BOUNDARY-LAYER FLOW STRUCTURES ASSOCIATED WITH PARTICLE REENTRAINMENT

Measurements of near-surface longitudinal and vertical wind velocity c omponents associated with particle reentrainment from a flat surface h ave been examined in a wind tunnel. Sparsely covered particle beds wer e used to assure that observed reentrainment events resulted primarily from the action of fluid forces. Characteristic velocity patterns wer e found to be associated with a majority of particle reentrainment eve nts examined. These characteristics have been categorized and examined as ensemble averages. The flow pattern most frequently observed durin g particle reentrainment was termed Ejection-Sweep (E-S) and is very s imilar to organized fluid motions previously observed in laboratory fl ows and in the atmospheric-boundary layer. A simple two-tiered E-S pat tern recognition scheme is described which strives to identify particl e 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 sec ond step is to use threshold values of pattern characteristics to acce pt or reject these first-tier patterns. Pattern recognition results ar e presented in terms of the ability to identify reentrainment events v ersus false identifications, and show an exponential growth in false i dentifications with an increasing number of reentrainment events ident ified.