Measurements have been made in a turbulent boundary layer modified by flow aligned vertical (sub-boundary layer) elements. Comparisons between the coherent structure (near-wall and outer-layer region) for both modified and canonical cases have been conducted in order to better understand the mechanism of skin friction reduction. Thus far we can report a modified near-wall convection velocity obeying inner scaling and a reduced spread of correlated events away from the wall. The outer-layer appears to be characterised by large-scale arch-like structures which produce a velocity field consistent with the heads of lifted near-wall horseshoe vortices. The modified case shows reduced convection velocity, increased frequency of occurrence and increased entrainment for this type of outer-layer event.

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