In mountainous rivers, the drag force acting on cobbles abundant in the riverbed surface is important in predicting behavior and response of the river. However there is little research for the drag coefficients of cobbles. This paper is to carry out the experiments for drag force of cobble and analyze the relation between the cobble shape and the drag coefficient. The effects of the shape factor on the drag coefficients when the long axis or the short axis of the cobbles are parallel to the direction of flow velocity were analyzed. The coefficient of drag force increased with the nominal diameter Reynolds number . The drag coefficients are greater in short axis than long axis. The coefficient of determination of the relation between and is greater in long axis than short axis. This means that the drag forces acting on the irregularly-shaped cobbles depend on the axis. A change of the drag force distribution has brought about the alternative swing of cobbles. For > 12,000, the amplitude of the swing has been increased sharply and especially was greater in short axis than long axis.

1. Achenbach, E. (1974). “Vortex shedding from spheres.” Journal of Fluid Mechanics, Vol. 62, No. 2, pp. 209-221.  

2. 

3. 

4. 

5. Lecoq, N., Anthore, R., Cichocki, B., Szymczak, P., and Feuillebois, F. (2004). “Drag force on a sphere moving towards a corrugated wall.” Journal of Fluid Mechanics, Vol. 513, pp. 247-264.  

6. 

7. Loth, E. (2008). “Drag of non-spherecal soild particles of regular and irregular shape.” Powder Technology, Vol. 182, pp. 342-353.  

8. 

9. O’Neill, M. E. (1968). “A sphere in contact with a plane wall in a slow linear shear flow.” Chemical Engineering Science, Vol. 23, pp. 1293-1298.  

10. 

11. Shin, S. S., Park, S. D., Lee, S. K., and Ji, M. G. (2015). “Estimating critical stream power by the distribution of gravel-bed materials in the meandering river.” Journal of Korea Water Resources Association, Vol. 45, No. 2, pp. 151-163 (in Korean).  

12. 

13. 

14. Yang, J. J., Park, S. D., Shin, S. S., and Woo, T. Y. (2014). “Characteristics of bed material shape of gravel bed rivers in Youngdong area.” Journal of the Geomorphologial Association, Vol. 21, No. 1, pp. 33-49.