Three-dimensional wake structures and aerodynamic coefficients for flow past an inclined square cylinder

Numerical investigation has been performed for flows past an inclined square cylinder in the range of Re≤300 to elucidate the characteristics of the laminar three-dimensional wake behind the cylinder depending upon Reynolds number and angle of incidence (θ). An immersed boundary method was used for implementation of the cylinder on a Cartesian grid system. Both Floquet stability analysis and full three-dimensional simulation were employed to detect the onset of the secondary instability leading to three-dimensional flow, and to provide quantitative flow data. It was revealed that mode A becomes more unstable for “symmetric” flow configuration, while mode C is dominant for asymmetric flows in the range of 10°≤θ≤25°. The most unstable three-dimensional modes predicted by the Floquet stability analysis were well confirmed by the full 3D simulations which were conducted at Re=150, 200, 250, and 300, with varying angle of incidence. The full 3D simulations also provided the key flow characteristics such as mean flow-induced force/moment coefficients and Strouhal number of vortex shedding. It was seen that they are sensitive to slight inclination of the square cylinder, and the Re effects are insignificant except for mean lift coefficient. Vortical structures in the wake, taken from the results of the Floquet stability analysis and the full 3D simulations, respectively, were visualized by Q-contours, revealing good agreement between the two.

▶ Inclination alters flow topology, resulting in change of flow characteristics. ▶For a given angle of incidence, Floquet analysis predicts the secondary instability. ▶ The 3D modes predicted by Floquet stability analysis are confirmed by full DNS. ▶ DNS computes force loading, moment, and St for each set of Re and angle of incidence. ▶ Re effects are insignificant on force loading except mean lift coefficient.