Composite active drag control in turbulent channel flows

TL;DR

This study introduces a composite drag control method combining opposition and spanwise opposed wall-jet forcing, achieving significantly higher drag reduction and net power savings in turbulent channel flows compared to individual methods.

Contribution

The paper presents a novel composite control strategy that synergistically combines opposition and spanwise wall-jet forcing for enhanced drag reduction in turbulent flows.

Findings

Maximum 33% drag reduction with CDC, surpassing individual methods.

Approximately 32% net power saving achieved with CDC.

CDC effectively suppresses near-wall turbulent structures, including vortices and streaks.

Abstract

A composite drag control (CDC) combining the opposition (OC) and spanwise opposed wall-jet forcing (SOJF) methods is studied in a turbulent channel flow via direct numerical simulation of the incompressible Navier-Stokes equations. A maximum drag reduction of about 33% is obtained for CDC -- much higher than that produced by either individual method (namely, 19% for SOJF and 23% for OC). Due to the small power input required for both OC and SOJF methods, a significant net power saving (about 32%) is achieved via CDC. Flow analysis shows that CDC can take advantage of both OC and SOJF methods to better suppress drag producing, near-wall turbulent structures -- vortices and streaks. In particular, due to the presence of the large-scale coherent swirls generated by SOJF, it is more effective than OC in suppressing the random turbulence. Moreover, due to the OC's role in suppressing random small-scale turbulence, CDC requires weaker large-scale coherent swirls than those using SOJF only -- hence decreasing the drag contribution associated with large-scale swirls. In summary, our results suggest prospects of employing composite control strategy for effective skin friction drag reduction, particularly at very high Reynolds numbers.