Modelling the aeroelastic response of slender structures to vortex-induced vibrations for fatigue analysis

• Applicant: Dr.-Ing. Francesca Lupi
• Subject Area: Applied Mechanics, Statics and Dynamics
• Term: from 2019 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 426322127

Slender structures subjected to vortex-induced vibrations experience an aeroelastic interaction in the lock-in range. The negative aerodynamic damping is the governing parameter for the onset of self-induced vibrations. However, available literature models still lack of a unified behaviour of the aerodynamic damping as a function of the oscillation. This is also reflected in codified methods to design slender structures in view of cross-wind actions. This evidence applies firstly for the maxima of the oscillation amplitudes, i.e. the maxima of the structural stresses. Additionally, concerning modelling of load processes for the analysis of fatigue strength, theoretical deficiencies cause even larger deviations between the prediction of fatigue life and its actual value.The research project aims to develop a model to predict the oscillation amplitudes throughout the lock-in range, thereby laying the foundation for modelling of fatigue processes. The starting point is the experimental investigation in a wind tunnel. The forced-vibration testing technique shall be used. The following parameters are considered:1) The wind velocity is varied in relation to the oscillation frequency, thereby exploring the resonance zone in the experiment. In this way, it is possible to determine the behaviour of the aerodynamic damping.2) The amplitude of the forced oscillation includes the effect of the Scruton number. In this regard, it has to be clarified if, in case of large oscillations, a limiting value of the oscillation occurs, which cannot be exceeded even in case of increasingly smaller Scruton numbers, or if the boundaries of the aerodynamic damping are asymptotically approached. 3) The turbulence intensity influences the spectral content of the excitation process by extending its spectral bandwidth and consequently enlarging the velocity range, in which lock-in occurs. As a result, resonance happens more often.A model for the calculation of the aeroelastic, vortex-induced vibration is developed based on this experimental background. The model is validated experimentally through wind tunnel tests in free-vibrations. Additionally, through these tests, the whole lock-in range – including its rises to the largest oscillation as well its drops from resonance – is investigated. In time domain, depending on the value of the Scruton number, intermittent - mathematically speaking quasi-periodic - oscillations occur with either a predominant stochastic or a harmonic character. They need to be modelled as well, in order to determine the effect on the fatigue life of both full resonance and intermittent situations throughout the whole resonance zone. Finally, the whole concept will be verified by means of aeroelastic wind tunnel tests in case of a realistic load bearing behaviour, especially in view of the mode shape.

• DFG Programme: Research Grants