Experimental studies of fluid turbulence during the entrainment of an individual gravel particle.

Abstract

Experiments were performed in a laboratory flume to investigate the relative importance of different turbulence mechanisms for the entrainment of an individual gravel particle into bed-load transport. The bed conditions tested in these experiments were a mobile gravel bed with a rough-to-smooth (R-S) transition, a fixed gravel bed with a R-S transition, and a fixed gravel bed with a smooth-to-rough (S-R) transition. One location of interest was in a region of steady uniform flow located away from the transition (SS location), and the other location of interest was in a region of steady non-uniform flow located near the roughness transition (T location). Fluid velocity measurements were taken directly above a tracer particle during entrainment into bed-load. Turbulence statistics of interest in this investigation included the downstream velocity (u), vertical velocity (w), downstream normal Reynolds stress (u’u’), vertical normal Reynolds stress (w’w’), and the Reynolds shear stress (u’w’). Conditionally averaged results show a strong correlation of particle entrainment with increases in u and decreases in w for all SS and T locations over the bed conditions tested. The fixed bed R-S transition results also show a correlation between initial motion and increases in u’u’, and u’w’ for the SS location but not the T location. The peaks seen in the trends of the conditionally averaged u results primarily occur prior to initial motion, and the time difference increases with distance above the bed. This suggests that inclined coherent structures are responsible for the entrainment of individual gravel particles.