Interaction of large-scale structures and injected small-scale structures in a turbulent boundary layer

In this experimental study, synthetic small-scale structures were injected in a turbulent boundary layer at different wall-normal positions over a flat plate via the immersion of a small cylinder with a diameter five percent of the boundary layer thickness. Two-dimensional, planar time-resolved particle image velocimetry (PIV) measurements were taken immediately downstream of the cylinder obtaining a field of 3.7δ-by-2.2δ in the streamwise-wall-normal plane. The turbulent statistics indicate that the level of turbulence suppression downstream of the cylinder is dependent on the height where the cylinder is placed. By performing conditional projection averaging of the data using large-scale structures obtained from spectral proper orthogonal decomposition (SPOD), the evolution of the cylinder wake downstream is observed to be affected both in amplitude and advection by local large-scale turbulent structures. Some of the large-scale structures are observed to be lessened in intensity downstream of the cylinder, depending on the wall-normal location of the cylinder. Additional comparisons between coherent structures for the cases with the immersed cylinder and the undisturbed boundary layer may be discussed, such as comparing SPOD modes and quadrant analysis.