Towards a general semi-empirical model for the aspiration efficiencies of aerosol samplers in perfectly calm air

Abstract

In previous papers we have described a body of experimental work, backed up by numerical studies, to acquire a large set of new data for the aspiration efficiencies of aerosol samplers of simple geometries in perfectly calm air and for a range of orientations. The samplers studied were idealized thin-walled and spherical blunt probes, and experiments were conducted for upwards, downwards and horizontal sampling. Taken as a whole, this is believed to be the most comprehensive data set yet obtained for aerosol sampling in calm air. In this paper, we have examined this complete data set—including both experimental and numerical data—and used it to construct a set of semi-empirical models for aspiration efficiency. The starting point for these is the basic physical model of Levin (1957) for the entry of particles into a point sink in calm air. Additional terms are introduced that account for the role of the sampler having finite dimensions and defined shape. The models are expressed in terms of groups of dimensionless combinations of variables that embody the effects of particle inertia, gravitational settling, sampler bluntness and sampler orientation with respect to the vertical. They are semi-empirical in that, although they thus embody physical mechanisms and ideas, the added terms (beyond the Levin-model starting point) containing those dimensionless quantities are incorporated into mathematical constructs that are entirely empirical. The constructs are such that they may be combined together in a single universal form that reverts back to the individual specific cases when those conditions are inserted. Agreement is generally good across the whole range of conditions studied. The small number of areas where agreement is less good may be explained in terms of experimental error or technical limitations in acquiring the experimental data.