Identifying turbulent skin-friction generation structures in channel flows via canonical correlation decomposition

Flow structures directly responsible for local unsteady skin-friction generation in turbulent channel flows are identified using the newly developed canonical correlation decomposition (CCD) method. The dominant structures take the form of streamwise streaks that are spanwise-localised around the position where the skin friction is targeted and exhibit progressively shorter streamwise extents as the mode number increases. The resulting CCD spectrum shows a clear low-rank behaviour; flow reconstruction using only the first 4 CCD modes recovers more than 80 % of the examined skin-friction variation, in contrast to inefficient skin-friction reconstructions using other correlation-based methods. When the opposition control technique is used to reduce drag, the application of CCD shows that drag reduction is achieved by lifting the original streak structures and generating thinner and shorter streaks with opposite phases underneath. These findings demonstrate that CCD isolates the causally relevant flow structures governing the turbulent skin-friction generation and modification, which is expected to find use in various drag control applications in wall-bounded turbulence.