Numerical and Experimental Study of Flow in Open Channels with Cylindrical Roughness

Abstract

In this study effect of the different types of surface roughness on a turbulent boundary layer was studied using 1D velocity measurements in a relatively high Reynolds free-Surface water channel. The roughness consisted cylindrical ribs with several P/D (P is the roughness spacing, D is the roughness height). The roughness elements were aligned in a direction transverse to the flow. The experiments involve one-dimensional flow velocity, water surface profiles and bed pressures and forces with different Reynolds numbers. A series of 12 tests of fully rough turbulent subcritical flow over two dimensional transverse repeated cylindrical ribs was undertaken in which the ribs were varied to give uniform rib spacing-toheight ratios of P/D = 5-20. The results show that the normalized velocity U+ for all the surfaces has significantly decreased comparing to smooth bed. The average values of the drag Cf and friction Ft coefficients are reduced with increasing of roughness distances on rough beds, also the amount of the drag and friction coefficients are the highest values on the cylindrical roughness with P/D= 15, H= 8 cm, D= 6 mm. Flow development in an openchannel with rough beds was investigated using FLUENT. The RNG k-ε turbulence model was used for subcritical flow cases. The volume of fluid (VOF) method was used to allow the free-surface to deform freely with the underlying turbulence. The main purpose of this study is to examine the effects of surface roughness on a turbulent boundary layer in a relatively different Reynolds numbers. The determination of velocity profile in turbulent open channels with rough bed is a difficult task due to the significant effects of the roughness. The velocity distributions in the log-region for the rough wall were determined and the roughness function U+, was evaluated compared to the smooth bed. The results show that the mean profiles of all the surfaces collapse well in the velocity defect form.