No abstract

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.

Particle image velocimetry has been applied to the study of a canonical turbulent boundary layer and to a turbulent boundary layer forced by transversal wall oscillations. This work is part of the research programme at the Politecnico di Torino aerodynamic laboratory with the objective of investigating the response of near-wall turbulence to external perturbations. Results are presented for the optimum oscillation period of 100 viscous time units and for an oscillation amplitude of 320 viscous units. As expected, turbulent velocity fluctuations are considerably reduced by the wall oscillations. Particle image velocimetry has allowed comparisons between the canonical and forced flows in an attempt to find the physical mechanisms by which the wall oscillation influences the near-wall organized motions.

The axisymmetric synthetic jet produced by an oscillating piston has been investigated experimentally. The study was performed in water using digital particle image velocimetry for the instantaneous velocity planar measurements. The near field, up to x/d 0 ≈ 6, was investigated for three different Reynolds numbers. The self-similarity of the flow field is already fully reached for the mean velocity profiles at x/d 0 ≈ 4 while longer distances are needed for the turbulent intensity profiles. The flow structure investigation shows that a single vortex ring or a train of vortex rings characterizes the near field during the blowing phase, depending on the jet flow parameters. The details of the flow field have also been described in terms of phase-averaged flow analysis and of triple decomposition of the instantaneous velocity field. The near flow fields investigated do not seem to be influenced by the Reynolds number.

The present research is based on the belief that the observation of the response of near wall turbulent organized motions to external perturbations is a powerful technique for investigating the physics of turbulence regeneration. The low speed streak behavior in the buffer layer of a flat plate turbulent boundary layer under the action of transversal wall oscillation has been statistically analyzed by observing particle image velocimetry velocity fields in a plane parallel to the wall at a distance of 20 wall units. It was found that the observed reduction in turbulence activity (velocity variance and Reynolds stress) is accompanied by an increase of the averaged velocity streaks width, spacing and waviness and by a reduction of their strength. The increase in streak width and spacing has been attributed mainly to coalescence processes between neighboring streaks. The occurrence of velocity streaks strength reduction, associated with the consequent decrease of the number of unstable low speed streaks, is believed to be the main reason leading to the reduction of turbulent activity. The interpretation of the present results supports the concept that the so-called streak regeneration cycle plays an important role in the near wall turbulence reproduction process.