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Mixed-mode instability of a miscible interface due to coupling between Rayleigh-Taylor and double-diffusive convective modes
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Image of FIG. 1.
FIG. 1.

Buoyancy-driven instabilities in a vertical Hele-Shaw cell at the miscible interface between two fluids: (a) Rayleigh-Taylor (, δ) = (0.82, 0.51) at t = 20 s, (b) double-diffusion (, δ) = (2, 1.96) at t = 600 s, (c) diffusive layer convection (, δ) = (1.25, 0.27) at t = 300 s, (d)−(f) three consecutive snapshots of the mixed mode dynamics at (, δ) = (0.89, 0.27) taken at 40, 50, 60 s after contact. Frames size: (a)−(c) 15 mm × 9 mm, (d)−(f) 11 mm × 9 mm.

Image of FIG. 2.
FIG. 2.

Classification of the various density profiles in the (, δ) parameter plane for δ < 1. Dynamics characteristic of RT, MM, and DLC are obtained experimentally for the points above the full line (red online) and those shown by diamonds (red online) and black squares, respectively. The filled circle (green online) and diamond (red online) are those corresponding to the images of Figs. 1(a) and 1(d)–1(f) , respectively. The sketches represent the typical density profiles in each region.

Image of FIG. 3.
FIG. 3.

Density profiles (a) on the full curve (red online) δ = of Fig. 2 for = 0.84 and (b) in the MM zone for δ = 0.3 and = 0.9. (c) Summarizes the zone of existence of RT, MM, and DLC modes depending on the relative value of Δρ and Δρ′ for δ = 0.3.

Image of FIG. 4.
FIG. 4.

(a) Sketch showing how the mixing length is measured as the distance between the two black horizontal lines passing by the furthest upmost and downmost points of the fingering zone while the amplitude of the deformation of the contact line is the distance between the two dashed (red online) horizontal lines passing by the furthest upmost and downmost points of the deformed interface. The temporal evolution of the mixing length and of the amplitude of the interface modulation are given on panels (b) and (c) for δ = 0.27 and = 0.8 (RT, crosses), 0.85 (MM, dotted line), 0.89 (MM, dashed line), 0.96 (MM, solid line), and 1.01 (DLC, squares). The mixing length continuously increases in time for all values of but its intensity decreases as increases. (c) The amplitude of the contact line modulation is equal to the mixing length in the RT mode, saturates to a constant value in the MM dynamics and vanishes in the DLC regime.

Image of FIG. 5.
FIG. 5.

Numerical concentration maps of species B for = 0.85 and in each column: δ = 0.8, 0.6, 0.4, and 0.2 on a width =16384, 16384, 8192, 8192 (aspect ratio constant) from left to right. Times of the first row are, respectively, from left to right: = 2 × 10, 2 × 10, 1.4 × 10, and 5 × 10; for the second row, we have = 1.1 × 10, 1.1 × 10, 5.8 × 10, and 4 × 10.

Image of FIG. 6.
FIG. 6.

Reconstructed numerical density maps ρ = for = 0.85 and δ = 0.2 in a system of width 8192 (aspect ratio preserved) on a grey scale ranging from the less dense in white and the denser in black. (top left) Initially, at = 0, a little sinusoïdal perturbation is added on the contact line between the denser solution of A on top of the less dense solution of B; (bottom left, = 30 000) As A diffuses faster than B, it creates a denser accumulation zone (black line) beneath the miscible interface while a less dense depletion zone (white line) appears above it. This effect is weakened at the tip of the protrusions because of curvature effects; (right, = 70 000) Across the lateral sides of the minima, A diffuses faster out than B enters creating denser sinking sides. At the tip of the minima, the concentrated inward flux of B and diluted outward flux of A lead to a mixed zone of intermediate density, which rises between the sinking denser sides. As a result, “Y” shaped sinking antennas are observed. A symmetric argument can be developed for the rising maxima.


Generic image for table
Table I.

Expansion coefficients α, diffusion coefficients of the solutes used in experiments.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Mixed-mode instability of a miscible interface due to coupling between Rayleigh-Taylor and double-diffusive convective modes