Empirical sensitivity of two-dimensional nonlinear wake–cylinder oscillators in cross-flow/in-line vortex-induced vibrations

Abstract

Phenomenological wake-cylinder oscillators have been extensively implemented for vortex-induced vibration (VIV) predictions. Although such models capture fundamental VIV phenomena, the maximum response estimations and comparisons with different experimental data reveal some quantitative discrepancies due to the model empiricism embedding some uncertainties through system variables. This vital issue has not been well addressed in the literature of VIV modelling. This paper presents a new comprehensive investigation into the sensitivity to empirical input variables of nonlinear wake-cylinder oscillators simulating the twodimensionally coupled cross-flow/in-line VIV and amplified mean displacements of a flexibly mounted circular cylinder in uniform flows. The fluid-structure coupling terms are advanced by accounting for the higher-order nonlinear effects of fluctuating lift-drag forces and steady-drag dynamic magnifications, depending on the relative flow-cylinder velocities. A random sampling and variance-based sensitivity studies are carried out using Monte Carlo simulations which are computationally efficient based on the reduced-order model. This enables a large series of parametric examinations. Individual contribution, relative importance,coupling and interdependence of multiple input variables affecting output uncertainties are qualitatively and quantitatively evaluated. The Reynolds number dependence is also captured by correlating the wake and hydrodynamic coefficients with experimental data. Parametric studies highlight greater variations in the predicted amplitudes and mean displacements of the cylinder two-degree-of-freedom VIV with a lower mass ratio. Numerical findings allow for the identification of a few most influential variables to be treated as the empirically tuned coefficients. The improved understanding of model versatility and sensitivity enhances the calibration confidence and the response predictability with a reduced computational effort.Highlights  Monte Carlo-based sensitivity analyses of nonlinear wake-cylinder oscillators are presented. Contribution, relative importance, coupling and interdependence of empirical variables are studied. Reynolds number dependence of empirical wake and hydrodynamic coefficients is accounted for. Most influential variables correspond to lift force wake oscillator governing cross-flow/in-line VIV. Input sensitivities and output uncertainties depend on system parameters and response amplitudes.

Flexible cylinder flow-induced vibration

“…Additionally, large-scale experiments in both deep water basin facilities and field 19,[43][44][45][46][47][48] helped to shed some light on the response of the slender structure with a very large aspect ratio close to those used in the real ocean. Furthermore, numerical simulations including both the reduced order model, such as wake oscillator, [49][50][51][52] and high-fidelity CFD [53][54][55][56][57][58] have used to explore a large structural parameter space as well as to visualize the wake topology in detail. 21 The uniform flexible cylinder in the uniform flow is one of the simplest models for the flexible cylinder VIVs.…”

Modelling of wake-induced vibrations of tandem cylinders with a nonlinear wake-deficit oscillator

“…and FD2 depend on the system relative velocities. Note that the total drag force associated with the vortex shedding consists of the mean and oscillatory components (Sumer and Fredsoe, 2006), the former contributing to the hydrodynamic damping and the in-line mean displacement of the cylinder, whereas the latter contributing to the in-line VIV (Srinil et al, 2018). In the absence of an in-line response as in the present study, only the mean drag component is accounted for through FD1 and FD2, which will render the hydrodynamic damping effect on the transverse response.…”

“…4 based on d = 4, 10.1 and 20.2. As the upstream cylinder response has been experimentally found to be largely independent from the downstream cylinder motion for a cross-flow response system with d  3, attention is placed on calibration of 2 or 2, as performed in Srinil et al (2018). Herein, an attempt is made to capture the dependence of the wake-deficit oscillator on key physical parameters d and Re, thanks to relevant WIV data of Assi et al…”

“…A phenomenological modelling of VIV of a single circular cylinder subject to a steady cross flow based on a wake oscillator has been considerably implemented and applied to both rigid and long flexible structures (Facchinetti et al, 2004;Gabbai and Benaroya, 2005;Gao et al, 2019;Opinel and Srinil, 2020;Srinil, 2010Srinil, , 2011Srinil et al, 2018;Srinil and Zanganeh, 2012;Violette et al, 2007;Srinil, 2014, 2016).…”

Optimization of the Wake Oscillator for Transversal VIV

“…The use of a single fluid oscillator for rigid cylinders mounted on elastic supports that allow displacements on the horizontal plane (2DOF) based on the relationship between the drag and lift coefficients was discussed in [15], and developed in [16,17]. Additionally, the relationship of the drag coefficient to the oscillation amplitude and the flow velocity, and also possible patterns embedded in the damping constant of the fluid oscillator were discussed in [18][19][20].…”