COMBINED EFFECTS OF GALLOPING INSTABILITY AND VORTEX RESONANCE

Some towers and most tall buildings have cross-sectional shapes that are aerodynamically unstable to transverse disturbances in a wind. The oscillation resulting from this form of instability is called galloping, and for systems in which the critical wind speed for the initiation of galloping is considerably higher than the wind speed for wake vortex resonance with the elastic system, a quasi-steady theory can successfully predict galloping behaviour. However, the parameters for towers and tall buildings typically place the critical galloping wind speed close to, or even lower than, the vortex-resonance wind speed, thus introducing mutual effects of the two phenomena, and making the quasi-steady theory inapplicable. In the present experimental investigation, these mutual effects are studied in a wind tunnel for a two-dimensional prism of square section in smooth flow, as a first step in seeking understanding of the phenomena. Interesting results are obtained, and some trends, dependent on the relative values of the two critical wind speeds, are indicated.