Extensive investigation of the influence of wall permeability on turbulence

A series of direct numerical simulations of turbulent porous-walled channel flows is performed to extensively investigate the influence of wall permeability on turbulence modification. The bulk mean Reynolds number is fixed at 3000, and porous media consisting of perforated plates are considered in the lower side of the channel. The mean-permeability Reynolds number is varied from 14 - 118 by varying the hole size of the perforated plates. A spectral analysis reveals the presence of two characteristic perturbation modes, namely, the streamwise perturbation mode originating from the Kelvin-Helmholtz (K-H) type of instability and the spanwise perturbation mode. When the mean permeability Reynolds number is relatively low, the streamwise perturbation model by the K-H instability is dominant, and this increases the coherence of the wall-ward turbulence motion, thus resulting in considerable turbulence enhancement. However, as the mean permeability Reynolds number increases further, the streamwise perturbations tend to decrease in strength, and the streamwise elongated high and low-speed streaky structure, the mean spacing of which is much longer than that over a smooth wall, is developed owing to the spanwise perturbation mode. In this regime, the turbulence enhancement effect is weakened because of an increased slippage velocity at the porous interface.