Effect of surface topology on the galloping instability of rectangular cylinders

摘要

The effect of geometry on the transverse galloping instability of rectangular cylinders was studied experimentally for Reynolds numbers between 1,000 and 10,000. In particular, a comparison was made between a rectangular cylinder with rounded corners and a smooth surface, and the same baseline geometry with added surface topology synthesized from two-dimensional Fourier-modes. The effects of the topology amplitude and wavelength were investigated. From measurements of the normal (galloping direction) force coefficient variation with angle-of-attack, it was found that the added surface topology generally had a destabilizing effect relative to the smooth cylinder. At the lowest Reynolds number, the smooth cylinder was stable, while the cylinders with added topology were unstable with respect to galloping. For Reynolds numbers from 5,000 to 10,000, the added topology did not cause a similar instability. However, there was a monotonic increase in the slope of the normal force coefficient at zero angle-of-attack with increasing surface height amplitude, thus moving the geometry closer to the instability threshold. This effect diminished as Reynolds number increased. Overall, for the range of parameters investigated herein, whenever the cylinders with topology were unstable to soft or hard galloping, the larger topology exhibited more favorable galloping resistance characteristics than the one with smaller topology. Topology wavelength was found to have no effect on the galloping behavior of the cylinder for Reynolds numbers below 7500, and a moderate increase in the normal force slope at zero angle of attack with decreasing wavelength for larger Reynolds numbers. The latter effect was associated with an increase in the angle of attack at which the cylinder could become unstable to hard galloping.