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COMPUTATIONAL METHODS FOR ENGINEERING SCIENCE
Edited by: B.H.V. Topping
Towards Numerical Prediction of Galloping Events of Iced Conductors
W.G. Habashi1, A. Borna1 and G. McClure2
1Computational Fluid Dynamics Laboratory, Department of Mechanical Engineering
W.G. Habashi, A. Borna, G. McClure, "Towards Numerical Prediction of Galloping Events of Iced Conductors", in B.H.V. Topping, (Editor), "Computational Methods for Engineering Science", Saxe-Coburg Publications, Stirlingshire, UK, Chapter 6, pp 139-165, 2012. doi:10.4203/csets.30.6
Keywords: galloping, numerical simulation, fluid-structure interaction, transmission lines, ice accretion, wind effects.
In this chapter, various types of transmission line instabilities are presented and their impact on reliability and serviceability of electrical networks discussed. The galloping event is shown as an important design criterion. By following a historical background, different mechanisms to describe this event are investigated and the benefits and drawbacks of each method are referenced. The important factors affecting galloping and the conditions that may lead to large instabilities are addressed. Moreover, a cost-effective computational methodology to study galloping as aeroelastic instability is presented, and the modules involved are described in detail. Numerical advances in predicting galloping events under different weather conditions and a variety of ice accretions are presented through various test cases. The first test case is that of the transverse vortex-induced vibration of a circular cylinder with low mass-damping. In the absence of direct Navier-Stokes (DNS) and large eddy simulation (LES) models, as they are computationally expensive, it is demonstrated that unsteady Reynolds-averaged Navier-Stokes (URANS) models can efficiently predict the salient features of the flow around freely vibrating bluff bodies, including the near wake structure, the shedding modes, and unsteady loading. The numerical results are compared with experiments and show very good agreement compared to the previous studies, in particular for capturing the upper branch of the response. The second test case deals with the galloping of an iced profile, for which an experimental benchmark test case is available. The instability of the profile at other incident velocities than the experiment is also investigated. The effects of incident velocity on the flow field, the aerodynamic loading, oscillations, and the galloping ellipses are examined. Finally, the fluid-structure interaction of windward and leeward conductors, in the speed range of 10-40 m/s, is studied under three icing conditions. The structural response of the conductors, effects of incident wind velocity on horizontal, vertical and torsional amplitudes of conductor motion, orientation of the galloping ellipses, in-the-wake structural response, structure of the vortex street, relation between vertical and torsional displacements, and the effect of various conditions on the onset of galloping are studied. The simulation results show that small ice accretions on conductors (of the order of 0.1 D), subjected to a favourable free-stream velocity, can lead to large undamped oscillations. It is demonstrated through a rime ice case that if the shape and amount of ice modify the conductor profile in such a way as to considerably amplify the aerodynamic loading, a galloping event is likely.
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