Refined Measurements of Turbulent Boundary Layer Statistics to Investigate Permeability Effects on Skin Friction*

Hydrodynamic and aerodynamic surfaces in nature often feature anisotropic permeability. Previous modelling endeavours show that these surfaces can contribute to friction drag reduction (DR) in wall-bounded turbulent flows by producing direction-dependent slip. This work investigates the impact of such surfaces on turbulent skin friction via laboratory boundary layer experiments in a large-scale water channel facility. Drag forces were measured directly in several experiments, and velocity fields were measured using particle image velocimetry (PIV). 

To improve spatial resolution in the near-wall region, a technique capable of single-pixel resolution in the wall-normal direction was used for calculations of the first and second-order turbulence statistics from the PIV images. The performance of this technique was verified using data from prior experiments and direct numerical simulations. Statistics were calculated for experimental turbulent boundary layer data over smooth and permeable surfaces using the single-pixel method.