This paper describes an experimental study of the manipulation of a fully developed
turbulent channel flow through large-scale streamwise vortices originated by vortex
generator jets distributed along the wall in the spanwise direction. Apart from the
interest in flow management itself, an important aim of the research is to observe
the response of the flow to external perturbations as a technique for investigating the
structure of turbulence. Considerable mean and fluctuating skin friction reductions,
locally as high as 30% and 50% respectively, were measured for an optimal forcing
flow intensity. Mean and fluctuating velocity profiles are also greatly modified by the
manipulating large-scale vortices; in particular, attenuation of the turbulence intensity
was measured. Moreover the flow manipulation caused an increase in longitudinal
coherence of the wall organized motions, accompanied by a reduced frequency of
burst events, demonstrated by a reduction of the velocity time derivative PDFs and
by an higher intermittency. A strong transversal periodic organization of the flow
field was observed, including some typical behaviours in each of the periodic boxes
originated by the interaction of the vortex pairs. Results are interpreted and discussed
in terms of management of the near-wall turbulent structures and with reference to
the wall turbulence regeneration mechanisms suggested in the literature.