Buoyant convection in an enclosure in time-dependent acceleration field

Abstract

A numerical and theoretical study is performed for transient buoyant convection in an enclosure, filled with a pure fluid or fluid-saturated porous medium, under time-dependent acceleration field. The vertical sidewalls are at two different temperatures, and the other walls are thermally insulated. The system is exposed to time-dependent acceleration fields on terrestrial(chapter 2) and null(chapter 3,4,5) background gravity field. Major emphasis is placed on the effect of unsteady acceleration field on the onset of thermal and flow field in a convection system. In chapter 2, a numerical investigate is made of natural convection of a paramagnetic incompressible fluid in a cubic enclosure having periodically-varying mechanical (Case I) and magnetizing (Case II) force under terrestrial background gravity field. The magnetizing force for Case II is induced by an electronic coil placed below the bottom wall. In both cases, resonance takes place when the frequency of oscillations matches the basic mode of internal gravity oscillations. At the resonance frequency, the amplitude of Nusselt number fluctuation is maximized and convective activities in the interior are intensified. It is worth pointing out that the occurrence of resonance does not depend on the kind of external forcing. The finding in the present work suggests that the resonant convection in a cubic can be accomplished in realistic experimental apparatus and engineering system by a relatively simple way by applying a time-periodic magnetic forcing. In chapter 3, the convective process of a pure-fluid cavity system in response to abrupt imposition of the gravity from the motionless null-gravity initial state is studied. A scale analysis is performed. Numerical solutions to the governing Navier-Stokes equations are acquired. A significant dynamical element in this flow layout is the presence of pre-existing cavity-scale horizontal temperature gradient. Due to this thermal configuration, strong bulk f...