Characterization of Turbulent Boundary Layer Dynamics via Active Manipulation of Large-Scale Structures

The dynamic response of a zero-pressure gradient turbulent boundary layer to an active flow control actuator was experimentally investigated. The experimental canonical turbulent boundary layer had a sufficiently low Reynolds number (Re_tau=690,Re_theta=1770) that there was no natural organized large-scale structure present. The plasma based actuator device was then placed inside the outer region of the turbulent boundary layer to introduce a periodic spanwise-uniform synthetic large-scale structure which was dynamically similar to naturally occurring large-scale structures. This novel actuation scheme then provides a new means by which to test the top-down view of turbulent boundary layer dynamics. The turbulent boundary layer response to the synthetic large-scale structure downstream of the actuator was investigated using a variety of measurement techniques, over a large streamwise extent. Phase-locked analysis was implemented to measure the development of the synthetic large-scale structure and the turbulent boundary layer response to this periodic perturbation. The interaction of large-scale velocity fluctuations, induced by the synthetic large-scale structure, and smaller-scale turbulent motions was quantified using a modulation coefficient, and a strong positive correlation within the inner and log-region of the boundary layer was observed. The phase speed of these induced large-scale velocity fluctuations was found to be nearly constant in the region below the actuator. The phase speed was also found to be nearly equal to the local mean velocity at the point of actuation, showing the superimposed effect of the convecting synthetic large-scale structure on near-wall turbulence dynamics. Additionally, in the presence of the synthetic large-scale structure, there was a measured increase in spanwise organization of velocity fluctuations that extended into the near-wall region. Overall, these results demonstrated the influence of top-down interactions in determining global turbulent boundary layer dynamics.