Time domain simulation of cross-flow and in-line vortex-induced vibrations

3105 ABSTRACT: A method for time domain simulation of combined cross-flow and in-line vortex-induced vibrations of slender structures with circular cross section is presented. Drag and lift forces are evaluated based on the incoming fluid velocity and the velocity and acceleration of the structure. The present method differs from the semi-empirical frequency domain and the Van der Pol wake oscillator models, particularly due to the way synchronization between vortex-shedding forces and structure motion is simulated. A synchronization model is presented, which allows the vortex-shedding frequency to adjust itself to the frequency of motion. This is also referred to as lock-in; a well-known and highly important feature of vortex-induced vibrations. The synchronization model, along with a small number of non-dimensional coefficients, must be determined empirically, by utilization of already established data. Since the method works in time domain, it is in principle possible to include non-linear structural effects, or study response due to transient flows such as waves, although this has presently not been investigated. The hydrodynamic force model is used in combination with a finite element model and time integration of the equation of motion to simulate combined in-line and cross-flow vortex-induced vibrations of a flexible cylinder in a stepped current. Key results such as dominating frequency, mode and standard deviations of displacement and curvature are compared to experimental observations. It is concluded that the model predicts vibration frequency and mode quite accurately. The predicted displacement and curvature is very realistic for the in-line direction, while slightly conservative for the cross-flow direction.

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