Effects of Sinusoidal Riblets on Turbulent Boundary Layer Flow Structures

We investigate experimentally the effects of sinusoidal microgrooves on the flow structures developing in a zero pressure gradient turbulent boundary layer. The sinusoidal, or 3D riblets, have been found to be more effective than the widely investigated straight riblets [1, 2]. Particle image velocimetry measurements carried out in the streamwise-wall normal plane allow us to infer on the effect of sinusoidal riblets on the near-wall structures, such as hairpin vortices, along with their vorticity and wall distance. In addition to the proposed uplift mechanism induced by the straight grooves, the sinusoidal pattern is expected to introduce a further share of wall normal displacement, in agreement with the further share of drag reduction. Implementing a vortex detection method based on the second invariant of the velocity gradient tensor, we provide statistical evidence for this further uplift, in the case of the sinusoidal groove, at least in the case characterized by the smaller amplitude. Furthermore, the available data suggest that the spanwise vorticity associated with these structures is attenuated as an effect of the groove profile.