Effects of oncoming flow turbulence on the near wake and forces of a 3D square cylinder

The aerodynamic forces and near wake of a three-dimensional (3D) square cylinder with a height-to-width ratio (H/d) of 5 were experimentally studied in a low-speed wind tunnel at a Reynolds number of 3.4 × 10⁴. This work aims to investigate the effects of turbulence intensity (=0.5%, 5.0% and 12.1%) on the forces and near wake. The wall boundary layer thickness is about 0.5d, unchanged in the three Ti values. A nominal two-dimensional (2D) square cylinder was also tested for comparison. The predominant vortex shedding frequencies of both 2D and 3D cylinders do not change with Ti. The increase of Ti reduces the aerodynamic forces and the spanwise correlation of fluctuating forces for the nominal 2D cylinder. On the contrary, it enhances both forces and spanwise correlation of the 3D cylinder. Proper orthogonal decomposition (POD) of PIV results suggests that with increasing Ti, the near wake becomes less organized for the nominal 2D cylinder, but gets more coherent and organized for the 3D cylinder. For the latter, the increase of Ti converts spanwise vortex shedding from in-phase to out of phase, especially near its free end.