Aerodynamic entrainment threshold: effects of boundary layer flow conditions

摘要: Experimental data are presented demonstrating the influence of boundary layer flow conditions on aerodynamic entrainment grains in absence intersaltation collisions. New methods proposed for (1) unambiguous determination threshold any grain population and (2) approximation probability density function (PDF) distributions shear velocity entrainment. In wind tunnel experiments, orderly spatial development within a developing over roughened surface flat plate constrains condition terms both mean fluctuating values. Initial dislodgements subsequent erosion from narrow strips loose, finely fractionated ballotini were recorded photographically as speed was increased. Boundary parameters, including average (U*t), calculated using momentum integral method. Direct observations show that sporadic oscillation preceded dislodgement. At slightly higher velocities most rolled their neighbours before entering saltation. spatially semi-organized flurries 50 or more followed by quiescent periods at airflow close to threshold. These provide strong circumstantial evidence linking nature pattern initial motions sweep events during fluid bursting process. For each fraction, values U*t found span an unexpectedly wide range decrease downwind leading edge turbulence intensity A probabilistic model is applied so incorporate effects changing turbulent regimes plate. Analysis strip curves gives objective definition usable approximations PDF required future stochastic treatment condition.