Modification of turbulent boundary layer structure using immersed wall-mounted cylinders

Spanwise arrays of wall-mounted cylinders with H/δ ≤ 0.2, where H is the cylinder height and δ is the boundary layer thickness, and tall enough to protrude into the logarithmic region were used to modify turbulent boundary layers (Re_τ=2400) in an attempt to affect the organization of the coherent vortical structures. Velocity fields were obtained downstream of the arrays by PIV and V3V. Averaged results yielded initial splitting and divergence of wakes downstream of cylinders followed by consistent initial pairing of neighboring wake structures at a location that scaled with the array spacing. Perturbation effects were limited to the log region. Instantaneous results showed that an array with 8D (0.4) spacing yielded relatively stable, long low speed regions while an array with 4D (0.2) spacing typically yielded additional pairings and a greater variability in spanwise location of downstream structures.