Time-resolved PIV of a turbulent boundary layer over a spanwise-oscillating surface

This work reports measurements of a turbulent boundary layer at Re$_{\theta } \approx $ 2500, over a resonant spanwise-oscillating surface driven by a linear electromagnetic motor. Time-resolved PIV measurements of velocity are presented and supplemented by hot-wire measurements of velocity and direct drag measurements of friction drag using a drag balance. A maximum of 16{\%} surface friction reduction, as calculated by the diminution of the wall-normal streamwise velocity gradient was obtained. The PIV laser beam was parallel to the plane of the oscillating surface at a height of $y^{+} \approx $ 15, hence, top-down views of the near-wall turbulence activity and the effect of the surface oscillation on its evolution were obtained. It has been shown that the imposition of a spanwise Stokes-like layer at a non-dimensional period of $T^+={Tu_\tau ^2 } \mathord{\left/ {\vphantom {{Tu_\tau ^2 } \nu }} \right. \kern-\nulldelimiterspace} \nu $ $\approx $ 100 at peak-peak oscillation amplitudes equal to or larger than the mean streak spacing enabled the direct manipulation of the quasi-streamwise near-wall structures and caused fundamental changes in their evolution leading to reductions, for example, in the near-wall values of the mean-square of the streamwise fluctuating velocity component.