Wall Turbulence Manipulation by Large Scale Streamwise Vortices

This paper describes an experimental study about 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 was 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 largely 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, evidenced 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 behaviors in each of the periodic boxes originated by the interaction of the vortex couples. Results were interpreted and discussed in terms of management of the near wall turbulent structures and with reference to the wall turbulence regeneration mechanisms postulated in the literature.