  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.
