An interesting transport phenomenon is observed through openings between two compartments separated by a thin, vented, horizontal partition such as those between containment areas in nuclear power systems, in other installations in the event of fire, passive cooling of heated structures, and in natural building ventilation. A heavier fluid located on the top of a lighter fluid and separated by a horizontal vent constitutes a gravitationally unstable system. Horizontal vents produce flow, which are unstable with irregular oscillatory behavior. The objective of the present work was to simulate such type of flow across a circular opening in a horizontal partition in the presence of a buoyancy force. Unsteady, axisymmetric Navier–Stokes equations were solved using the in‐house CFD code based upon the well‐established finite volume methodology. In terms of temporal differencing a second‐order accurate Crank–Nicolson scheme was used. Interpolation to cell faces for the convective terms was performed using a third‐order QUICK scheme and a second‐order central differencing was used for the viscous terms. Pressure‐velocity coupling was based on the SIMPLE procedure. The upper chamber was filled with salt water and the lower chamber with fresh water, creating a density differential between the two chambers. Opposing forces at the interface created a gravitationally unstable system, and an oscillating exchange of fluid developed. Several cases were numerically simulated and classified with respect to an aspect ratio L/D, where L represents the thickness of the partition and D represents the diameter of the opening in the partition. The flow coefficients and pulsation frequency have been determined. The effect of opening aspect ratio on frequency and flow coefficient was observed. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.21023
