Theoretical and experimental investigation of double-diffusive instability at a sharp interface

The stability of an initially sharp interface between two homogeneous layers of fluid differing in the concentration of two dissolved quantities is examined. A simplified mathematical model is used. This study takes into account a nonlinear, time dependent temperature distribution by developing the time dependent velocity perturbation using a Fourier series with time dependent coefficients, and solving the resulting system of equations numerically.; The initial behavior of the system and the horizontal wave number of disturbances depend on the thermal and solute Rayleigh numbers, and on the ratio of the diffusivities of the two dissolved quantities. The growth of the disturbance is found to be confined to a region near the interface between the two fluid layers, which can be defined as the penetration depth of the instability. This depth is found to increase as the conditions became more stable.; Several cases near the lower limiting condition for salt-fingering at an interface, are investigated. The behavior of the instability near the limit at which the upper layer is initially more dense, is also studied.; In the experimental investigation, the stability of a two-layer system initially with a sharp interface which separates two gravitationally stable layers is studied. The top fluid layer is a sediment-laden (lighter), while the bottom fluid layer is a sediment-free sugar solution (heavier). The resulting vertical motion has been investigated. We find that merely due to molecular diffusion and settling, a local region (just above the still-sharp interface) is formed. This layer is gravitationally unstable (a local Rayleigh-Taylor instability). With time the instability increases and reaches some critical value at which this local region breaks-down and a new local region starts to form.; A wide range of particle size distributions with different concentrations is investigated. Three different internal structural formations resulted: (i) a distinct columnar structure (FINGER) with a clear and sharp mushroom head. (ii) a cloudy, globular and vertical gravity-driven bulk convective motion of the upper suspension moving as a whole (a settling density current). (iii) individual particle motions. Bulbous tips (mushroom-like heads) form on the sediment fingers, and develop into vortex rings.