• accelerating flow
• Large-Eddy Simulations
• rectangular cylinder
• upstream-edge rounding
• Wind tunnel

The present work investigates the high-Reynolds-number flows around rectangular cylinders of different chord-to-depth ratios (from 1 to 5), which can be assumed as simplified models for complex architectures, e.g., tall buildings and bridge decks. The synergic approach of Large-Eddy Simulations and wind tunnel tests favours a deeper insight into the physics and the flow features of these phenomena. Despite the simple geometries, these flows are characterized by considerable vortex shedding and high variability. The flow features of the 5:1 rectangular cylinder are ruled by the velocity fluctuations on the mean separated shear layers. The upstream-edge rounding and the angle of attack play a fundamental role in governing the flow features. In particular, the former parameter is negligible in wind-tunnel tests when it is below r/D=0.0360; conversely, its further increase reduces the starting slope of the shear layers and the mean recirculation length. The 3:1 and 4:1 rectangular cylinders present an intermittent flow reattachment, but the upstream-edge rounding favours permanent reattachment. In addition, great relevance is given to unsteady phenomena, such as sudden flow accelerations caused by thunderstorm outflows. During these accelerating flows around square cylinders, the presence of constant-frequency time cells causes a not-constant vortex-shedding Strouhal number, which could affect the wind-induced loading of structures in thunderstorm conditions.