Turbulent drag reduction by traveling waves of spanwise forcing

etc15 Tracking Number 324

Several techniques based on spanwise focing were introduced in the last decade \cite{du-symeonidis-karniadakis-2002, quadrio-ricco-viotti-2009} in the form of traveling waves of spanwise forcing as generalization of the spanwise-wall oscillation technique to reduce skin-friction drag in a turbulent channel flow. Here we examine all their variants (including one that has never been considered before), by addressing the type of forcing (wall movement versus body force) as well as the traveling direction (streamwise versus spanwise) of the waves. We carry out a DNS-based study within an unified framework, to compare their capability to reduce skin-friction drag and, more importantly, net energy savings. The present results confirm the potential for drag reductions for every considered forcing. The best-performing spanwise traveling wave, in terms of either drag reduction or net energy saving, is found to be the one with infinite wavelength, i.e. still the spanwise wall oscillation. The streamwise-traveling waves consistently offer the best performance, especially in terms of net savings. The conditions under which body-force based control can be meaningfully compared to wall based control are discussed.