On the retention of inertialess particles in the near wake region of a two-dimensional flat plate placed normal to an aerosol-laden freestream

The dispersion of inertialess particles in the recirculation zone immediately behind long, two-dimensional flat plates positioned perpendicular to an aerosol-laden freestream was investigated both experimentally and numerically. The physical situation corresponds to the release of particles from a two-dimensional line source positioned upstream of the flat and consequent entrainment of those particles into the near wake region. After the sudden interruption of the aerosol generation process, the particles persist in the near wake, with the number concentration decaying exponentially with time. The experiments were carried out in a low-turbulence wind tunnel. An optical detection system was developed for the fast observation and recording of changes in the particle concentration in the near wake of the plate. The dimensionless residence time for fine inertialess smoke particles in the near wake region H(= τU/D, where τ is the time constant for the exponential concentration decay, U is the undisturbed freestream air velocity, and D is the width of the obstruction) was measured in the range of Reynolds numbers (Re) from about 1000 to 10,000. H was found to be approximately constant at about 9.5 for Re above about 4000. These results were in agreement with a simple model and with numerical calculations based on a two-equation, steady-flow, k — ε approach with coefficients chosen to reflect to some degree the actual physical situation which also includes vortex shedding.