Perturbing vortex packets in a turbulent boundary

A zero pressure gradient turbulent boundary layer of Re-tau = 2500 was perturbed by a single spanwise array of finite cylinders mounted on the bounding surface and extending through the logarithmic region. The cylinder height was H/delta = 0.2 (H+ = 500), where delta is the boundary layer thickness, with an aspect ratio (Lambda R) (height/ diameter) of four. Streamwise-spanwise (x-y) planes of the flow were examined by particle image velocimetry (PIV) up to 7 delta downstream at a wall-normal location of z(+) = 300 for cylinder array spacings ranging from 0.2 delta to 0.8 delta. Average streamwise velocity fields showed a splitting, then merging pattern of cylinder wakes which occurred further downstream as the cylinder spacing increased. Based on measurements at the furthest downstream location, both the spanwise variation of average streamwise velocity and the Fourier content in the instantaneous fields suggested that the case with 0.6 delta cylinder spacing, which matched the dominant spanwise scale in the unperturbed flow, yielded the most persistent downstream flow organization. A flying PIV method was implemented to track specific packet structures over a range -2< x/delta < 7 with respect to the cylinder array, corresponding to a time scale of 12.4/U-delta Packets approaching the 0.2 delta spacing array first lost their organization but then regained it a distance 2 delta downstream, suggesting that a persistent outer layer organization propagated inwards into the log region. For arrays with larger spanwise spacing, approaching packets were generally redirected into the spanwise location midway between cylinders and sometimes enhanced.