Boundary layer response to periodic spanwise perturbation by an array of obstacles

Hot-wire and stereoscopic particle image velocimetry (SPIV) measurements were performed on zero pressure gradient turbulent boundary layers perturbed by a single array of cylinders extending into the log region. The array spacing had a strong effect on the resulting vortex packet organization. With $0.6\delta $ array spacing, incoming vortex packets are frequently enhanced and remain stable over a downstream distance of $7\delta $. On the other hand, a $0.2\delta $ spacing strongly disturbed incoming packets initially, but the incoming organization frequently reappeared by $2\delta $ downstream. Measurements indicate that the effect of the perturbation is strongest in the mid-span location between array elements. Furthermore, hot-wire measurements suggest that the strongest velocity perturbations migrate upwards with increasing downstream distance. SPIV results in streamwise-spanwise planes at different heights will be compared to show how structural aspects of the perturbation vary with downstream distance.