A comparison of collisions of saltating grains with loose and consolidated silt surfaces

A particle tracking velocimetry system was used to study the trajectories of saltating sand particles as they impacted either consolidated ( solid) or unconsolidated ( loose) silt surfaces in a wind tunnel. The tunnel friction velocity varied between 0.26 and 0.3 m s(-1). The average coefficient of restitution, defined as the ratio of postcollision velocity to precollision velocity, was found to be epsilon = 0.64 for the loose bed and epsilon = 0.79 for the solid bed, respectively. The average ratio of energy loss to impact energy was found to be E-L/E-0 = 0.58 for the loose bed and E-L/E-0 = 0.37 for the solid bed, respectively. These indices demonstrate that the loose bed absorbs more momentum and energy from the impacting sand particle. The average ejection angle was lower at 18 degrees for the loose bed than 23 degrees for the solid bed. In the loose surface, crater formations were observed to form with each impact. Surface profile measurements suggest an average crater volume of >0.1 mm(3). For both beds, the coefficient of restitution decreases with the particle impact speed. In the case of the solid bed, the dependence on impact speed is in good agreement with a model of two colliding spheres with identical material properties. With regard to the loose bed, a model of the impacting particle motion as it plows through and plastically deforms the bed material is tested. If the ratio of the horizontal to vertical forces on the particle as it plows through the bed is taken as a linear function of impact speed, the model is in good agreement with measured data. This suggests that compaction of the surface may occur along with the plowing and displacement of loose bed material.