NUMERICAL INVESTIGATION OF RECTANGULAR RIB ROUGHNESS EFFECTS ON FLOW RESISTANCE IN OPEN-CHANNEL FLOW

Abstract

A series of numerical simulations are undertaken to investigate the flow resistance of a rough open-channel flow. The roughness is generated by periodically spaced bars and transversely glued on the channel bottom. The free surface flow is modeled by using the finite volume method and the Reynolds stress model coupled to the two-phase model. The results obtained show that for a given bar thickness, the maximum flow resistance is obtained when the ratio of cavity length to roughness height is around 12.5. For several longitudinal thicknesses of the bars, it is found that the flow resistance and the generated roughness are higher when the longitudinal thickness-to-height ratio of the bars is less than unity. For given streamwise spacing bars, the friction factor and the generated roughness are inversely proportional to the longitudinal thickness of the bars. By including the geometric dimensions of the roughness elements and their spacing, a relationship between the friction factor and the generated roughness is derived. A good agreement was found with the existing expressions in the relevant literature. It is postulated that the extension of the proposed formula to the large-scale roughness reproduces acceptable results.