Author(s): Mirali Mohammadi
Linked Author(s):
Keywords: Open channel flow; Friction factor; Depth-averaged velocity; Boundary shear stress
Abstract: In open channels, estimation of the local and global friction factor is one of the most importan tasks and it sometimes is very difficult. This difficulty arises from the existence of the secondary flow cells and the boundary induced turbulence, both of which affect the distributions of depth-averaged velocity and boundary shear stress, and consequently the local friction factors. A series of measurements was undertaken for velocity and boundary shear stress in fully developed turbulent uniform flow for a smooth open channel. Equations relating the transverse distribution of apparent shear force across the channel boundary for various flow conditions are presented. The experimental overall friction factors were evaluated from the stage-discharge relationships applying the Darcy-Weisbach equation. Using the measured local depth-averaged velocity, U d, and the local bed shear stress, τ b, the local depth-averaged bed friction factor, f b, can be calculated for each lateral position by applying f b =8τ b /ρU d 2. The lateral distribution of local depth-averaged friction factors show that the distributions are close together for low flow discharges in both sub critical and supercritical flows, but they are more variable for the higher flow discharges. In other words, the normalized local friction factors, f b /f, illustrate that the discrepancy in the higher flow rates are larger than the lower flow rates. More importantly, the Figures show that the local friction factor values tend towards a more constant distribution at high flow rates as the Froude numbers increase. It can also be seen that the local friction factor is found to be decreased from the sidewalls to the channel centerline.
Year: 2004