Streamwise evolution of drag-reduced turbulent boundary layer over a travelling wavy wall

Travelling wave control is a promising technique for reducing turbulent skin friction by suppressing turbulence in boundary layers. This study presents experimental observations of the streamwise evolution of turbulence over a travelling wavy wall. Particle image velocimetry measurements were performed at multiple downstream locations. The travelling wave was generated by oscillating a rubber sheet to attain properties that are known to achieve drag reduction. The results reveal a two-stage process where the drag-reduction mechanism qualitatively changes in the streamwise direction. In the upstream region (up to approximately two wavelengths from the start of control), turbulent fluctuations are rapidly reduced, however, being limited to the near-wall region. Further downstream, the suppression effect diffuses in the wall-normal direction, leading to a modification on the edge of the boundary layer. The diffusion process of the turbulence-suppression effect is consistently interpreted within the framework of an internal boundary layer, whose development follows a power law. Two-point correlation analysis indicates that the wave crests initially disrupt the near-wall streaky structures, and subsequently reorganise into a characteristic state with a shorter streamwise coherence length downstream. While fully developed states have been studied previously, this work presents the streamwise development of turbulence suppression within a finite length, informing the design of practical drag-reduction devices.