Journal
JOURNAL OF FLUID MECHANICS
Volume 700, Issue -, Pages 77-104Publisher
CAMBRIDGE UNIV PRESS
DOI: 10.1017/jfm.2012.97
Keywords
drag reduction; turbulence control; turbulent boundary layers
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Funding
- Japan Society for the Promotion of Science (JSPS)
- Grants-in-Aid for Scientific Research [23760176] Funding Source: KAKEN
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Harmonic oscillations of the walls of a turbulent plane channel flow are studied by direct numerical simulations to improve our understanding of the physical mechanism for skin-friction drag reduction. The simulations are carried out at constant pressure gradient in order to define an unambiguous inner scaling: in this case, drag reduction manifests itself as an increase of mass flow rate. Energy and enstrophy balances, carried out to emphasize the role of the oscillating spanwise shear layer, show that the viscous dissipation of the mean flow and of the turbulent fluctuations increase with the mass flow rate, and the relative importance of the latter decreases. We then focus on the turbulent enstrophy: through an analysis of the temporal evolution from the beginning of the wall motion, the dominant, oscillation-related term in the turbulent enstrophy is shown to cause the turbulent dissipation to be enhanced in absolute terms, before the slow drift towards the new quasi-equilibrium condition. This mechanism is found to be responsible for the increase in mass flow rate. We finally show that the time-average volume integral of the dominant term is linearly related to the drag reduction.
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