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Dry galloping of inclined cables has been shown to have a strong relation to the critical Reynolds number. This study concerns the occurrence of galloping of an elliptical cylinder at critical Reynolds numbers under normal wind and an assessment of the quasi-steady assumption on predicting these vibrations. A series of static and dynamic wind tunnel tests are carried out to measure the wind pressure on a static cylinder and displacement of a three-degree-freedom vibrating cylinder. The static aerodynamic force on the cylinder shows the occurrence of reattachment which lowers the drag coefficient and increases the lift coefficient at critical Reynolds numbers. This phenomenon gives the possibility of satisfying the across-wind galloping criterion by adversely changing the aerodynamic force with the angle of attack. Meanwhile, unsteady and steady amplitude galloping are observed in dynamic tests in a certain range of Reynolds number and angle of attack. The observations indicate that the galloping is across-wind dominated and strongly depends on the Reynolds number. Finally, quasi-steady predictions of galloping instability are compared with the observed galloping. Most of the observed occurrences of galloping are in the predicted unstable range, but several other cases for which galloping was predicted do not exhibit large vibrations. This implies that the quasi-steady assumption does not work well in predicting the galloping of elliptical cylinders at critical Reynolds numbers.

The excitation mechanism of vibrations of circular cylinders in the critical Reynolds number range remains unclear. These vibrations have been observed in wind tunnels many times but rarely in the field. The surface roughness of the cylinder might be a reason for this difference. Aiming to reveal the effect of surface roughness on the aerodynamic forces and vibrations in the critical Reynolds number range, seven circular cylinders with various value of surface roughness were covered with abrasive papers and tested in stationary and elastically mounted wind tunnel tests. The results show that the surface roughness significantly influences the aerodynamic forces in the critical Reynolds number range by reducing the range of transitions on boundary layers. These influences include suppressing the jumps in lift coefficients and other phenomena that relate to a bistable state occurring only for less rough cylinders. Therefore, sufficient surface roughness can mitigate the vibrations in the critical Reynolds number range by suppressing the bistable state in which the vibrations appear.

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