Decoupling the roles of porosity and permeability in thermally driven Rayleigh–Taylor instability: a REV-scale lattice Boltzmann study

Rayleigh–Taylor instability (RTI) in porous media affects mixing and transport in carbon geological storage, solar ponds, and thermal-energy systems. This study numerically investigates thermally driven RTI between two miscible fluids in a homogeneous porous medium at the representative elementary volume (REV) scale using a thermally coupled dual-distribution-function lattice Boltzmann method (LBM). Porosity and permeability are prescribed separately within the REV formulation for systematic comparison, while their physical correlation through pore structure is acknowledged. The results show that porosity mainly affects instability onset and early development, whereas permeability more strongly controls the transition to convection and the subsequent flow intensity. For ε = 0.6 and Ra = 10 6 , a condition-dependent transition occurs near K = 5 × 10 −4 ; below this value, transport remains predominantly diffusion-dominated under the present numerical conditions. The near-clear-fluid limiting case ε = 0.999 exhibits a more channelized and directionally organized flow pattern than the intermediate-porosity cases. Mixing-layer thickness, mixing efficiency, normalized kinetic energy, and monitoring-point data consistently indicate that the response becomes progressively less sensitive to further increases in permeability at high K . A fifth-order polynomial is used only as an empirical interpolation of the porosity-dependent temperature response under a representative high-permeability condition within the investigated range. These findings clarify the separate and coupled roles of porosity and permeability in thermally driven porous-medium RTI. The reported transition is specific to the present geometry, Rayleigh number, initial perturbation, and boundary conditions and should not be interpreted as a universal material constant.