Formation of low-speed ribbons in turbulent channel flow subject to a spanwise travelling wave

Turbulent flow control with skin-friction drag reduction subject to spanwise travelling wave (STW) induced by Lorentz force is investigated by direct numerical simulation (DNS) in a channel. The results show that STW produces a set of distinct longitudinal vortices, suppressing the regeneration of near-wall turbulence structures. It is also shown that the formation of low-speed ribbons by STW is associated with these longitudinal vortices, which weaken the sweep and ejection events in the near-wall region. At the same time, the production of counter-gradient Reynolds stresses is increased, leading to up to 30% of turbulent drag reduction by STW.