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DNS of shear-induced drag reduction

Results from DNS of turbulent incompressible channel flow subjected to spanwise-wall motion will be discussed, from both a statistical and structural perspective. The apparent paradox that application of a mean strain can cause a turbulent flow to become LESS energetic will be quantified, via histories of mean skin-friction drag and evolution of the various terms in the Reynolds-stress budgets. A kinematic model of the alterations to near-wall turbulence structures associated with this behaviour will be proposed. The challenge to such (quasi-streamwise-vortex-based) models represented by Chernyshenko & Baig's (2005) numerical experiments on near-wall streak generation will be highlighted.