Experimental Analysis of Mixed Configuration Riblets for Turbulent Boundary Layer Control*

This study investigates the characteristics of mixed and uniform blade riblets in a turbulent boundary layer under various flow conditions, represented by different non-dimensional spacing (s+), with results compared to those of a flat plate. Particle Image Velocimetry (PIV) and Micro Particle Image Velocimetry (micro-PIV) techniques are employed to examine the near-wall flow behavior of both riblets and flat plate. The log-law method is used to determine shear velocity using regular PIV data, while data from micro-PIV were utilized to calculate near-wall viscous and Reynolds shear stress and vorticity. For regular blade riblets, an increase in s+ beyond the optimum s+ leads to elevated near-wall shear stress and shear velocity, resulting in increased drag. In contrast, mixed riblets configuration demonstrates lower shear velocity and drag reduction across a broad range of s+ values when compared to the flat plate. Additionally, at high s+, the regular blade riblets exhibit high vorticity close to the wall, while the mixed riblets configuration shows the lowest vorticity compared to both the flat plate and regular blade riblets.