Influence of Particle Shape on Drag Coefficient for Commonly Occuring Sandy Particles in Coastal Areas

Abstract

A well defined relationship connecting settling velocity with sediment geometry and ambient properties is an essential pre-requisite for coastal and hydraulic engineering studies. An established relationship for settling velocity of sandy particles assuming spherical shape geometry is available in the literature. In reality, the sediment particles need not be spherical at all times, which influences settling velocity that is strongly biased to the drag coefficient. Based on quantitative comparison with measured data collected at Oahu Islands located in the Hawaiian archipelago, USA this work provides a relationship between drag coefficient and particle shape factor for sand grains viz; sand, sandy loam and fine sandy loam typically found in coastal environment (typical size ranges from 0.05 to 2.0 mm). The particle Reynolds number and shape factor are evaluated for each grain. The drag coefficient evaluated as function of nominal diameter and Reynolds number show a positive correlation over a wide range of shape factors used in this study. A comprehensive correlation has been developed of the drag coefficient for non-spherical particles as a function of Reynolds number and particle shape. Further a regression analysis was performed on the functional dependence of drag coefficient on particle shape. Based on this study, it could be advocated the validity of Krumbien shape factor holds well for the above characterized grain size and various particle shapes considered. Hence, the settling velocity of particles has a functional dependence on estimated drag coefficient with important implications for modeling sediment transport and swash zone hydrodynamics.