AERODYNAMIC ENTRAINMENT THRESHOLD - EFFECTS OF BOUNDARY-LAYER FLOW CONDITIONS

Experimental data are presented demonstrating the influence of boundar y layer flow conditions on aerodynamic entrainment of grains in the ab sence of intersaltation collisions. New methods are proposed for (1) t he unambiguous determination of aerodynamic threshold for any grain po pulation and (2) approximation of the probability density function (PD F) distributions of threshold shear velocity for aerodynamic entrainme nt. In wind tunnel experiments, the orderly spatial development of flo w conditions within a developing boundary layer over the roughened sur face of a flat plate constrains the aerodynamic threshold condition in terms of both mean and fluctuating values. Initial grain dislodgement s and subsequent erosion from narrow strips of loose, finely fractiona ted ballotini were recorded photographically as wind speed was increas ed. Boundary layer parameters, including average threshold shear veloc ity (Ut), were calculated using the momentum integral method. Direct observations show that sporadic oscillation of grains preceded dislodg ement. At slightly higher velocities most grains rolled over their nei ghbours before entering saltation. Initial entrainment in spatially se mi-organized flurries of 50 or more grains was followed by quiescent p eriods at airflow velocities close to threshold. These observations pr ovide strong circumstantial evidence linking both the nature and spati al pattern of initial grain motions to sweep events during the fluid b ursting process. For each grain fraction, values of Ut were found to span an unexpectedly wide range and to decrease downwind from the lead ing edge of the plate as turbulence intensity increased. A probabilist ic entrainment model is applied to the aerodynamic threshold condition so as to incorporate the effects of changing turbulent flow regimes o ver the plate. Analysis of strip erosion curves gives both an objectiv e definition of the threshold condition and usable approximations of t he PDF for Ut required by the model and for future stochastic treatme nt of the threshold condition.