Hossein Zanganeh

Characterization of Variable Hydrodynamic Coefficients and Maximum Responses in Two-Dimensional Vortex-Induced Vibrations With Dual Resonances

A phenomenological model and analytical-numerical approach to systematically characterize variable hydrodynamic coefficients and maximum achievable responses in two-dimensional vortex-induced vibrations with dual two-to-one resonances are presented. The model is based on double Duffing and van der Pol oscillators which simulate a flexibly-mounted circular cylinder subjected to uniform flow and oscillating in simultaneous cross-flow/in-line directions. Depending on system quadratic and cubic nonlinearities, amplitudes, oscillation frequencies and phase relationships, analytical closed-form expressions are derived to parametrically evaluate key hydrodynamic coefficients governing the fluid excitation, inertia and added mass force components, as well as maximum dual-resonant responses. The amplification of the mean drag is ascertained. Qualitative validations of numerical predictions with experimental comparisons are discussed. Parametric investigations are performed to highlight the important effects of system nonlinearities, mass, damping and natural frequency ratios.

Coupled Axial/Lateral VIV of Marine Risers in Sheared Currents

Modelling and prediction of vortex-induced vibrations (VIV) of marine risers is a challenging task due to the associated multi degrees of freedom in both cross-flow/in-line directions and the multi-mode fluid-structure interactions. In addition, the axial motion and its geometrically nonlinear coupling with lateral responses can be significant, especially at higher-order modes. Nevertheless, several papers in the literature dealing with VIV predictions have often overlooked such aspects. Therefore, this study aims to investigate and understand the effect of axial or longitudinal motion through a theoretical model and numerical approach in time domain. Attention is paid to VIV of vertical risers subjected to linearly sheared currents. To capture a three-dimensional aspect of the flexible cylinder experiencing VIV, a semiempirical model is developed consisting of nonlinear equations of cross-flow, in-line and axial structural oscillations which are coupled with the distributed van der Pol-type wakeoscillators modelling the fluctuating fluid lift/drag forces. The mean drag force is also taken into account. These model equations are numerically solved via a space-time finite difference scheme, and the obtained numerical results highlight several aspects of VIV of elastic cylinders along with the axial motion effects. Apart from the validation of the numerical model with published experimental results, this study reveals how the effect of axial motion and its nonlinear coupling with the two lateral cross-flow/in-line motions can be very important. These depend on the flow velocity, the fluid-structure parameters, the single or multi-mode lock-in condition, and the standing-wave or travelling-wave feature. We recommend that the axial response should be accounted for in VIV analysis and prediction model.

VORTEX INDUCED VIBRATION OF CIRCULAR CYLINDER WITH TWO DEGREES OF FREEDOM: COMPUTATIONAL FLUID DYNAMICS VS. REDUCED-ORDER MODELS

Computational fluid dynamics (CFD) studies capturing vortex-induced vibration (VIV) phenomena in a wide range of both the hydrodynamics and the structural parameters are important, because the analysis outcomes can be applied to numerical prediction codes, complement experimental measurement results and suggest a modification of some practical design guidelines. Nevertheless, in spite of many published studies on VIV, CFD studies for two dimensional coupled cross-flow/in-line VIV even with two degrees of freedom (2-DoF), are still quite limited. More CFD studies which can control the equivalence of system fluid-structure parameters in different directions with reduced uncertainty are needed to improve the numerical model empirical coefficients and capability to effectively match numerical predictions and experimental outcomes.This paper presents a CFD study on the 2-DoF VIV of elastically mounted circular cylinder with a low mass ratio (m* = 2.55). The Reynolds number is fixed to be 150 and the reduced flow velocity parameter is varied by changing the cross-flow natural frequency. To model the problem, two-dimensional Navier-Stokes equations coupled with linear structural equations in the in-line and cross-flow directions are solved. Particular attention is paid to the determination of maximum attainable amplitudes and the associated instantaneous lift and drag forces and hydrodynamic coefficients. These results are compared with the obtained results from alternative numerical prediction outcomes using new reduced-order models with four nonlinearly coupled wake-structure oscillators (Srinil and Zanganeh, 2012). Some qualitative and quantitative aspects are discussed. Overall, the important VIV characteristics are captured including the dual-resonance and figure-of-eight trajectories. Through the flow visualization study, it is found that as the dual-resonance is excited, a P+S wake pattern appears.Copyright

Modeling Pile Setup in Natural Clay Deposit Considering Soil Anisotropy, Structure, and Creep Effects: Case Study

AbstractThis paper reports the results of a numerical investigation of the behavior of a natural soft clay deposit under the installation of a case study pile. The case study problem included installation of an instrumented close-ended displacement pile in a soft marine clay deposit, known as Bothkennar clay, in Scotland. The site has been used for a number of years as a geotechnical test bed site, and the clay has been comprehensively characterized with both in situ tests and laboratory experiments. The soft soil behavior, both after pile installation and subsequent consolidation, was reproduced by using an advanced critical-state-based constitutive model that accounts for the anisotropy of soil fabric and destructuration effects during plastic straining. Furthermore, a time-dependent extension of the model was used to study soil creep and the significance of its consideration in the overall pile-installation effects. The simulation results were compared against field measurements; furthermore, for compa...

Two-Dimensional Coupled Vortex-Induced Vibration of Circular Cylinder: Prediction and Extraction of Hydrodynamics Properties

This paper discusses two-dimensional coupled vortex-induced vibration of circular cylinder.

EXPERIMENTAL INVESTIGATION OF TWO-DEGREE-OF-FREEDOM VIV OF CIRCULAR CYLINDER WITH LOW EQUIVALENT MASS AND VARIABLE NATURAL FREQUENCY RATIO

This contribution looks at an experimental investigation of two-degree-freedom VIV of circular cylinder with low equivalent mass and variable natural frequency ratio