Experiments on the aerodynamic behaviour of square cylinders with rounded corners at Reynolds numbers up to 12 million

The influence of the angle of incidence and corner radius on the aerodynamics of square-section cylinders is studied by means of wind tunnel experiments. Two different corner radii (r∕D=0.16 and 0.29) were investigated at three angles of incidence (α=0°, −22.5° and −45°). Steady and unsteady global forces and local surface pressures were measured in the high-pressure wind tunnel in Göttingen. The Reynolds number was varied up to values as high as 12×106, thereby spanning the known flow state regimes up to high transcritical. The results demonstrated that a decrease of the cylinder's bluffness induced lower maximum drag coefficients and r.m.s. values, as well as higher Strouhal numbers in all flow state regimes. Furthermore, the critical Reynolds numbers shifted to significantly lower values. For the cylinder configurations at α=0° no upper transition or transcritical flow state was present up to ReD=12 million. A decrease in the angle of incidence resulted in a significant reduction of the length of the supercritical flow state, a shift of the drag force, Strouhal number and base pressure to higher values and an increase of the critical Reynolds numbers. The cylinders at non-zero angles of incidence all displayed a clear critical flow state, at which two discontinuous transitions were observed, accompanied by jumps in the CD and CL values and the Strouhal number. Only three out of six studied configurations experienced hysteresis, where for the high corner radius configuration at α=0° a particularly broad hysteresis effect was measured.