A MATHEMATICAL STUDY OF AEROSOL SAMPLING BY AN IDEALISED BLUNT SAMPLER ORIENTED AT AN ANGLE TO THE WIND: THE ROLE OF GRAVITY

In this paper a mathematical approach is adopted to model the flow of air and airborne particles into an idealised spherical aerosol sampler whose single circular inlet is oriented at an angle α to the undisturbed freestream. From detailed knowledge of the airflow, the paths of suspended particles in the flow were calculated and the aspiration efficiency of the sampler determined. In tracing the particle paths both inertial and gravitational forces were considered, and values of α up to 60° were investigated. The results were compared with data obtained for the same sampling system in wind tunnel studies, and were generally in good agreement. It was found that the common practise of neglecting gravity when predicting aspiration is only reasonable in certain situations, for example, for samplers facing the airflow the effects of gravity upon aspiration were found to be negligible even at low wind speeds of O(10 −1 ) m s −1 . However, as the sampler orientation to the airflow increases above zero this assumption was found to lead to increasing errors. In the case of an orientation of 60° to the airflow errors in aspiration efficiency greater than 10% were found to occur even in wind speeds as large as 6 m s −1 for the samplers considered.