PARTICLE DISPERSION IN THE DEVELOPING FREE SHEAR-LAYER .2. FORCED FLOW

In this study we analyse the dispersion mechanisms of small water particles in an acoustically forced plane, turbulent mixing layer. When compared to the naturally developing flow, the excited mixing layer is shown to exhibit drastic changes in the cross-stream particle concentration evolution, with the particles now dispersing laterally at larger rates than that of the longitudinal momentum of the turbulent gas glow. The particle dispersion is shown to occur as a size-selective process characterized by the existence of an intermediate particle size range for which the lateral dispersion is maximized. Unlike in the natural flow evolution, the forced shear layer does not possess a non-dimensionalization rendering particle size independent dispersion properties. It is demonstrated that this behaviour results from the non-similarity of the developing gas motion. The mixing layer is shown to have inhomogeneities both in the droplet concentration and in the droplet-size probability density distribution function. Instantaneous flow visualizations as well as spectral analysis of laser extinction measurements show the presence of a coherent organization in the particle concentration field resulting from the large-scale eddies characterizing the underlying turbulent gas flow. Conditional, phase-average sample techniques are used to analyse the structure of this coherent particle dispersion field. The dispersion is shown to be controlled by an array of large streaks that emanate from the undisturbed spray, engulfing areas which are almost depleted of droplets. The data from the conditional sampling measurements are in good agreement with preliminary results from a simplified Eulerian model of the particle motion, showing the potential that this formulation can have for analysing this type of flow.