Experimental Study of Particle Clouds in Stagnant Water

Laboratory experiments were conducted to investigate the evolution of particle clouds in stagnant water. Nozzle size and released mass of particles were formed to an aspect ratio of $L_o/d_o$, where $L_o$ is a length of sand particles occupied in a pipe with a nozzle diameter of $d_o$. Characteristic length and time scales of $L_i$ and ${\tau }^{*}$ were found to classify the initial regime (sand jets), the transient regime (particle cloud with trailing stem), particle cloud without trailing stem, and swarm of particles. Image analysis indicated that the transition regime occurred for $1<{\tau }^{*}<5$ and swarm of particles occurred for ${\tau }^{*}>15$, where ${\tau }^{*}$ is the characteristic time scale for cloud classification. This classification was found to be applicable for $0.1375\mathrm{mm}\le D_{50}<0.507\mathrm{mm}$, where $D_{50}$ is the mean particle size. It was found that the formation of trailing stem can change the characteristics of transient particle clouds. Power law correlations were developed for progression depth, width, and frontal velocity of particle clouds with $L_o/d_o$ using length and time scales based on initial parameters of particle clouds. The effect of particle size on evolution of particle clouds was considered using Stokes number $S_t$. Wider particle clouds were observed for small $S_t$ because small particles have a higher tendency to follow water eddies. The effects of nondimensional release height of sand particles $\eta$ on spreading of particle clouds were also investigated. A relatively wide range of $\eta$ was tested for $1\le \eta \le 26.5$. For small $L_o/d_o$ (i.e., $L_o/d_o\le 1.5$), experimental results indicated a direct correlation between $\eta$ and the width of particle clouds. An adverse relationship was found for $1.5<L_o/d_o<15$.