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Description
This study examines the spatiotemporal evolution of laminar plumes propagating through a vertically density-stratified porous medium under Darcy flow conditions at high Peclet numbers ($\mathrm{Pe} \gg 1$). Density stratification is ubiquitous in natural subsurface environments and plays an important role in controlling plume migration, spreading, and mixing. Such conditions arise in a wide range of applications, including saline intrusion, contaminant transport, CO$_2$ dissolution and sequestration, and thermally or compositionally stratified groundwater systems. Despite its importance, the influence of ambient density stratification on plume dynamics in porous media remains incompletely understood.
For this study, laboratory-scale experiments were conducted using dyed saline solutions injected into a saturated porous medium composed of uniformly packed glass beads. A controlled vertical density gradient was imposed to generate stable stratification conditions in the ambient fluid. Both buoyant and dense plumes were examined to explore the competing effects of buoyancy forces and background density gradients on plume evolution (Fig. 1). The imposed stratification modifies the balance between vertical buoyant motion and horizontal spreading, leading to distinct plume morphologies and transport pathways.
Non-intrusive optical visualization based on the dye attenuation technique was employed to capture the temporal evolution of concentration fields. High-resolution imaging served exclusively as a diagnostic tool, enabling quantitative analysis without disturbing the flow. Image processing in Matlab allowed precise detection of plume boundaries and systematic extraction of plume parameters, including plume length, effective width, and lateral and vertical spreading rates. These metrics enabled robust comparison across a range of stratification strengths and flow conditions.
Results demonstrate that increasing ambient density stratification significantly inhibits vertical plume propagation while enhancing lateral confinement and spreading. In contrast, weaker stratification permits greater vertical migration, plume elongation, and enhanced mixing. Experimental observations reveal a clear inverse relationship between plume length and stratification strength, which is well captured by a best-fit scaling trend (Fig. 2) , indicating consistent plume behavior across varying regimes. Depending on the relative strength of buoyancy forces and background stratification, plume dynamics transition from stratification-dominated transport to porous-media-controlled dispersion.
These findings highlight the critical role of ambient density structure in shaping macroscopic transport behavior, even in homogeneous porous media. By linking laboratory observations to dimensionless parameters governing variable-density flow, this work provides improved insight into mixing and dispersion processes in stratified porous systems. The results are directly relevant to environmental and geophysical applications such as CO$_2$ sequestration, contaminant transport, saline intrusion, and geothermal plume evolution, where density contrasts and stratification strongly influence plume persistence, spreading, and mixing.
| Country | India |
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