Stability and transition of buoyancy-induced flows in a stratified medium

Abstract

An experimental and theoretical investigation has been carried out to determine the effect of a stable ambient thermal stratification on the developing buoyancyinduced flow adjacent to a flat vertical surface dissipating a uniform heat flux. The nature of the resulting base flow and its instability characteristics, linearized for two-dimensional disturbances, were analysed for Prandtl numbers Pr of 6·7 and 0·733, for several levels of ambient stratification. Stratification was found to cause initial stabilization of the flow but later destabilization downstream. Disturbance growth rates, frequency filtering and amplitude distributions across the boundary region were calculated. These aspects of the disturbance field were measured in a flow generated by an electrically heated metal foil, with artificially introduced two-dimensional disturbances, in water (Pr = 6–7). The experimental results are in very good agreement with the calculations. Measurements of natural transition indicate that a stable ambient stratification delays the onset of transition. A tentative transition-correlating parameter is generalized to include the effect of stratification.