Large Eddy Simulation of incident flows around polygonal cylinders

Abstract

In this study, we carry out large eddy simulation (LES) of incident flow around polygonal cylinders of side number N = 5 − 8 at Reynolds number Re= 104 . In total, six incidence angles (α) are studied on each cylinder ranging from face to corner orientations, thus covering the entire α spectrum. Special focus is put on the time-mean aerodynamic forces including lift, drag and vortex shedding frequencies as well as the near wake flow features. It is found that because of y-plane asymmetry of polygonal cross sections at most incidence angles, the flow separation characteristics and hence the induced base pressure distribution and the aerodynamic forces exhibit unique and complex dependence on α and N. While the general inverse relation of drag coefficient and Strouhal number previously proposed from experimental observations at principal orientations still holds at arbitrary α, the variation of the two is found to be non-monotonic on both α and N. We also found that compared to the absolute time mean shear layer length measured from the final separation point, the extent of them stretched to the wake, measured from the cylinder centre, is a powerful scaling factor for all the quantities investigated, including the wake characteristic length scales. In particular, the difference between the top and the bottom shear layer (due to geometrical asymmetry at arbitrary α) describes the variation of the non-zero time mean lift coefficient reasonably well, whose sign varies with N non-monotonically.