Large-scale turbulence effects on flow dynamics around and aerodynamic forces on a square cylinder

Turbulence effects on the aerodynamics of a square cylinder have been widely investigated due to their fundamental significance in both flow physics and engineering applications. However, the influence of large-scale turbulence on shear layer unsteadiness, and its consequences on flow structure and aerodynamic forces has received insufficient attention. The present study explores these effects, considering turbulent flows with turbulence intensities up to 20% and integral length scales up to four times the characteristic length of the obstacle. A reduced-order model and measurable indicators of flow dynamics are employed to investigate the underlying mechanisms quantitively. The findings reveal that large-scale, high-intensity freestream turbulence amplifies the root mean square (rms) flapping amplitudes of shear layers by provoking and superposing a set of low-frequency unsteadiness with energy levels comparable to that of Karman vortex shedding. The alteration in shear layer behavior results in (1) an extended region of high rms pressures around the square cylinder and (2) intermittent shear layer reattachment, followed by an intermittent weakening of the vortex shedding. These effects lead to a significant increase in rms pressure coefficients on the lateral and leeward surfaces, as well as an intermittent suppression of lift forces. Two new flow patterns were observed during periods of weakened flow dynamics: (1) vortices forming above the lateral surfaces shed downstream directly without interacting with the shear layer on the other side; and (2) Karman vortices in the wake region break down before shedding downstream.