Turbulence Characteristics Over Permeable Walls (Invited)

In order to understand the effects of the wall permeability on turbulence near a porous wall, a series of flow field measurements are carried out for turbulent flows in a channel with a porous bottom wall by a two-component PIV (particle image velocimetry) system. The porous media used are three types of foamed ceramic layers which have the same porosity of 0.8 but different permeability. Firstly the measurements of the x (streamwise)-y (wall normal) planes are performed. Then, the x-z (spanwise) planes are measured to discuss the turbulence structures. Using the x-y plane data, it is confirmed that the flow becomes more turbulent over the porous wall and tends to be turbulent even at the bulk Reynolds number of Re=1300 in the most permeable wall case tested. Corresponding to laminar to turbulent transition, the magnitude of the slip velocity on the porous wall is found to increase drastically in a narrow range of the Reynolds number. This transitional behaviour of the slip velocity can be generally correlated with the permeability Reynolds number. To discuss the effects of the wall roughness and the wall permeability, detailed discussions are made with the zero-plane displacement and the equivalent wall roughness for porous media. The results clearly indicate that the turbulence is induced by not only the wall roughness but the wall permeability. An experimental correlation for the mean velocity distribution is then proposed. The measurements have also revealed that as Re or the wall permeability increases, the wall normal fluctuating velocity near the porous wall is enhanced due to the effects of the wall permeability. This leads to the increase of the turbulent shear stress resulting in higher friction factors of turbulence over porous walls. For understandings of the turbulent vortex structure near porous walls, based on the PIV experimental data of the x-y planes, a probability density analysis of fluctuating velocities, statistical quadrant and quadrant-hole analyses of the Reynolds shear stress are performed. From the discussions on those analyses, a conceptual scenario of the development of the vortex structure near a permeable wall is proposed for a moderate permeability Reynolds number case. It explains the reason why the near-wall long streaky structure tends to vanish near a porous wall with increasing wall permeability. Finally, the spanwise turbulent flow structures are discussed using instantaneous and statistical measurement data of the x-z planes. At a small permeability Reynolds number, low speed and high speed regions, which form a similar streaky structure to that of solid-wall turbulence, are observed. In case of a large permeability Reynolds number, although some large scale striped patterns are observed, they are confirmed to be different from those of the solid wall turbulence structure. Disappearance of the streak structure is confirmed by the larger correlation distance and the absence of oscillations in spanwise autocorrelation of the streamwise velocity.