample of the velocity sensor and the hydrodynamic specifications of the measurement location [14, 15]. The area of the space being examined by the flowmeter is a threedimensional volume and the flowmeters sample a limited volume of the flow [16]. The mean velocity is