Flow Leakage and Kelvin-Helmholtz Instability of Turbulent Flow over Porous Media

In the present paper, turbulent flow in a composite porous-fluid system including a permeable surface-mounted bluff body immersed in a turbulent channel flow is investigated using pore-scale large eddy simulation. The effect of Reynolds number (Re) on the flow leakage from porous to non-porous regions, Kelvin-Helmholtz (K-H) instabilities, as well as coherent structures over the porous-fluid interface are elaborated. Results show that more than 52% of the fluid entering the porous blocks leaks from the first half of the porous region to the non-porous region through the porous-fluid interface. As the Re number increases, the flow leakage decreases by 24%. Flow visualization shows that the Re number affects the size of counter-rotating vortex pairs and coherent hairpin structures above the porous block. Moreover, turbulence statistics show that by reducing the Re number, turbulence production is delayed downstream; at the Re=14400, it begins from the leading edge of the porous block (X/D=0), while at the Re=3600, turbulence production is postponed and starts nearly at the middle of the porous block (X/D=4.6). Finally, the distribution of pressure gradient for the three Re numbers confirms the occurrence of the K-H instability vortices over the porous-fluid interface. For Re = 3600, the K-H instability vortices show a linear growth rate in the vertical and horizontal directions with the slope of 0.136 and 0.05, respectively. However, by increasing the Re from 3600 to 14400, the growth rate slop in the horizontal direction decreases by nearly 33.8%, while in the vertical direction, it increases by 201%.