Experimental investigation of air flow around blunt aerosol samplers

Abstract

To evaluate better the aerosol collection behavior of blunt samplers, the flow field around a two-dimensional cylindrical sampler and a spherical sampler is investigated experimentally. The velocity along the symmetric axis of the cylindrical sampler is measured with LDV. The measurements are in very good agreement with viscous numerical calculations. The stagnation distance and the location of the separation point on the spherical sampler is studied by flow visualization. Smoke is employed to illuminate the fluid flow around the sampler. When the sampler faces the wind and the ratio of suction velocity to free-stream velocity U s/ U 0 > 1, the experimental measurements are in excellent agreement with theoretical inviscid calculations. For U s/ U 0 ≤ 1, the theoretical predictions are higher than the measurements. When the sampler is oriented at an angle to the oncoming flow, the stagnation distance on a symmetric plane is consistent with the distance predicted by a theoretical inviscid calculation. Unfortunately, on an asymmetric plane, the experimental method breaks down and accurate results cannot be obtained. An appropriate experimental technique for this asymmetric study remains to be developed. The separation point on a spherical sampler at different orientations and different velocity ratios is also experimentally examined. With suction, the farther the sampler rotates, the stronger the influence of the suction on the location of the separation point. After rotating to angles greater than 90 °, the separation point reaches an asymptotic value. This asymptotic value depends on the velocity ratio. At different velocity ratios, higher suction rates push the separation point further back, and the location of the separation point and its asymptotic value moves backward with an increase of the velocity ratio. When the sampler opening locates past the separation point, the flow near the opening entrance becomes turbulent, and laminar or inviscid flow models are inadequate to calculate aspiration efficiency