Deflecting Velocity Rod for Flow Measurements in Small Channels

Abstract

A hinged rod, when immersed in flowing water, deflects against its weight due to the hydrodynamic force of water. A relationship was derived to calculate the average flow velocity in small channels from the deflection of the rod, length and thickness of the rod, density of the rod's material, flow depth, and an experimentally determined rod's velocity coefficient. The rod's velocity coefficient was found to be invariant with flow velocity (V) and Reynolds number (R) for the range investigated in this study (V ⩽ 60 cm/s and 25,000 ⩽ R ⩽ 60,000). The calculated velocity compared very well with the measured velocity, with an average error of only 0.7%. Analysis suggested that a rectangular rod made of seasoned wood (density ⩾ 0.7) with length = 100 cm, width = 2 to 4 cm, and thickness = 3 to 5 cm will provide a simple but acceptable technique for determination of average flow velocity in small prismatic channels having a flow velocity ⩽1 m/s and flow depth ⩽45 cm with less than 5% error.