Feedforward control of very-large-scale motions using wall deformation

This work experimentally investigated the active control capabilities of wall-normal surface deformations for targeting very-large-scale motions (VLSMs) of a turbulent boundary layer (TBL) at a friction Reynolds number of 2600. The control system used a feedforward control scheme based on velocity measurements using a real-time particle image velocimetry (RT-PIV). A circular active surface with a diameter D roughly equal to the boundary layer height δ was designed to achieve smooth deformations with a peak amplitude of 0.07δ. Preliminary investigations characterized the frequency response of the actuator, which was followed by characterizing the actuator’s impact on the TBL when operated at a constant amplitude and frequency. These tests included deformation amplitudes of 0.02δ, 0.04δ, 0.06δ and actuation frequencies of St = 0.05, 0.10, 0.15, 0.20 where the Strouhal number is based on D and the free stream velocity U_∞. Finally, the feedforward control applied an opposition strategy with different gain values using the RT-PIV measurement of streamwise velocity fluctuations. The impact of the actuation on the TBL was analyzed using a spatial filter to separate the large-scale from the small-scale motions. The active surface showed the largest attenuation of the large-scale motions in the logarithmic region.