High spatial resolution particle-image velocimetry (PIV) and micro-particle tracking velocimetry (μ-PTV) measurements have been performed to analyze the flow structures in a turbulent flat plate boundary layer undergoing spanwise traveling transversal surface waves. The scope of this investigation is to examine the connection between friction drag reduction and the structural properties of the instantaneous flow fields manipulated by the surface wave. The experiments were conducted for a momentum based Reynolds number of Reθ=2250 at dimensionless wave amplitudes of A+=6, 7, 8, and 11, a wave period of T+=94, and a wave length of λ+=3738. Local drag reduction of up to 4.9% at 0.087 boundary layer thicknesses downstream of the moving surface was measured. The Reynolds shear stress in the logarithmic region (50≤y+≤250) is suppressed due to the surface waves, whereas the wall-normal velocity fluctuations are enhanced in the outer layer. The turbulent contribution term in the Fukagata, Iwamoto, and Kasagi (FIK) friction drag decomposition  is reduced by 2.7% due to the dampened Reynolds shear stress above the wave crest. The analysis of the large-scale uniform momentum zones (UMZs) indicates that the population of these zones above the wave trough is nearly identical compared to the non-actuated turbulent boundary layer, while above the wave crest the number of UMZs is decreased by 3.5%. The conditional statistics of the modal velocity based on the near-wall velocity fluctuation evidence that there are strong links between the near-wall flow events and the UMZs. The near-wall flow structures of high fluctuating intensity are less correlated to the large-scale UMZs due to the spanwise surface wave motion, which diminishes the momentum transfer from the outer large-scale energetic structures to the near-wall flow, and thus, leads to the friction drag reduction.
- Friction decomposition
- Friction drag reduction
- Traveling surface wave
- Turbulent flat plate boundary layer
- Uniform momentum zones