Turbulent boundary layers over substrates with streamwise-preferential permeability

Substrates with streamwise-preferential permeability are theorized to reduce drag in turbulent boundary layers by relaxing the no-slip condition. This study presents the first experimental validation of this concept in airflow using a 3D-printed structure. Contrary to predictions, results show an increase in drag driven by the inverse wall-normal Forchheimer coefficient, agreeing with recent direct numerical simulation data on acoustic liners. The findings highlight the challenge of modeling porous substrates for drag predictions, and suggest that translating their abstraction into physical realizations relevant for practical applications would result in structures similar to riblets.