Interaction of synthetic jets with a fully developed turbulent channel flow

The effects of the interaction of a spanwise array of inclined circular synthetic jets with a turbulent channel flow have been experimentally studied. The array is arranged in a convergent couple configuration, where each single jet is oriented at +/- 45 degrees in the plane perpendicular to the mean channel flow. The actuator system is derived from a commercial engine for airplane models where the oscillating piston gives rise to the periodic blowing and suction phases needed for the generation of a synthetic jet. The interaction produces couples of counter-rotating large-scale longitudinal vortical structures organized periodically in the spanwise direction. These vortices persist along the channel up to long distances from the injection section and produce an attenuation of the wall turbulence. Reductions in the mean skin friction of up to 15% and in the turbulence fluctuations ( wall shear stress and streamwise velocity) of up to 12% are obtained. The efficiency of the interaction depends on the piston oscillation frequency. The near-wall activity is influenced to a great extent by the action of the large-scale vortices. The variable interval time averaging ( VITA) events are reduced and the integral time macroscales are increased while the probability density function ( PDF) analysis shows higher levels of intermittency. The interpretation of these effects is given in terms of regeneration mechanisms of the near-wall structures. A comparison between the present control scheme and the forcing obtained using continuous jets has also been made. In the case of synthetic jets, the forcing effects appear weaker with respect to those obtained using continuous jets, at least for the tested conditions. Drag reduction and turbulence attenuation are in fact lower when synthetic jets are used as a forcing technique.