The effects of vent aspect ratio oil oscillatory flow regime through a horizontal opening were studied numerically. The physical model consisted of a vertical rectangular enclosure divided into two chambers by a horizontal partition. The partitions contained it slot that connected the two chambers. The upper chamber contains cold air and the lower chamber contains hot air. A density differential due to the different temperatures drives the interaction between the two cliambers. The opposing forces at the interface between the two chambers create a gravitationally unstable system, and all oscillating exchange of fluid develops. Results were obtained for cases with L/D = 1.0.5, and 2.0, where L represents the thickness of the partition and D represents the slot width of the opening in the partition. Results indicate that the flow exchange increases with partition thickness L/D = 0.5 and decreases for L/D = 2. The frequency of the oscillatory flow pattern is also examined for the different cases. Sudden bursts of upflow with a corresponding downflow have been documented and compared with experimental observations in the literature. The time traces of velocity and temperature fields for this flow regime reveal interesting mechanisms, which have been explained. (c) 2008 Elsevier Ltd. All rights reserved.