Title:

Finger Growth to Mixed Layers in Double Diffusive Instability

Speaker:

Dr. Mohammad Mohaghar

Affiliation:

Civil Engineering at Georgia Tech

When:

Tuesday, November 4, 2025 at 1:00:00 PM   Add to Calendar

Where:

MRDC Building, Room 4211

Host:

Cyrus Aidun and Ellen Mazumdar
cyrus.aidun@me.gatech.edu

Abstract

Salt-driven double-diffusive instability (DDI) is recognized as a canonical pathway by which transitions of unstable fingers into fully mixed layers are caused by opposing temperature and salinity gradients in natural and engineered environments. However, quantitative experimental data and characterization of the mixing transition within the finger-growth regime remain limited. A laboratory data set is presented that captures both large-scale finger-growth statistics and detailed finger-scale dynamics for three salinity contrasts (ΔS = 350, 450, 550 ppm) at ΔT = 5 K. Cold, fresh water was injected through a clear tube at the base of a 20 cm-tall acrylic tank with a sealed upper surface beneath warmer, saltier water. Simultaneous particle-image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) were employed; a temporal resolution of 0.4 s, a velocity-vector spacing of 286 µm, and a scalar-field resolution of 35 µm were achieved, yielding fully coupled velocity–concentration fields throughout the finger evolution. Ensemble-averaged finger-tip trajectories from hundreds of DDI fingers were observed to collapse onto a single non-dimensional curve for each case, with growth rates that matched the predictions of linear-stability theory within 2%. A modest decline in mixing efficiency with increasing salinity was observed in the mixed-material area as a function of nondimensionalized time. Finger-scale maps of vorticity, enstrophy, scalar dissipation, and salt flux were found to indicate that peak circulation and enstrophy coincide with the interval of maximum vertical buoyancy flux, thereby indicating a brief period of strongest transport. At ΔS = 550 ppm, a zig-zag ascent is triggered by intensified shear along the finger flanks: with each 5–10 mm lateral excursion, a secondary finger is spawned and the horizontal salt flux is significantly increased relative to lower-salinity-difference conditions. The first quantitative experimental results revealing how salinity contrast controls finger growth, mixing efficiency, and the transition from coherent fingers to mixed layers in double-diffusive instability are provided by these measurements.

Biography

Dr. Mohammad Mohaghar is a Research Faculty member in the Environmental Fluid Mechanics Laboratory at Georgia Tech’s School of Civil and Environmental Engineering. He earned his Ph.D. in Mechanical Engineering from Georgia Tech. His work uses advanced diagnostics (PLIF, PIV, tomographic-PIV) along two complementary paths: (1) hydrodynamic instability and turbulence: double-diffusive and shock-driven mixing and passive-scalar transport in turbulent boundary layers; and (2) biological and bio-inspired fluid dynamics: swimmer hydrodynamics and soft-robotic propulsion. He has contributed to projects supported by the National Science Foundation (NSF), the Air Force Office of Scientific Research (AFOSR), and Los Alamos National Laboratory (LANL).