THE EFFECTS OF BLUNTNESS AND ORIENTATION ON 2-DIMENSIONAL SAMPLERS INCALM AIR

This article uses numerical simulation to investigate the effect of sa mpler bluntness, particle size, and sampler orientation on aspiration efficiency in calm air. The procedure is to first numerically solve th e velocity field around the sampler in calm air and then to trace the particle trajectories and calculate the the aspiration efficiency. Two samplers are studied: a two-dimensional parallel plate and a two-dime nsional blunt cylinder. The variation of aspiration efficiency with pa rticle size shows two minima between two asymptotic values. When the s amplers are facing upward, the asymptotic values are 1 for very small particles and the ratio of particle settling velocity to suction veloc ity for very large particles. At other orientations, the horizontal-fa cing and the downward-facing, the asymptotic value for large particles is 0. The sampler bluntness has an important effect in the region of particle size where there is competition between the particle inertia and the fluid drag force (i.e., 5 mum < d < 100 mum in our case). A bl unt sampler always has higher aspiration efficiency than does a sharp- edged sampler in this region of particle sizes. For very small particl es and very large particles, the aspiration efficiencies approach asym ptotic values and the sampler bluntness has little effect. The results also show that the sampler orientation affects the predicted aspirati on efficiency.