Drag reduction of three-dimensional riblets on a flat plate turbulent boundary layer

We performed an experimental investigation of the turbulent flow on a flat plate presenting micro roughness. In addition to the typical longitudinal micro-grooves, commonly referred to as riblets, we also investigate the performance of “three-dimensional” riblets, i.e. presenting a sinusoidal pattern. Further to the typical cross-section parameters that characterize longitudinal riblets, namely depth ($h$) and spacing ($s$) of the micro grooves, the sinusoidal riblets add two more parameters: the wavelength ($\lambda$) and the amplitude ($A$). In our study, we consider a parabolic profile ($s/h$=0.7) for the cross-section of the micro-grooves and we study two different sinusoidal riblets varying the amplitude ($A$=0.6mm and $A$=0.15mm), for a fixed value of the wavelength $\lambda/s$=64mm. Our load cell measurements show a consistent effect of the amplitude of the sinusoidal riblets on the friction drag reduction. In particular, while the longitudinal riblets feature drag reductions of the order of 7.7\% at $s^+$=13 (in good agreement with Bechert et al. 1997), the sinusoidal riblets can achieve values as large as 10\% for similar values of $s^+$. Stereoscopic-PIV measurements show the different near wall structure of the flow, when the three-dimensional riblets are employed.