Particle motion in the vicinity of a bulky sampling head operating in calm air

A study is made of the aspiration of particles into a bulky sampling head with a sampling orifice located at a general point on its surface. The particles are assumed to be suspended in an ideal fluid that is at rest apart from the motion caused by the action of suction into the head. The sampling head considered is a two-dimensional circular cylinder with the orifice modelled as a line sink. The work extends that by Davies and Peetz (1954) and Davies (1967), where the aspiration of particles into the top and bottom of a two-dimensional cylindrical sampling head were considered. It is found that the region around the head can be divided into four regions, namely, where particles (i) enter the sampling head, (ii) deposit on the head, (iii) pass by the head, and (iv) where no particles enter and hence forms a shadow. The size and shape of these regions is found to depend not only on the position of the sampling orifice on the surface of the head, but also on the value of a nondimensional parameter k. This parameter is a multiple of the ratio of the particle settling velocity times the sampler size to the rate of aspiration of the fluid by the sampler. For all positions of the sampling orifice, the shadow zone is found to extend to infinity for large values of k, which corresponds to small suction rates, but becomes finite as k decreases.