Influence of a large-eddy breakup device on the frictional drag in a turbulent boundary layer

A direct numerical simulation of a spatially developing turbulent boundary layer with a large-eddy breakup (LEBU) device was performed to investigate the influence of the LEBU device on the near-wall turbulence and frictional drag. The LEBU device, which is thin and rectangular in shape, was located at 80% of the boundary layer thickness (δ). The LEBU device reduced the skin-friction coefficient (C f ) up to 17%. The breakdown of the outer structures passing through the LEBU device reduced the energy of the long wavelength motions ( λ z + > 200 ) along the wall-normal direction. The reduction of C f mainly arose from the contribution of the Reynolds shear stress by the difference in the spatial coherence of the outer high- and low-speed structures. We investigated the relationship between the large-scale motions and the velocity–vorticity correlations ( v ω z and − w ω y ) , which directly contribute to C f . The contributions of v ω z and − w ω y accounted for 80% of the total C f reduction. The amount of the C f reduction induced by vortex stretching arose from the contributions of the intense positive u L ( u L + ≈ 2 ) and the relatively weak negative u L ( u L + ≈ − 0.7 ). Owing to the negative contribution of the advective vorticity transport, the total C f reduction was obtained under the negative u L even though drag reduction was achieved by the suppression of vortex stretching under the intense positive u L .