Investigating Flow Resistance for Uniform and Random Rough Surfaces

This study aims at investigating flow resistance of uniform roughness in the largescale roughness condition and examining flow resistance and effective roughness of random roughness. Uniform roughness are constructed with three distribution patterns and different concentrations ranging from 8% to 32%. Influence of distribution concentrations and patterns on wave resistance as well as the total flow resistance are investigated with the help of the two equation turbulence model. For random rough surfaces, a new parameter namely coarseness parameter as well as two fundamental parameters namely steepness parameter and standard deviation are proposed to characterize roughness of gravel surfaces and to calculate flow resistance. A novel approach is employed to study these fractal based parameters. The resulting quantitative relationship is validated using the water-worked gravel surfaces. In addition to the fractal analysis based on the second-order structure function, multifractal analysis is adopted to characterize water-worked gravel surfaces. There is a close relationship between the strength of multifractality reflecting the heterogeneity and the development of a stable armour layer on the surface.