THERMAL AND FLUID-FLOW INSTABILITIES IN BUOYANCY-DRIVEN FLOWS IN OPEN-ENDED CAVITIES

The basic features of the buoyancy-driven convection in an open-ended cavity are analyzed and an in depth presentation of the related results are given in this work. It is shown that, at higher Rayleigh numbers, a one to one relationship exists between the frequency of the periodic oscillations in the Nusselt number and the central vortex's oscillations and location inside the cavity. In fact, for any given type of oscillatory pattern (e.g. asymmetric sinusoidal or distorted W), the frequency of the oscillations of the Nusselt number (or the central vortex inside the cavity) increases linearly as the Rayleigh number increases. For air, this linear relationship was explicitly determined. Furthermore, it is determined that, for cases which Ra $ ̌9 × 105, the central vortex's oscillation frequency and/or amplitude becomes much higher and it starts to deviate from any type of repeatable pattern. It is proposed that it is this irregular high frequency and/or amplitude of the central vortex coupled with the flow separation around the mid-section of the lower block that triggers the transition to turbulent flow. © 1990.