Influence of spanwise transversal surface wave on coherent structures in turbulent boundary layers

Coherent structures in an actively drag-reduced zero-pressure gradient turbulent boundary layei are investigated using particle-image velocimetry (PIV) and micro-particle-tracking velocimetry (mu-PTV). The friction drag reduction is achieved by spanwise traveling waves via wall-normal deflection of an aluminum surface generated by an electromagnetic actuator system. For a momentum thickness based Reynolds number Re-theta = 1200 flow, the measurements yield a maximum local drag reduction of 5.3%. The coherent structures in the streamwise vertical and streamwise horizontal planes are measured by phase-averaged Ply. The spanwise traveling transversal surface waves manipulate the coherent structures by interrupting the regeneration cycle of the wall-bounded turbulence. The Reynolds stresses are enhanced in the outer part of the boundary layer above the wave trough. The turbulence production is shifted off the wall during the upward motion of the wall due to the wall-normal momentum induced by the transversal surface waves. This finding is substantiated by the distributions of the wall-normal vorticity fluctuations and two-point correlations. A secondary spanwise flow induced by the waves generates a net spanwise fluid transport in the direction of the wave propagation. This secondary flow prevents the low-speed streaks to follow the downward motion. It rearranges them in the outer region of the boundary layer resulting in the shift of the near-wall turbulence production off the wall. (C) 2019 Elsevier Masson SAS. All rights reserved.