Mean velocities and turbulence statistics around a finite wall-mounted square cylinder in turbulent boundary layer with polymer solution injection

This study investigates the influence of polymer (polyethylene oxide: PEO) solutions injected through a wall slit on the flow around a finite wall-mounted square cylinder with an aspect ratio of 7.5, defined as the ratio of its height to its width, installed perpendicular to a flat-plate turbulent boundary layer. Experiments were performed in a closed-loop water tunnel at a Reynolds number of 5980. The cylinder was placed downstream of the polymer injection slit, where the drag reduction rate exceeded 60 % and the ratio of the boundary layer thickness to the cylinder height was 0.23. The diffusion process of PEO was visualized using PLIF (planar laser-induced fluorescence), and the velocity field was measured using PIV (particle image velocimetry) to obtain the mean velocities and turbulence statistics around the cylinder. PLIF results showed that rolled-up structures were generated periodically in the cylinder’s wake region regardless of the injected fluids (water and PEO solutions). The inclination angle of these structures depended on the type of fluids, leaning upstream in the water case and downstream in the PEO injection case. PIV results revealed that alternating upward and downward flows appeared in the wake region, with the upward flow contributing to the development of the rolled-up structures. Mean velocities and turbulence statistics showed that, in the PEO injection case, the saddle point shifted upward, leading to the appearance of high-speed fluid near the wall and resulting in the distinct difference in Reynolds shear stress distribution compared with the Newtonian case.