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

The role of streamwise length scales (lambda x) 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 FukagataL73 (2002)]. The total skin friction generation associated with motions scaled with local boundary layer thickness delta of lambda x 3 delta and lambda x 3 delta) 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 lambda x 3 delta 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.

Automated summary

The role of streamwise length scales in turbulent skin friction generation is investigated. It is found that both the motions scaled with local boundary layer thickness and the large-scale ejection and sweep events contribute significantly to turbulent skin friction. The reduction of turbulent skin friction can be achieved by modifying both the large- and small-scale quadrant events.