Turbulent boundary layers over a porous surfaces

Turbulent flow over the porous surface is one of the least documented problems, despite having the potential to affect a wide range of applications. This study presents an experimental investigation of turbulent boundary layer over open cell porous foam where the pore size and foam thickness are systematically varied. Hot-wire anemometry, Planar and Stereoscopic particle image velocimetry (PIV) are used for flow characterisation and floating element drag balance for drag measurement. All the measurements are performed over a range of freestream velocities at a fetch of 3.5m in a turbulent boundary layer that has developed over the porous media. The skin-friction coefficient is found to be considerably affected by the porosity as well as the thickness of the porous foam. However, at higher freestream speeds, a Reynolds number invariant skin-friction behaviour appears to exist, similar to rough walls. Hot-wire and PIV data shows that it is very difficult to isolate a clear logarithmic region in the mean velocity profiles. Preliminary assessment of the mean flow and turbulence statistics also indicates that outer layer similarity may be invalid for the surfaces considered here. Further analysis of the energy spectra as well as the spatial structure of the flow will be presented.