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FLUID NONLINEARITIES EFFECT ON WAKE OSCILLATOR MODEL PERFORMANCE
Last modified: 2017-11-30
Abstract
Vortex-induced vibrations (VIV) need to be accounted for in the design of marine structures such as risers and umbilicals. If a resonance state of the slender structure develops due to its interaction with the surrounding fluid flow, the consequences can be severe resulting in the accelerated fatigue and structural damage. There is a range of VIV models and specific software available in the modern engineering practice. In general, two important aspects need to be balanced in any applied method: accuracy of predicting the development of the resonance state in given conditions, and computational time to get reliable results. The existing software is able to provide either accurate prediction by complex CFD estimations of the fluid characteristics around the structure, or a relatively fast result, but with a probability of risk of underestimation. In both cases, when the decision needs to be made about VIV prevention, designers tend to apply high safety factors. That means that there is a limited trust in the existing models, and, hence, there is a room to improve computational approach to VIV problem.
Wake oscillator models allow to estimate the fluid force acting on the structure without complex and time consuming CFD analysis of the fluid domain. However, contemporary models contain a number of empirical coefficients which are required to be tuned using experimental data. This is often left for the future work with the opened question on how to calibrate a model for a wide range of cases and find out what is working and is not.
The current research unveils the strategies to pass the barrier of calibration and to modify the wake oscillator models proposed in [1] to deliver an adequate prediction of the system behaviour. The improved model is initially designed for 2 degrees-of-freedom rigid structures employing other versions of nonlinear damping including those suggested by Landl [2] or by Krenk and Nielsen [3]. The main improvements made in this work are an identification of the delay between theoretical and real resonances, and also a variation in the applied damping in fluid equation. The priority during calibration is given to predicting peak displacements of a structure. In this work, the authors show how one of the existing models can be adapted through calibration and becomes applicable to a variety of cases. The validation is performed using the data from different experimental set-ups such as [4-5]. This work is aimed to be one of the steps on the way to introduce modern, refined wake oscillator models into wide engineering practice.
References:
[1] Postnikov, A., Pavlovskaia, E.E., Wiercigroch, M. (2016) CFD calibrated wake oscillator model for vortex induced vibrations, International Journal of Mechanical Sciences, DOI: 10.1016/j.ijmecsci.2016.05.019.
[2] Landl, R. (1975) A mathematical model for vortex-excited vibrations of bluff bodies. Journal of Sound and Vibration, 42(2), 219-234.
[3] Krenk, S. and Nielsen, S.R.K. (1999) Energy balanced double oscillator model for vortex-induced vibrations. Journal of Engineering Mechanics, 125(3), 263-271.
[4] Stappenbelt, B. and Lalji, F. (2008) Vortex-induced vibration super-upper response branch boundaries. International Journal of Offshore and Polar Engineering, 18(02), 99 – 105.
[5] Williamson, C.H.K. and Jauvtis, N. (2004) The effect of two degrees of freedom on vortex-induced vibration at low mass and damping. Journal of Fluid Mechanics, 509, 23 – 62.
Wake oscillator models allow to estimate the fluid force acting on the structure without complex and time consuming CFD analysis of the fluid domain. However, contemporary models contain a number of empirical coefficients which are required to be tuned using experimental data. This is often left for the future work with the opened question on how to calibrate a model for a wide range of cases and find out what is working and is not.
The current research unveils the strategies to pass the barrier of calibration and to modify the wake oscillator models proposed in [1] to deliver an adequate prediction of the system behaviour. The improved model is initially designed for 2 degrees-of-freedom rigid structures employing other versions of nonlinear damping including those suggested by Landl [2] or by Krenk and Nielsen [3]. The main improvements made in this work are an identification of the delay between theoretical and real resonances, and also a variation in the applied damping in fluid equation. The priority during calibration is given to predicting peak displacements of a structure. In this work, the authors show how one of the existing models can be adapted through calibration and becomes applicable to a variety of cases. The validation is performed using the data from different experimental set-ups such as [4-5]. This work is aimed to be one of the steps on the way to introduce modern, refined wake oscillator models into wide engineering practice.
References:
[1] Postnikov, A., Pavlovskaia, E.E., Wiercigroch, M. (2016) CFD calibrated wake oscillator model for vortex induced vibrations, International Journal of Mechanical Sciences, DOI: 10.1016/j.ijmecsci.2016.05.019.
[2] Landl, R. (1975) A mathematical model for vortex-excited vibrations of bluff bodies. Journal of Sound and Vibration, 42(2), 219-234.
[3] Krenk, S. and Nielsen, S.R.K. (1999) Energy balanced double oscillator model for vortex-induced vibrations. Journal of Engineering Mechanics, 125(3), 263-271.
[4] Stappenbelt, B. and Lalji, F. (2008) Vortex-induced vibration super-upper response branch boundaries. International Journal of Offshore and Polar Engineering, 18(02), 99 – 105.
[5] Williamson, C.H.K. and Jauvtis, N. (2004) The effect of two degrees of freedom on vortex-induced vibration at low mass and damping. Journal of Fluid Mechanics, 509, 23 – 62.