Turbulent boundary layer structures downstream of a square cylinder

Particle image velocimetry is used to investigate the coherent structures within a turbulent boundary layer (TBL) perturbed by a slender square cylinder. A moderate Reynolds number of Reτ=3691 is considered. The cylinder is immersed in different vertical regions of the TBL, from the viscous sublayer to the logarithmic layer, depending on the value of the cylinder–wall gap ratio within the range of G/D=0.0−2.0. Here, G is the distance between the wall and the lower surface of the cylinder, and D is the cross-section size of the square cylinder. The results reveal a substantial influence of the TBL statistics and structures. Utilizing velocity correlation and proper orthogonal decomposition (POD), we highlight the intriguing influence of large-/very large-scale turbulent structures induced by the cylinder. When the cylinder is positioned on or in close proximity to the wall, periodic shedding of vortices is not apparent. In the presence of a cylinder, the large-/very large-scale turbulent structures are reduced in size compared with those in the undisturbed TBL, owing to the interaction between the flow around the cylinder and the boundary layer turbulence. The lower modes of the POD in the disturbed TBL are similar to those of the undisturbed flow field. However, the large-/very large-scale turbulent structures are increasingly affected by the strengthening of the shedding vortices with increasing G/D when the cylinder is located in the logarithmic region. As a result, the Kármán vortex street becomes the dominant structure associated with the higher POD modes at sufficiently high cylinder–wall gap ratios.