Experimental study of turbulent structure over permeable walls with a refractive-index-matching technique

High-resolution particle velocimetry (PIV) measurements were conducted to explore turbulent flow overlying idealized permeable walls. The measurements successfully captured the overlying flow as well as the flow within the pore spaces with the specific goal of investigating the flow interactions across the permeable interface. A refractive-index matching (RIM) technique was employed to gain full optical access to the nearwall and subsurface flow and a number of idealized wall models were fabricated by casting acrylic. The permeable walls consisted of two and five layers of cubically packed uniform spheres (d=25.4mm), which provided 48% of porosity. In addition, an impermeable rough wall with identical topography was considered as a baseline of comparison in order to explore the structural modifications imposed by the permeability in the nearwall region. First-and second-order velocity statistics at two specific locations provided a quantitative assessment of such modifications of the local flow. A double-averaging approached (Nikora et al., 2007) allowed investigation of the global representation of the flow and to assess conventional scaling parameters.