Diverging transition pathways to turbulence in regular and random porous media

Flows in porous media are critical to many industrial processes, yet the transition from laminar to turbulent flow in the intermediate regime Re∈[200,300] remains poorly understood. This study employs direct numerical simulations based on the immersed boundary-lattice Boltzmann method to investigate this transition within two three-dimensional porous structures: regular cubic packing (RCP) and random packing (RP), under periodic boundary conditions. By analyzing vortex structures, velocity fluctuations, and turbulent kinetic energy from 27 monitoring points, we identify distinct transition pathways for each structure. For RCP, transition occurs abruptly within a narrow range of Re∈[200,300], characterized by a sudden onset of global velocity fluctuations. In contrast, the transition in RP is more gradual, beginning with small periodic fluctuations at Re = 220 and evolving to fully irregular turbulence by Re = 300. These findings underscore how pore-scale geometry fundamentally alters the transition mechanism in porous media flows.