Hydrodynamic Coefficients for Vortex Induced Vibrations of Slender Beams

Abstract

Empirical codes for prediction of vortex induced vibrations need reliable data for hydrodynamic coefficients. Such data are almost exclusively based on measured forces on rigid cylinders that have forced harmonic motions in cross-flow (CF) or in-line (IL) directions. This type of experiment is not able to capture all effects that could be important for realistic cross section motions of slender beams due to two reasons: 1. Slender beams will normally have combined IL and CF oscillations. 2. Higher order frequency components will normally be present for vibrating beams. It is difficult to measure local forces on short segments of flexible beams in laboratory tests due to the small diameters. The most convenient instrumentation is to use a large number of strain gauges or accelerometers along the beam. Proper data processing will then give reliable data for the motions, which means that the trajectory of cross sections can be found. Hence, the following set of experiments can be carried out in order to find hydrodynamic coefficients under realistic VIV conditions: 1. Experiments with a slender flexible beam and processing of recorded strains or accelerometers to identify cross section trajectories. 2. Measurement of forces on a rigid cylinder section with forced motions. Reynolds number, amplitude ratio, orbit shape and non-dimensional frequency must be identical in the flexible beam and rigid cylinder tests. Such experiments have been carried out, and the results are presented in terms of hydrodynamic coefficients for combined CF and IL oscillations. Coefficients are found for the primary CF and IL frequencies, but also for higher order frequency components. Results are presented and discussed in relation to well known results from pure CF and IL oscillations. One way of verifying that the coefficients have been correctly identified, is to apply the coefficients in an empirical response model and compare analysis results to the observation. This step has, however, not been carried out so far.

An investigation into the hydrodynamics of a flexible riser undergoing vortex-induced vibration

“…Soni (2008), Yin and Larsen (2010) studied forced oscillations of a smooth, rigid cylinder, where the oscillations replicated the motion at various points on a flexible riser as measured from flexible riser model tests. Based on these experiments, the hydrodynamics of flexible risers undergoing VIV in uniform and shear flows were studied.…”

Improved In-Line Vortex-Induced Vibrations Prediction for Combined In-Line and Cross-Flow Vortex-Induced Vibrations Responses

“…3. This database is based on the results from the PhD work of Soni [8], and is updated based on comparisons with the model tests on a vertically tensioned riser [19] [20]. The coefficients should still be considered as preliminary.…”

“…Realistic orbits measured from a flexible beam VIV model tests were applied in forced motion VIV model tests [7]. This experimental method was first applied by [8], and further used by [7] and [9]. Both non-periodic time history of the motions and representative periodic motions were used in the experiments, see Fig.…”

Improved In-Line VIV Prediction for Combined In-Line and Cross-Flow VIV Responses

“…Realistic orbits measured from a flexible beam VIV model tests were applied in forced motion VIV model tests [9]. This experimental method was first applied by [4], and further used by [9] and [10]. Both non-periodic time history of the motions and representative periodic motions were used in the experiments, see Figure 2.…”