Velocity profiles and friction coefficients in circular open channels

Velocity distributions and friction coefficients in a circular open channel have been investigated experimentally. To measure three-dimensional velocity components, a stereoscopic particle image velocimetry system was used. Based on the measured data, velocity distributions and wall friction coefficients were obtained using the non-linear least-squares fitting method. Unlike in rectangular channels, the velocity distribution varies significantly since the side walls affect the flow in a different manner as the flow depth changes. The present experimental data exhibit different characteristics whether the flow depth is higher or lower than 50% of the pipe diameter. The main effect of the flow depth on the velocity distribution is the ratio of mean to maximum velocities and the position of maximum velocity. Both values reduce if the flow depth is below 50%. Wall friction and Manning coefficients also differ from the generally estimated values as the flow depth is reduced by 50%.