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 nearwall turbulence and frictional drag. The LEBU device, which is thin and rectangular in shape, was located at 80% of the boundary layer thickness (delta). 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 ( lambda(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 (< nu omega(z)> and <-w omega(y)>), which directly contribute to C-f. The contributions of < v omega(z)> and <-w omega(y)> accounted for 80% of the total C-f reduction. The amount of the Cf reduction induced by vortex stretching arose from the contributions of the intense positive u(L) ( u(L)(+) approximate to 2) and the relatively weak negative u(L) (u(L)(+) approximate to 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).