Response of a turbulent boundary layer to a synthetic periodic large-scale structure

The dynamic response of a zero-pressure-gradient turbulent boundary layer (TBL) to a large-scale perturbation in the outer region was investigated experimentally. The baseline TBL had a moderate Reynolds number such that there were no naturally occurring energetic large-scale structures (LSS) present. An active plasma-based actuator was then placed in the outer region of the TBL to introduce a periodic, spanwise-uniform, synthetic LSS. Thus this novel actuation scheme provides a new tool by which to experimentally examine an idealised case of outer–inner layer interaction within the TBL. The TBL response to this synthetic LSS was investigated using a combination of planar particle image velocimetry and spanwise offset hot-wires, over a large streamwise extent downstream of the actuator device. Phase-locked analysis was implemented to isolate and measure the streamwise development of the imposed synthetic LSS and resulting phase-dependent changes in turbulence amplitudes throughout the TBL. A strong correlation was observed between large-scale motions near the wall, which were found to be linearly superimposed from the outer-region synthetic LSS, and a periodic modulation of the turbulence amplitude. Further, this periodic modulation was found to be linked to phase-dependent changes in both the production and transport of turbulence driven by the induced streamwise and wall-normal large-scale motions. The phase speed of these induced large-scale motions, coupled with intermittent changes to spanwise coherence near the wall, also revealed a clear, but transient, effect of the outer-region synthetic LSS on the near-wall cycle dynamics. Overall, these results serve to characterise the influences, and limitations, of idealised outer–inner interactions on global TBL dynamics, and highlight considerations for the design and optimisation of future flow control techniques.