Flying PIV investigation of vortex packet evolution in perturbed boundary layers

A turbulent boundary layer with zero pressure gradient of Reτ=2500 was perturbed using a single spanwise cylinder array with H/δ=0.2 (H+=500) and aspect ratio H/D=4 mounted on the bounding surface. Both fixed location and tracking measurements were performed using particle image velocimetry (PIV) in streamwise-spanwise planes up to 7δ downstream at a wall-normal location of z+=300. The spacing of the cylinder array was varied from 0.2δ to 0.8δ, and the 0.6δ spacing case, which matched the dominant spanwise mode in the unperturbed flow, showed the most stable downstream organization which persisted beyond 7δ in the streamwise direction. The flying PIV method allowed us to track specific vortical structures in the streamwise direction of range-2<x/δ<7, corresponding to a time scale of order 12δ/U∞. The flying PIV results showed that approaching packets lost their organization immediately downstream of the 0.2δ array but regained it 2δ downstream on a time scale of order 3δ/U∞, suggesting that the outer layer organization propagated inward into the log region. With larger spacing, the cylinders sometimes enhanced the organization of existing packets and generally redirected packets into the spanwise location midway between cylinders. The interactions between packets and cylinder wakes were discussed in three scenarios based on the relative spanwise distance between the cylinder and upstream packet. 1