THE ACCUMULATION AND DISPERSION OF HEAVY-PARTICLES IN FORCED 2-DIMENSIONAL MIXING LAYERS .1. THE FUNDAMENTAL AND SUBHARMONIC CASES

This paper presents detailed computational results for the dispersion of heavy particles in transitional mixing layers forced at both the fu ndamental and subharmonic frequencies. The results confirm earlier obs ervations of particle streaks forming in the braid region between succ essive vortices. A scaling argument based on the idealization of the s patially periodic mixing layer as a row of point vortices shows that t he formation of these concentrated particle streaks proceeds with opti mum efficiency for St similar or equal to 1. It thereby provides a qua ntitative basis for experimental and numerical observations of prefere ntial particle dispersion at Stokes numbers of order unity. Both the m odel and full simulation furthermore exhibit oscillatory particle moti on, as well as the formation of two bands of high particle concentrati ons, for larger Stokes numbers. The particle dispersion as a function of time and the Stokes number is quantified by means of two different integral scales. These show that the number of dispersed particles doe s not reach a maximum for intermediate Stokes number. However, when th e distance is weighted, optimum dispersion is observed for Stokes numb ers around unity. By tracing the dispersed particles backwards in time , they are found to originate in inclined, narrow bands that initially stretch from the braid region into the seeded free stream. This sugge sts that particle dispersion can be optimized by phase coupling the in jection device with the forcing signal for the continuous phase. In th e presence of a subharmonic perturbation, enhanced particle dispersion is observed as a result of the motion of the vortices, whereby a larg er part of the flow field is swept out.