On the aspiration characteristics of large-diameter, thin-walled aerosol sampling probes at yaw orientations with respect to the wind

Abstract

Experiments were carried out in a large wind tunnel to investigate the aspiration efficiencies of thin-walled aerosol sampling probes of large diameter (up to 50 mm) at orientations with respect to the wind direction ranging from 0 to 180 degrees. Sampling conditions ranged from sub- to super-isokinetic. The experiments employed test dusts of close-graded fused alumina and were conducted under conditions of controlled freestream turbulence. For orientations up to and including 90 degrees, the results were qualitatively and quantitatively consistent with a new physical model which takes account of the fact that the sampled air not only diverges or converges (depending on the relationship between wind speed and sampling velocity) but also turns to pass through the plane of the sampling orifice. The previously published results of Durham and Lundgren (1980) and Davies and Subari (1982) for smaller probes were also in good agreement with the new model. The model breaks down, however, for orientations greater than 90 degrees due to the increasing effect of particle impaction onto the blunt leading edge of the probe body. For the probe facing directly away from the wind (180 degree orientation), aspiration efficiency is dominated almost entirely by this effect.