Dynamics of finger-type convection in double-diffusive instability
Abstract
Finger-type convection in double-diffusive instability (DDI) controls mixing and scalar transport in many stratified flows, yet a quantitative, finger-resolved description of the transient growth, transport, and saturation pathways has been limited. Here, finger-type DDI is analyzed in a sealed-surface laboratory facility using synchronized planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) at fixed thermal contrast ΔT=5∘C and three salinity contrasts, ΔS=350, 450, and 550 ppm, complemented by a matched high-resolution three-dimensional DNS. A systematic fingertip detection and tracking framework generates ensemble growth curves. Fingertip growth follows a sequence of three stages (acceleration, quasi-steady propagation, and decay). The peak growth rates increase monotonically with ΔS, and nondimensional fingertip-height histories collapse onto a common trend. The peak growth rates are reproduced by DNS and agree with linear stability analysis, establishing experiment--DNS--theory consistency in the intermediate regime. The mixed-material area increases with time, initially following a common nondimensional trend before transitioning to ΔS-dependent interaction and breakdown. Finger-scale measurements reveal the formation of a symmetric vortex ring at the fingertips for ΔS=450 ppm, inducing vertical-aligned transport. At ΔS=550 ppm the roll-up becomes asymmetric: stronger buoyancy amplifies shear, destabilizes the vortex ring, and produces a zig-zag/lateral-drift mode that enhances the lateral transport. Finally, the evolution of the buoyancy anomaly links the growth-rate phases to a time-dependent force balance in which increasing buoyancy drives acceleration, shear-induced resistance regulates quasi-steady propagation, and dilution with top-boundary influence yields late-stage fingertip deceleration.
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BibTeX
@misc{mohaghar2026dynamics,
title = {{Dynamics of finger-type convection in double-diffusive instability}},
author = {Mohammad Mohaghar and Anirban Bhattacharjee and Suhas S. Jain and Donald R. Webster},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2605.00335},
eprint = {2605.00335},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2605.00335},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Dynamics of finger-type convection in double-diffusive instability
AU - Mohammad Mohaghar
AU - Anirban Bhattacharjee
AU - Suhas S. Jain
AU - Donald R. Webster
PY - 2026
DA - 2026/09/04
PB - arXiv
DO - 10.48550/arxiv.2605.00335
UR - https://arxiv.org/abs/2605.00335
AB - Finger-type convection in double-diffusive instability (DDI) controls mixing and scalar transport in many stratified flows, yet a quantitative, finger-resolved description of the transient growth, transport, and saturation pathways has been limited. Here, finger-type DDI is analyzed in a sealed-surface laboratory facility using synchronized planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) at fixed thermal contrast ΔT=5^∘C and three salinity contrasts, ΔS=350, 450, and 550 ppm, complemented by a matched high-resolution three-dimensional DNS. A systematic fingertip detection and tracking framework generates ensemble growth curves. Fingertip growth follows a sequence of three stages (acceleration, quasi-steady propagation, and decay). The peak growth rates increase monotonically with ΔS, and nondimensional fingertip-height histories collapse onto a common trend. The peak growth rates are reproduced by DNS and agree with linear stability analysis, establishing experiment--DNS--theory consistency in the intermediate regime. The mixed-material area increases with time, initially following a common nondimensional trend before transitioning to ΔS-dependent interaction and breakdown. Finger-scale measurements reveal the formation of a symmetric vortex ring at the fingertips for ΔS=450 ppm, inducing vertical-aligned transport. At ΔS=550 ppm the roll-up becomes asymmetric: stronger buoyancy amplifies shear, destabilizes the vortex ring, and produces a zig-zag/lateral-drift mode that enhances the lateral transport. Finally, the evolution of the buoyancy anomaly links the growth-rate phases to a time-dependent force balance in which increasing buoyancy drives acceleration, shear-induced resistance regulates quasi-steady propagation, and dilution with top-boundary influence yields late-stage fingertip deceleration.
ER -