Turbulent boundary layer around 2D permeable and impermeable obstacles

An experimental investigation of the turbulent boundary layer around 2D permeable and impermeable obstacles was carried out using high-resolution particle image velocimetry (PIV) in a large-scale refractive-index-matching (RIM) flume. The flow over three rectangular obstacles was studied including an impermeable model (case 1), a model porous through only its flow-facing surface (case 2), and an obstacle porous through both its flow-facing and upper surfaces (case 3). The ratio of the height (h) to the incoming boundary layer thickness (${\delta }_0$) was $h/{\delta }_0\approx 1/4$. Measurements were performed at a Reynolds number Re = 70,000 based on the freestream velocity and ${\delta }_0$. The results highlight the impact of permeability on the mean flow, separation bubble, Reynolds shear stress, as well as the level and production of turbulent kinetic energy. In particular, the momentum deficit and flow reattachment downstream of the obstacle were highly sensitive to the permeability. Similarly, the second-order statistics of the velocity in the vicinity of the three obstacles exhibited large variations. It is shown that constraining and channeling the flow through only the flow-facing surface of the obstacle (case 2) result in lower momentum deficit, turbulence levels, and Reynolds shear stress in the intermediate and far wake regions. Overall, the results highlight the distinctive effects of obstacle permeability on the turbulent boundary layer.