Low-Frequency Variations of Force Coefficients on Square Cylinders with Sharp and Rounded Corners

Square lighting poles with sharp or round corners have been observed to exhibit large amplitude oscillations with a frequency that is near that of their first mode. This frequency is one order of magnitude lower than that of the vortex shedding frequency for the observed wind speeds. As such, lock-in resonance between the vortex shedding and the poles motions has been ruled out as the cause of these oscillations. In this work, numerical simulations of flows over square cylinders with different aspect ratios and cross-sectional shapes have been carried out to explore and quantify low-frequency variations of the aerodynamic forces and their sources. The results show that low-frequency components are a basic aspect of the flow over finite length cylinders and are associated with the three-dimensional flow characteristics near the free end of the cylinder. This aspect is related to the loss of synchronization of the vortex shedding which, in turn, results in variations of the mean drag coefficient along the cylinder. This ability to numerically simulate the flow over a lighting pole and detailed results, as presented here, could be used to overcome wind tunnel test limitations imposed by the size of the test section and blockage ratio, and of meeting geometric similarity requirements for high aspect ratio configurations. Consequently, results from such simulations could play an important role in improving code specifications which, to date, have been based on wind tunnel simulations.