Measurements and improved model of vortex-induced vibration for an elongated rectangular cylinder

This paper deals with vortex-induced vibrations of a rectangular 4:1 cylinder with a twofold purpose. First, it provides experimental results as a benchmark for checking the predictions of mathematical models and computational techniques. In particular, a wide wind tunnel campaign on a sectional model in smooth flow yielded the oscillation amplitudes in the lock-in range for several Scruton numbers. The dominant frequency of the velocity fluctuations in the wake of the oscillating body was measured. The second purpose of the paper is the improvement of a current approach for assessing vortex-induced vibrations of bridge decks. For this reason, prior to starting the analysis an overview of the existing mathematical models is reported. Afterwards, it is shown through wind tunnel tests that the single-degree-of-freedom model developed by Scanlan at the beginning of the 1980s is unsuitable to predict the VIV response of bridge decks for Scruton numbers different from that at which the model parameters were identified. Finally, an improved version of the model is proposed, allowing to approximate accurately the limit-cycle amplitudes for the entire range of Scruton numbers tested. The identification of the aeroelastic parameters of the improved version of the model requires only three decay-to-resonance tests for three different Scruton numbers. From the engineering point of view, this represents an important advantage, especially when the mass and the damping of the structure are uncertain or dampers have to be designed, as it makes the assessment of the sensitivity to vortex-induced vibration of bridge deck cross sections less time-consuming and expensive. In the future, it will be necessary to verify that the functional form postulated for the aeroelastic parameters of the model is valid not only for elongated rectangular cylinders but also for bridge deck cross sections.