Capture envelope of an exhausted opening under cross draft: a numerical approach.

The performance of an exterior hood is known to be affected by the cross draft (1, 2). Based on the knowledge from a classical "Rankin's nose" or "semi-infinite body" problem, in which the opening shrinks to a sink instead of a finite opening, the exhausted airflow combining the cross draft forms a capture envelope in front of the hood(3). All streamlines within the envelope lead to the hood opening, those outside of the envelope lead to infinity. Therefore, contaminant released inside the envelope tends to be captured by the hood; otherwise, it tends to escape beyond capture. In this study, the hood is modelled by a circular opening on an infinite wall. The capture envelopes formed by the combination of the exhaust airflow and cross draft are computed numerically based on the potential flow theory. The general geometric pattern of the envelope then is determined by compiling the results from extensive computations. A recent study conducted in a wind tunnel also justified the existence of capture envelopes and their consistency with computation based on the potential flow theory( 4 ).