تعيين ضريب زبري لوله هاي موجدار زهكشي براي شرايط واقعي جريان

Determination of Roughness Coefficient for Corrugated Drainage Pipes Based on Real Flow Conditions

The roughness coefficient of the corrugated drainage pipes (the nominal coefficient) that is found in the literature is applicable for uniform hydraulically rough turbulent flow. While, the flow condition in these pipes is turbulent spatially varied flow but either hydraulically rough or in the transition region. In spatially varied flow, tiny jets enter the pipe normal to the main flow direction and penetrate into the main stream which in turn drastically affect the velocity distribution across the channel and hinder the flow movement in the main flow direction. The first effect necessitates the application of the momentum correction factor, while the second one increases the nominal friction coefficient of the pipe. In this research the influence of the mentioned factors were examined using an experimental setup. Based on the governing equation of the spatially varied flow with increasing discharge in circular pipes the water surface profiles were determined and compared with the observed ones. The comparison indicated that computed profiles, based on assuming uniform velocity distribution and applying the nominal roughness coefficient, differ from that of observed ones. However, modifying the roughness coefficient and momentum correction factor produced good agreements between them. Consequently, based on the computed roughness coefficient and momentum correction factor, average values for the mentioned coefficients were proposed.