Double-diffusive convection with imposed vertical mass flux

Abstract

Vertical advection and double-diffusive convection are both processes affecting the stratification of the oceans. The effect of vertical advection on double-diffusive convection is investigated by considering the situation in which a horizontal layer of fluid has imposed upon it a destabilizing vertical temperature gradient, a stabilizing vertical salinity gradient and a uniform vertical mass flux. The effect of the imposed vertical mass flux is to remove the stabilizing salt gradient, but not the destabilizing temperature gradient over most of the depth of the layer. This arises from the fact that heat diffuses much more quickly than does salt. The Peelet numbers γT = Wod/κT and γS = Wod/κS where κT and κS are the molecular diffusivities of heat and salt respectively, d is the depth of the layer, Wo the imposed vertical velocity, are dimensionless measures of the relative importance of advection and diffusion. Both the linear stability theory and the laboratory experiments show a decrease in the critical value of the Rayleigh number for the onset of oscillatory convection when |γS| \u3e 0, over that when γS = 0. The calculation is valid for |γS| ≤ 1. The laboratory experiments were conducted for 0 ≤ |γS| ≤ 3.2, where Rc was found to be reduced by as much as 20%