Effect of gravity modulation on natural convection in a vertical slot

Abstract

Abstract The effect of a sinusoidal gravity modulation on natural convection of fluids with constant property, in vertical slots of aspect ratio of 20 and 15, is studied numerically by a finite difference method. The gravity modulation is characterized by the nondimensional amplitude Rg and nondimensional frequency ωf. We find a low frequency g-modulation has a strong effect on the fluid flow but little effect on the heat transfer rate. The flow of air usually enters a periodic state after two cycles of g-modulation, much faster than that of large Prandtl number fluids. The response is generally in the synchronous mode, except at ωf = 10 and Rg = 1 for air where it appears to be subharmonic. For a multicellular primary flow of air, g-modulation at ωf = 25 and Rg = 1 changes the multicellular primary flow to a unicellular one, demonstrating significant stabilizing effect. For high Prandtl number fluids, a flow pattern, consisting of horizontal secondary cells, preceded the onset of the commonly observed vertical secondary cells. At low modulation frequencies, stability of convection is enhanced as demonstrated by the increase in the critical Grashof number, by approximately 154% at Rg = 1.6 and ωf = 40. The temperature remains unchanged within the cycle of the modulation, totally different from the case of air, and gives a critical gradient approximately 0.56 at the center.