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

Abstract

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 (Cf) up to 17%. The breakdown of the outer structures passing through the LEBU device reduced the energy of the long wavelength motions (λz+>200) along the wall-normal direction. The reduction of Cf 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 (vωz and −wωy), which directly contribute to Cf. The contributions of vωz and −wωy accounted for 80% of the total Cf reduction. The amount of the Cf reduction induced by vortex stretching arose from the contributions of the intense positive uL (uL+≈2) and the relatively weak negative uL (uL+≈−0.7). Owing to the negative contribution of the advective vorticity transport, the total Cf reduction was obtained under the negative uL even though drag reduction was achieved by the suppression of vortex stretching under the intense positive uL.