Separation angles and wake features of incident flow around polygonal cylinders*

In this study, large eddy simulation (LES) is used to investigate shear layer separation of incident flow around polygonal cylinders of side number N=5-8 at Reynolds number Re=10⁴ . In total, six equally distributed incidence angles (α) are studied on each polygon between the face and the corner orientations, thus covering the entire α spectrum. The position of separation points and the instantaneous behaviour of the separated shear layers are studied in detail. It is found that the separated shear layers are highly dynamic manifesting a flapping motion of various condition depending amplitude, which is a key factor that causes flow reattachment and results in a second separation, in addition to the first separation in some incidence angles. The strength of this flapping motion is found to be associated with shear layer penetration distance and shear layer thickness growth rate. The separation angles, which are fixed on the corners, can be calculated by the proposed empirical/analytical equations which are found to be in good agreement with available experimental results. Furthermore, a wake deflection angle is introduced and shown to be a good scaling factor for lift, drag and Strouhal number. Finally, the critical separation angle, which corresponds to the longest shear layer penetration distance, the minimum drag and maximum lift conditions is determined and estimated with analytical and empirical equations.