Effects of Impact Energy and Aspect Ratio on the Motion of Particle Clouds in Stagnant Water

A series of laboratory experiments was conducted to investigate the effects of impact energy and other initial controlling parameters on the motion of particle clouds in stagnant water. Experiments were performed for two median sand diameters of D₅₀ = 0.52 mm and 0.74 mm and nozzle diameters of d_o = 6 mm and 8 mm. Sand masses were converted to an equivalent pipe length with the same diameter as the nozzle, L_o, and a wide range of aspect ratios, L_o/d_o, between 2 and 93 was tested. The impact energy of sand particles was controlled by the release height of sand particles, and it was quantified by the nondimensional release height, η, ranging from 1 to 21.5. It was found that particle clouds with higher impact energy had smaller concentration and velocity decay rates. This indicated that by increasing the release height, the momentum transfer between sand particles and the ambient water decreases. The time-series of instantaneous sand velocity were used to determine velocity fluctuations and turbulence intensity of sand particles, and a direct correlation was found between sand velocity fluctuations and aspect ratio in particle clouds. The effects of impact energy on the anatomy of the resulted particle clouds were examined in this study. It was found that the cloud width increased dramatically when the impact energy of sand particles with high aspect ratios (i.e., L_o/d_o > 39) increased. Furthermore, the dispersion of sand particle began earlier as the kinetic energy of sand particles increased at the water surface.