Cratering of a particle bed by a subsonic turbulent jet: Effect of particle shape, size and density

Abstract

The effect of particle properties on scour hole or crater formation, which occurs when a localized jet impinges on a bed of particles, is explored. Individual particle properties, such as particle density, size, and shape, are isolated and a parametric study is performed demonstrating the transient and steady crater growth for several particles. By keeping the particle diameter constant and changing the particle shape, this work shows that the effects of shape are significant. Uniform non-spherical (e.g. cylinders) and irregular frit shaped (e.g. crushed glass) particles are investigated. Non-spherical particles generally have higher friction, and the resulting decrease in recirculation rate of particles along the crater walls generally produces a narrower but deeper crater relative to spheres of the same size. A modification to the densimetric Froude number, which scales the steady crater depth to jet and particle properties is developed that accounts for particle shape. A new description of particle size called the erosion diameter is introduced, which is the average of the longest dimension a particle will align in a shear flow with its orthogonal, and results in the best scaling. The short-term crater depth grows faster for larger particles, which is counter to the long-term growth, and is explained by the early behavior being dominated by the number of particles that can be eroded. The transient crater depth growth is fit by an arctangent fitting function, which predicts the steady crater depth, and is compared to the logarithmic growth function that is generally used for early crater growth. For very large spherical particles, a high velocity is necessary for cratering and a newly identified particle–particle based cratering mechanism dominates. Additionally, cohesion reduces the crater depth and width and the steady crater depth does not scale with non-cohesive particles, but a further modification to the densimetric Froude number to account for cohesion is proposed.