Influence of a large-eddy breakup device on turbulent boundary layer

Abstract

Direct numerical simulation (DNS) of a spatially developing turbulent boundary layer (TBL) flow with a large-eddy breakup (LEBU) device was performed to investigate the changes of the structural characteristics by the LEBU device and its influence on the near-wall behavior, especially focusing on the frictional drag. The LEBU device was positioned through an immersed boundary method and its wall-normal position was 80% of the boundary layer thickness (δ). In the present study, the LEBU device reduced the skin-friction coefficient $(C_f)$ up to 17%. Comparing to w/o the LEBU case, the swirling motions and all components of the turbulent intensity decreased behind the LEBU device not only in the outer region but also in the near-wall region. These trends are corresponding with the change of the relatively long-wavelength energy of the intensity of the streamwise fluctuation. The Reynolds shear stress is reduced in all wall-normal locations because the ejection and sweep events decreased, but latter ones showed more reduction where the maximum $C_f$ reduction was occurred. Behind the LEBU device, the length scale in the spanwise direction is reduced in the outer region. Also the strength of the large-scale circulations which facilitate the inner-outer interaction decreased. Conditional correlation showed that the area of the near-wall footprints of the outer high-speed structures decreased but that of the outer low-speed structures increased relatively.