Experimental studies of particle transport in air flows near bluff bodies, with and without aspiration

This paper reviews experiments which have been carried out to investigate several features of the transport in the vicinities of bluff bodies of particles large enough to exhibit inertial effects. The work is stimulated by its relevance to the behavior of particulate contaminants in occupational and other environmental situations. For simple bluff body flows without aspiration, we have concentrated on effects in the near wake. Flow visualization showed clearly the effects of 'de-mixing' as the inertial parameter (St) increased, eventually to the point where particles are unable to enter the near wake cavity. Studies of the residence times of large particles injected at the base of a disc-shaped plate also showed strong dependence on St, and the results were consistent with an explanation based on the role of inertial forces in the macroscopic mean recirculating near wake flow. Bluff body flows with aspiration were studied mainly from the point of view of aerosol sampling. New experiments were developed to enable measurement of aspiration efficiency (A), reflecting the ability of particles to be transported from the external freestream and through the sampling orifice. The results for A as functions of St and entry orifice orientation with respect to the freestream (α) showed strong trends for the forwards-facing orientations (α < 90°), but much less so for large orientations (α > 90°). All the trends were most marked for large St, reflecting the importance of inertia in the aspiration process. Comparison of results for α = 90° and 180° respectively shows good agreement with current aspiration efficiency models.