Large-scale flow organization of wall-bounded turbulence over flush-mounted rotating discs

Rotating discs, flush-mounted within the wall beneath a turbulent boundary layer, affect the large-scale flow dynamics. An organized array of rotating discs is a surrogate for the transverse-oscillating wall concept that is known to reduce turbulent friction drag, since the convecting flow encounters a travelling wave, particularly in the spanwise centre of the discs. This work experimentally assesses the flow manipulation of this wall-based actuation method using planar and stereoscopic particle image velocimetry (PIV). Experiments were conducted in a developing turbulent boundary layer, at Re_τ ≈ 910, and with an optimized viscous-scaled sizing and layout of the discs following the direct numerical simulation (DNS) study of Ricco & Hahn (2013). Planar PIV in the streamwise-wall-normal plane over the spanwise centre of the discs revealed a reduction of the in-plane Reynolds stresses, suggesting a suppression of the near-wall turbulence auto-generation process. Wall-parallel planes of velocity data at a height of 70 viscous units above the wall revealed two distinct types of streamwise-oriented regions, comprising low- and high-momentum pathways. These spanwise alternating regions were also captured using the stereo-PIV measurements downstream of the disc-array. It was observed that the mean boundary layer flow is pulled closer to the wall in the disc center, resulting in a higher mean velocity and a less intense streamwise Reynolds stress for a given wall-normal height. With this effect being maximum in the disc center, while being absent between the discs, this type of flow manipulation could be optimized in terms of turbulence suppression (and potentially in terms of friction drag reduction at high Reynolds numbers), by considering larger discs.