A Mathematical Study of the Sampling Characteristics of a Thin-walled Sampler Operating in Calm Air

Abstract

A study is made of the sampling characteristics of a thin-walled cylindrical sampler operating at various orientations in calm air. A mathematical approach is adopted to model the problem which gives an accurate description of the flow field in the vicinity of the sampling inlet. The paths of the particles in the air are then traced and the sampling efficiency investigated. In tracing the paths of the particles both inertial and gravitational effects are considered. The surface formed by the limiting particle trajectories, separating the sampled and unsampled particles, in the undisturbed flow determines the aspiration efficiency of the sampler. If the sampler is vertical, then the situation is symmetrical and the cross section of the surface formed by the limiting particle trajectories, e.g., A ₀ is known to be circular. For all other orientations of the sampler the symmetry is lost, and the shape A ₀ is then unknown and in this paper is investigated for various situations. The size range of particles investigated is extended to include particles that lie outside the Stokes regime and the sampling efficiency of the sampler determined for such particles. This efficiency is compared with the value obtained by assuming Stokes law and the two are found to be significantly different when the local particle Reynolds number is large.