Wind Tunnel Investigations of An Inclined Stay Cable With A Helical Fillet

CHAPTER 5: CONCLUSIONS

The aerodynamics of a stay cable model fitted with a helical fillet and inclined to the oncoming flow were studied to evaluate the effect of adding a helical fillet at the surface of a stay cable in relation to its response to strong winds. A helical fillet with a double helix representative of what can be found on stay cables in Europe and North America was affixed to the surface of the HDPE tube of an existing cable model. The helical fillet had a rectangular cross section 2.3 mm high and 2.4 mm wide, a pitch equivalent to 3.2 cable diameters, and a helix angle of 45 degrees.

The experiments revealed that a stay cable with a representative surface roughness, cross- sectional shape, and helical fillet inclined at 60 degrees to the flow can experience wind-induced vibrations with large amplitudes in smooth or turbulent flow for a low level of structural damping. The oscillations observed appeared to be self-limited in amplitude and could be mitigated with an increase of structural damping equivalent to a Scruton number of 5.1. The cable model with the helical fillet experienced a drag crisis almost as pronounced as for the smooth cable but at a lower Reynolds number range.

The experiments revealed also that the aerodynamic forces at the source of the vibrations were highly sensitive to a rotation of the cable model on its axis. Measurements of the external diameter of the smooth cable model have shown a maximum eccentricity equivalent to 1 percent of the mean diameter, which appeared to be sufficient to influence the aerodynamics of the cable model even with the helical fillet in place. Such deviation from the mean cable diameter was deemed representative of field conditions. The trajectory of the cable motion in relation to the prevailing wind direction was also found to be a parameter of importance in the study of cable behavior and stability. During the previous studies on inclined cable galloping, it had not been possible to reach any conclusions on this aspect since the cable-wind plane and rotation of the cable on its axis could not be evaluated separately.

For a stay cable inclined at 45 degrees with a helical fillet, no large vibrations were observed for the range of wind speed investigated, 4 to 36 m/s, for several experimental conditions including spring-plane rotation and rotation of the cable on its axis. The rate of change of the mean aerodynamic force coefficients with wind speeds for a model inclination of 45 degrees were found to be lower than for the 60-degree inclination cases.

Complementary tests on the cable model without the helical fillet in smooth and turbulent flow at an inclination of 60 degrees revealed large vibrations in accordance with the results of previous studies on the same cable model. The vibrations were equivalent and sometimes larger in amplitude than what was previously reported and did not happen at the same wind speeds. This emphasized the importance of the shape of the polyethylene tube covering the stay cable, a local 1 percent deviation from the mean diameter, which was sufficient to amplify the wind excitation.