Boundary-layer flow structures associated with particle reentrainment

Measurements of near-surface longitudinal and vertical wind velocity components associated with particle reentrainment from a flat surface have been examined in a wind tunnel. Sparsely covered particle beds were used to assure that observed reentrainment events resulted primarily from the action of fluid forces. Characteristic velocity patterns were found to be associated with a majority of particle reentrainment events examined. These characteristics have been categorized and examined as ensemble averages. The flow pattern most frequently observed during particle reentrainment was termed Ejection-Sweep (E-S) and is very similar to organized fluid motions previously observed in laboratory flows and in the atmospheric boundary layer. A simple two-tiered E-S pattern recognition scheme is described which strives to identify particle reentrainment events objectively based on flow characteristics alone. The first step is to identify potential E-S patterns using criteria which identify a characteristic longitudinal acceleration, and the second step is to use threshold values of pattern characteristics to accept or reject these first-tier patterns. Pattern recognition results are presented in terms of the ability to identify reentrainment events versus false identifications, and show an exponential growth in false identifications with an increasing number of reentrainment events identified.