Yaw Effects on Galloping Instability

Quasisteady theory of galloping oscillations of slender prismatic structures, such as transmission lines, stay cables, etc. is extended to cover flows approaching at a yaw angle. From the independence principle, aerodynamic forces are found to reduce with the cosine squared of the yaw angle, while dynamic angles of attack increase inversely with the cosine. The combined effects result in a simple universal galloping response characteristic for arbitrary yaw angles, in terms of a dimensionless reduced amplitude and reduced velocity. This depends on the aerodynamic characteristics of the cross section and the mode shape. It is found that yaw shifts the instability towards higher velocities, but lower amplitudes. This, in general, improves the stability of the system, however, hard oscillators consequently also require lower starting amplitudes. Predictions are compared with calculated response characteristics based on measured static data for a yawed D-section and the agreement is fair.