This paper reports the results of a study of the transient removal of contaminant particle from enclosures containing an obstacle. We study specially a phenomena occur sometimes called the step effect. This phenomenon may occur if the size of the obstacle is small enough in comparison with the length or height of the enclosure. These results are the basic instruments for finding a model for contaminant particle removal from an enclosure containing an obstacle. A numerical CFD code is developed and validated with different cases, and then proper two- and three-dimensional cases are modeled. The size of the obstacle affect the order of magnitude of the convection-diffusion terms in the Navier-Stokes equations, hence results in different phenomena while removing the particles. It may end to a simple removal of the particles from the enclosure or it may contain two or three steps in removal, which is due to increase in scale of magnitude of the convection terms in the Navier-Stokes equations. The results of this paper and Ref. [3] may be compacted into one whole theory, describing the particle removal efficiency from an enclosure as a function of obstacle position and size.

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