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Effects of cylinder array height on tracked vortex packets in a turbulent boundary layer

The effect of obstacle height was investigated for cylindrical arrays embedded within the log region of a turbulent boundary layer with Reτ = 2500. The array spacing was set at 0.6δ to match the most probable spanwise spacing of hairpin packets. An array that extended to the top of the log region (H = 500) strongly amplified the 0.6δ mode throughout the log region with the strongest relative amplification closest to the wall at z = 125. A shorter array with H = 125 affected the flow up to z = 300 such that energy contained in spanwise scales below 0.6δ was decreased and energy in larger spanwise scales was increased. Cross correlation of flying PIV fields suggested that the shorter array enhanced the streamwise alignment of incoming packet structures in contrast to the taller array which shifted incoming packets into spanwise zones midway between the cylinders.