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Distribution of Local Friction Factor in Flat-bed Part-full Pipe Channels

Congress: 2008
Author(s):
Dr Mirali Mohammadi Department of Civil Engineering, Faculty of Engineering, The University of Urmia, P O Box 165, Urmia 57169-33111, Iran. and The University College of Science & Technology (Elm-o-Fann), Urmia, Iran. e-mail: m.mohammadi@mail.urmia.ac

Keyword(s): Keywords: Part-full pipe channel, flat-bed pipe channel, turbulent flow, local friction factor, 2D isovels, depth-averaged velocity, boundary shear stress.
AbstractAbstract: The relationship between circular pipes running full and resistance coefficients has been well documented in the literature. The influence that the free surface exerts on the resistance parameters has also been examined by a number of researchers. This paper focuses on the experimental study on the distribution of local friction factor, f, of circular pipe channels flowing part-full with flat-bed as in practice, it occurs in sewer pipes and culverts. Several series of measurements were conducted to explore the hydraulic characteristics of partly full flat bed pipe channels by measuring velocity and boundary shear stress. Also to understand the effect of cross sectional channel shape on the distribution of depth-averaged velocity in the experimental channels, cases with different flow rates were examined. The fluid flow was in turbulent flow regime and fully developed, which is often flows in practical channels. Using Surfer software, the contour plots of 2D isovels are drawn interpolating among averaged depths and velocities at a given position, obtained from superposing the various profile sections. It can be seen that the isovels are parallel to the channel boundary in a region close to the bed, and almost symmetric about the centerline, but not completely. A strong water surface phenomenon is observable throughout the isovels plots. Consequently, it is said that the presence of secondary currents and cross sectional shape effects should be responsible, in this case. The transverse distribution of depth-averaged velocity and boundary shear stress across the channel section for various flow conditions are presented. Local boundary shear stress around wetted perimeter, τb, and depth-averaged velocity, Ud, data are used to evaluate the local Darcy-Weisbach friction factor, f, parameter. The results illustrate that the friction factor is shown to be a function of flow depth as well as being dependent upon the Froude number, Fr. Therefore, local friction factor does not remain constant across the channel. Moreover, the common technique of assuming a constant friction factor over the entire section of the channel has been found to have little theoretical justification. The results of the present study indicate that the global friction factor decreases as flow discharge increases and channel bed slope getting steeper in the experiments that included subcritical and supercritical flow conditions. However, it can be seen that the effect of walls is so high at low Froude numbers. In general, the results indicate that the perturbations in the distribution of local friction factor are quite considerable in steeper channels as Froude number increases. It is also occurred for the distributions of boundary shear stress as well as depth-averaged velocity results at the same flow conditions. The information contained in this paper should prove valuable to practicing engineers and numerical modelers alike.
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