Large-scale and small-scale contribution to the skin friction reduction in a modified turbulent boundary layer by a large-eddy break-up device

Abstract

The role of streamwise length scales (λx) in turbulent skin friction generation is investigated using a direct numerical simulation data set of an incompressible zero pressure gradient turbulent boundary layer and the spectral analysis based on the Fukagata- Iwamoto-Kasagi (FIK) identity by Fukagata et al. [K. Fukagata et al., Phys. Fluids 14, L73 (2002)]. The total skin friction generation associated with motions scaled with local boundary layer thickness δ of λx < 3δ and λx > 3δ is assessed. The FIK-identity-based spectral analysis is further extended to include the quadrant analysis of Reynolds shear stress. This allows one to relate the turbulent skin friction generation to the quadrant events of Reynolds shear stress, which plays a central role in the momentum transport in turbulent wall-bounded flows. The small-scale ejection and sweep events (λx < 3δ) contribute to a significant portion of turbulent skin friction. However, it is found that the large-scale ejection and sweep events with streamwise length scales at λx > 3δ are equally important. The turbulent skin friction reduction associated with the modification of largeand small-scale quadrant events is studied, using well-resolved simulation data sets of a large-eddy break-up (LEBU) device in a turbulent boundary layer. The results reveal that LEBUs modify both the large- and small-scale ejection and sweep events, yielding a substantial turbulent skin friction reduction.