Heating a salinity gradient from a vertical sidewall: linear theory

When a body of fluid with a vertical salinity gradient is heated from a single vertical wall, instabilities have sometimes been observed experimentally (Thorpe, Hutt & Soulsby 1969; Chen, Briggs & Wirtz 1971; Tsinober & Tanny 1986). We present a linear stability analysis for this configuration and show that for strong salinity gradients the stability of the fluid to infinitesimal disturbances is governed by a single non-dimensional parameter, \[ Q = {(1-τ)^6g(αΔ T)^6}{νκ_Sl^2(-̄_z)^5} \] where g is the acceleration due to gravity, α the coefficient of thermal expansion, β the density change due to a unit change in the salinity, Δ T the change of temperature at the wall, S̄z the vertical salinity gradient, l the horizontal lengthscale ( k T t ) ½ , v the kinematic viscosity, (κ T t ) ½ the diffusivity of heat, k S the diffusivity of salt and τ the salt/heat diffusivity ratio. This non-dimensional parameter is related to the Rayleigh number, however, it involves two different lengthscales; the penetration depth of the thermal front, l , and the height by which a heated element of fluid would rise in the salinity gradient, gα Δ T/(- β S̄z) . This analysis is valid when the ratio of the vertical lengthscale to the horizontal lengthscale is small. This analysis gives good agreement with the published experimental results.