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Aeroelastic instability of a structural angle section Slater, Jonathan Ernest


Angle section members, used in open engineering structures, have been known to experience large amplitude oscillations when exposed to normal atmospheric winds, and in a few instances failure has been reported. The bluff geometry together with low natural frequency makes these members susceptible to aeroelastic vibrations of a vortex resonant or galloping nature. The thesis aims at studying the nature of the aerodynamic forces and the resulting instabilities for the safe design of the structures. It presents information on the aerodynamics and dynamics of the angle section during stationary, plunging, torsional and combined plunging-torsional conditions. From the measurements on stationary angle models, it is possible to predict the critical vortex resonant wind speeds for various angles of attack. The large variations of the unsteady aerodynamic coefficients indicate the dependence of the resonant instability on model orientation. Incorporating the stationary aerodynamic loadings, the quasi-steady analysis is able to predict the galloping instability and resulting amplitude and buildup time response. The absence of torsional galloping during the experiment is substantiated by the theory which shows the instability to occur only at high wind speeds or for systems with very low damping. The dynamical study demonstrates that structural angle sections are susceptible in general, to combined plunging and torsional vibrations. The nature of the instability depends on such system parameters as damping, natural frequency, angle of attack, section size, etc. However, due to the existence of two distinct families of virtual hinge points, it is possible to represent the motion as predominantly plunging or torsion. Furthermore, the frequency of the coupled motion as well as the type and range of the instability are found to be similar to those in the single degree of freedom. This makes it possible to obtain pertinent information by studying, both experimentally and theoretically, the plunging and torsional degrees of freedom, separately. During plunging resonance, the angle section experiences a vortex capture phenomenon where the shedding frequency is controlled by the cylinder motion over a finite wind speed range. On the other hand, the torsional vibration shows a vortex control condition over a large velocity range where the vortex shedding governs the frequency of oscillation and follows the stationary model Strouhal curve. Compared to the stationary and torsional results, the fluctuating pressures on the angle surface during plunging resonance are substantially larger in magnitude with less amplitude modulation and phase variation. Consequently, the unsteady aerodynamic coefficients increase with this instability. During resonance in either degree of freedom, the vortex velocity and longitudinal spacing remain essentially unaltered, however, the wake width experiences substantial increase with plunging motion. It appears that the torsional resonance has virtually no effect on the vortex shedding or wake characteristics.

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