Abstract
In this study, we carry out large eddy simulation 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 center, 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.
Highlights
The phenomenon of Kármán vortex shedding is a major concern in design of slender and bluff-body structures exposed to the wind and ocean current flows
We focus on polygonal cylinders of N = 5, 6, 7, 8, using three-dimensional large eddy simulation (LES) to investigate the effect on the aerodynamic forces and near wake flow subjected to the complete range of angle of attack α wrt U
In order to study the effect of incidence angle α on flow characteristics, 6 equal-spaced αs are studied for each cylinder between the corner and the face orientations, which effectively cover all possible orientations
Summary
The phenomenon of Kármán vortex shedding is a major concern in design of slender and bluff-body structures exposed to the wind and ocean current flows. In a continuing wind tunnel PIV experiment, Wang et al 1 presenting detailed data of the wake of the polygonal cylinders 3 ≤ N ≤ 16 with face and corner orientation at Re= 1.6 × 104, studied the dependence on N and the cylinder orientation of the mean velocity, the Reynolds stresses, and the coherent vortex structures in the near wake. They showed that the circulation of individual shed vortex grows to its maximum value at the vortex formation length (L∗f ) and decays downstream due to the combined effect of viscosity and vortex cancellation. We will focus on the time-mean aerodynamic quantities and their correlations with the characteristic length scales extracted from the mean velocity field
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