ANALYSIS OF RESISTANCE TO FLOW IN OPEN CHANNELS WITH ARTIFICIAL ROUGHNESS ELEMENTS

Abstract

This thesis presents the results of an experi-mental investigation concerning the resistance to flow in rigid boundary open channel with artificial roughness elements in the form of spheres arranged with different concentrations. The studies were carried out in a tilting flume of 15 m. long, 40 cm wide and 60 cm high with 25 mm spherical roughness elements. Recently it has been found that the velocity distribution in extremely rough pipes and channels deviates from the logarithmic law obtained by Prandtl and Karman especially near the boundaries. To explain this deviation, a mathematical model has been obtained and the model has been tested with the help of experimental data. The examination of velocity distribution data over the roughness elements with various concentration has shown that the deviation from logarithmic law occurs only in a distance of the order of roughness height while at larger distances from the bed the velocity distribution coincides with the logarithmic velocity distribution law. This deviation can be explained on the basis of an additional mixing length which as a function of relative roughness, shape and concentration of the roughness elements. A relationship has been obtained for additional mixing length as a function of roughness concentration, 4 relative roughness and roughness shape . From the velocity distribution data, an expression has also been obtained for resistance coefficient.