Vortex-induced vibration of cylindrical structures

Vortex-induced vibration (VIV) of cylindrical structures is a classical topic within fluid-structure interaction (FSI). In offshore engineering, it often causes the fatigue of slender structures, such as risers, mooring lines and pipelines. Detailed understanding of this FSI phenomenon and an efficient prediction of such self-excited and self-sustained oscillations are required for the reliable estimation of the fatigue damage and the development of VIV suppression techniques.Over the past few decades, VIV has been extensively studied and the majority of the existing publications in the literature are experiments or semi-empirical modelling. In contrast, FSI simulations by combining high-fidelity computational fluid dynamics (CFD) and computational structural dynamics (CSD) solvers have received less attention. The main objective of this thesis is to investigate VIV of elastically mounted rigid cylinders and flexible cylinders using fully three-dimensional (3D) FSI simulations. Apart from important VIV aspects, such as response amplitude, response frequency and fatigue damage etc., the present research is also focussed on the aspects which have not been fully addressed by previous studies such as correlation lengths and time-dependent 3D flow structures.Two-degree-of-freedom (2DOF) VIV of an elastically mounted circular cylinder with varying in-line (IL) to cross-flow (CF) natural frequency ratios (f* = fnx/fny) is first studied using a 3D CFD approach. Numerical simulation is carried out for a constant mass ratio m* = 2 at a fixed Reynolds number Re = 500. The reduced velocity Vr ranges from 2 to 12. Three natural frequency ratios are considered, i.e., f* = 1, 1.5 and 2. The structural damping is set to zero to maximise the response of the cylinder. The main objective of the first study is to investigate the effect of f* on the 2DOF VIV responses and the 3D characteristics of the flow. It is discovered that there is a significant increase in the vibration amplitude and the peak amplitude shifts to a higher reduced velocity when f* increases from 1 to 2. A single-peak cross-flow response is observed for the identical in-line and cross-flow mass ratios when f* = 2. Dual resonance is found to exist over the range of f* studied.;The preferable trajectories of the cylinder in the lock-in range are counterclockwise figure-eight orbits, whereas clockwise orbits primarily occur in the initial branch. The number of clockwise orbits decreases as f* increases from 1 to 2. Oblique figure-eight trajectories appear at Vr = 6, 7 and 8 when f* = 1. The third harmonic component which is observed in the lift fluctuation increases with f*. The correlation decreases in the lock-in range and reaches its minimum value around the transition region between the lock-in and post-lock-in ranges. Three vortex shedding modes (2S, P + S and 2P) appear in the present simulation. A dominant P + S mode is associated with the oblique figure-eight trajectories. Variation of vortex shedding flows along the cylinder is observed leading to the poor correlation of the sectional lift forces.Then, a numerical investigation of VIV of a vertical riser subject to uniform and linearly sheared currents is presented. The model vertical riser tested at the MARINTEK by ExxonMobil is considered. The predicted numerical results are in good agreement with the experimental data. It is found that the dominant mode numbers, the maximum root mean square amplitudes, the dominant frequencies and the fatigue damage indices increase with the flow velocity. Dual resonance is found to occur at most of the locations along the riser. At some locations along the riser, a third harmonic frequency component is observed in the CF response and a frequency component at the CF response frequency is found in the IL response apart from the frequency component at twice the CF response frequency. The majority of the vortex shedding shows a clear 2S pattern, whereas a 2P mode is observed near the position where the maximum vibration amplitude appears. The higher IL fatigue damage in the second study emphasises the importance of the IL fatigue damage analysis especially in the design of low flow velocity or low mode number applications.The third study is on VIV of two tandem flexible cylinders at different spacing ratios (Sx/D) at a fixed Reynolds number Re = 500 using a two-way FSI method. The main objective is to investigate the effect of spacing on the hydrodynamic interactions and the VIV responses of these cylinders. It is found that the responses of the two tandem flexible cylinders are similar to the classical VIV responses when Sx/D is small.;Once Sx/D is large enough for the vortices to be completely detached from the upstream cylinder, the response of the upstream cylinder is similar to the typical VIV response whereas the downstream cylinder undergoes wake-induced vibration (WIV). The characteristics of the response of the downstream cylinder in the present study are similar to those of the first two response regimes. The third response regime is not observed for the flexible downstream cylinder with both ends fixed. The two changes in the phase relation between the cross-flow displacements of the two tandem flexible cylinders are discovered to be linked with the initial-upper branch transition and the upper-lower branch transition, respectively. The correlation lengths of the two tandem flexible cylinders decrease significantly in the transition range between the upper and lower branches. Three vortex shedding modes (2S, P + S and 2P) have been identified in the present study. It is found that the upper-branch 2P mode is associated with large-amplitude vibration of the upstream cylinder and the P + S mode is related to large-amplitude vibration of the downstream cylinder for Sx/D = 3.5 and 5. On the other hand, the lower-branch 2P mode leads to small-amplitude vibration of the downstream cylinder in the post-lock-in range at Sx/D = 2.5. The relative phase shifts of the sectional lift coefficients on different spanwise cross sections can be attributed to the variation of the vortex shedding flow along the flexible cylinders and these phase shifts result in poor phasing between the forces and the displacements which is related to the decrease of the correlation lengths.Vortex-induced vibration (VIV) of cylindrical structures is a classical topic within fluid-structure interaction (FSI). In offshore engineering, it often causes the fatigue of slender structures, such as risers, mooring lines and pipelines. Detailed understanding of this FSI phenomenon and an efficient prediction of such self-excited and self-sustained oscillations are required for the reliable estimation of the fatigue damage and the development of VIV suppression techniques.Over the past few decades, VIV has been extensively studied and the majority of the existing publications in the literature are experiments or semi-empirical modelling. In contrast, FSI simulations by combining high-fidelity computational fluid dynamics (CFD) and computational structural dynamics (CSD) solvers have received less attention. The main objective of this thesis is to investigate VIV of elastically mounted rigid cylinders and flexible cylinders using fully three-dimensional (3D) FSI simulations. Apart from important VIV aspects, such as response amplitude, response frequency and fatigue damage etc., the present research is also focussed on the aspects which have not been fully addressed by previous studies such as correlation lengths and time-dependent 3D flow structures.Two-degree-of-freedom (2DOF) VIV of an elastically mounted circular cylinder with varying in-line (IL) to cross-flow (CF) natural frequency ratios (f* = fnx/fny) is first studied using a 3D CFD approach. Numerical simulation is carried out for a constant mass ratio m* = 2 at a fixed Reynolds number Re = 500. The reduced velocity Vr ranges from 2 to 12. Three natural frequency ratios are considered, i.e., f* = 1, 1.5 and 2. The structural damping is set to zero to maximise the response of the cylinder. The main objective of the first study is to investigate the effect of f* on the 2DOF VIV responses and the 3D characteristics of the flow. It is discovered that there is a significant increase in the vibration amplitude and the peak amplitude shifts to a higher reduced velocity when f* increases from 1 to 2. A single-peak cross-flow response is observed for the identical in-line and cross-flow mass ratios when f* = 2. Dual resonance is found to exist over the range of f* studied.;The preferable trajectories of the cylinder in the lock-in range are counterclockwise figure-eight orbits, whereas clockwise orbits primarily occur in the initial branch. The number of clockwise orbits decreases as f* increases from 1 to 2. Oblique figure-eight trajectories appear at Vr = 6, 7 and 8 when f* = 1. The third harmonic component which is observed in the lift fluctuation increases with f*. The correlation decreases in the lock-in range and reaches its minimum value around the transition region between the lock-in and post-lock-in ranges. Three vortex shedding modes (2S, P + S and 2P) appear in the present simulation. A dominant P + S mode is associated with the oblique figure-eight trajectories. Variation of vortex shedding flows along the cylinder is observed leading to the poor correlation of the sectional lift forces.Then, a numerical investigation of VIV of a vertical riser subject to uniform and linearly sheared currents is presented. The model vertical riser tested at the MARINTEK by ExxonMobil is considered. The predicted numerical results are in good agreement with the experimental data. It is found that the dominant mode numbers, the maximum root mean square amplitudes, the dominant frequencies and the fatigue damage indices increase with the flow velocity. Dual resonance is found to occur at most of the locations along the riser. At some locations along the riser, a third harmonic frequency component is observed in the CF response and a frequency component at the CF response frequency is found in the IL response apart from the frequency component at twice the CF response frequency. The majority of the vortex shedding shows a clear 2S pattern, whereas a 2P mode is observed near the position where the maximum vibration amplitude appears. The higher IL fatigue damage in the second study emphasises the importance of the IL fatigue damage analysis especially in the design of low flow velocity or low mode number applications.The third study is on VIV of two tandem flexible cylinders at different spacing ratios (Sx/D) at a fixed Reynolds number Re = 500 using a two-way FSI method. The main objective is to investigate the effect of spacing on the hydrodynamic interactions and the VIV responses of these cylinders. It is found that the responses of the two tandem flexible cylinders are similar to the classical VIV responses when Sx/D is small.;Once Sx/D is large enough for the vortices to be completely detached from the upstream cylinder, the response of the upstream cylinder is similar to the typical VIV response whereas the downstream cylinder undergoes wake-induced vibration (WIV). The characteristics of the response of the downstream cylinder in the present study are similar to those of the first two response regimes. The third response regime is not observed for the flexible downstream cylinder with both ends fixed. The two changes in the phase relation between the cross-flow displacements of the two tandem flexible cylinders are discovered to be linked with the initial-upper branch transition and the upper-lower branch transition, respectively. The correlation lengths of the two tandem flexible cylinders decrease significantly in the transition range between the upper and lower branches. Three vortex shedding modes (2S, P + S and 2P) have been identified in the present study. It is found that the upper-branch 2P mode is associated with large-amplitude vibration of the upstream cylinder and the P + S mode is related to large-amplitude vibration of the downstream cylinder for Sx/D = 3.5 and 5. On the other hand, the lower-branch 2P mode leads to small-amplitude vibration of the downstream cylinder in the post-lock-in range at Sx/D = 2.5. The relative phase shifts of the sectional lift coefficients on different spanwise cross sections can be attributed to the variation of the vortex shedding flow along the flexible cylinders and these phase shifts result in poor phasing between the forces and the displacements which is related to the decrease of the correlation lengths

Cross-cultural Adaptation of Mandarin-Speaking Undergraduate Students in the United States

ABSTRACT Studying in the United States is many Chinese students\u27 dream, but those who manage to achieve it encounter two main problems. The first one is the language barrier and the second one cultural shock. Chine is the highest provider of international students with 30% from statistics of 2017/2018 class. The primary language spoken by Chinese is the Mandarin language. All the other languages come in as a second language to most Chinese, including English. When they travel out of their country to further education as students, they are hampered by language barriers as they navigate through their struggle. China is also a country of rich ancient culture still upheld to date compared to other places in the world where cultures have changed or merged like in the U.S. When Chinese students come to America, they meet diverse cultures that are very different from those accustomed to. This cultural difference poses great challenges for the Mandarin speakers, which we describe as cultural shock. This project sets out to outline ways that will help international students adapt quickly to the new environment. The research involved interviewing some Chinese students to understand their cross-cultural experiences and compile information that will help develop a handbook that will guide these students in their stay in America. The research found out that these two problems affect Chinese students on a large scale, interfering with their stay, social life, and education. The two issues have lowered their interactions and self-confidence due to their inability to speak fluently and their reluctance to fit in a new cultural environment. The project recommended several guidelines to help these international students from China adapt to life in the U.S. These guidelines include psychological preparation before they travel and provide the necessary information to quicken their understanding of new ways of life

Numerical simulation of vortex-induced vibration of a vertical riser in uniform and linearly sheared currents

This paper presents a numerical study on vortex-induced vibration (VIV) of a vertical riser subject to uniform and linearly sheared currents. The model vertical riser tested at the MARINTEK by ExxonMobil is considered. The predicted numerical results are in good agreement with the experimental data. It is found that the dominant mode numbers, the maximum root mean square amplitudes, the dominant frequencies and the fatigue damage indices increase with the flow velocity. A standing wave response is observed for the single-mode in-line (IL) and cross-flow (CF) vibrations. Dual resonance is found to occur at most of the locations along the riser. At some locations along the riser, a third harmonic frequency component is observed in the CF response and a frequency component at the CF response frequency is found in the IL response apart from the frequency component at twice the CF response frequency. The majority of the vortex shedding shows a clear 2S pattern, whereas a 2P mode is observed near the position where the maximum vibration amplitude appears. The higher IL fatigue damage in the present study emphasises the importance of the IL fatigue damage especially in the design of low flow velocity or low mode number applications

CFD simulation of vortex-induced vibration of a vertical riser

Three-dimensional fluid-structure interaction (FSI) simulations are conducted on a vertical riser with a length-to-diameter ratio L/D = 481.5. Important vortex-induced vibration (VIV) parameters including the amplitude responses, orbital trajectories, oscillation frequencies and vorticity contours are presented. The computational fluid dynamics (CFD) simulation results are in good agreement with published experimental data. The riser exhibits a dual-resonant response. Two different vortex shedding mode is observed, i.e., 2P and 2S modes. 2P mode is associated with the maximum transverse amplitude and 2S mode is observed elsewhere along the riser

Three-dimensional numerical simulation of two-degree-of-freedom VIV of a circular cylinder with varying natural frequency ratios at Re = 500

The two-degree-of-freedom (2DOF) vortex-induced vibration (VIV) of a circular cylinder with varying in-line to cross-flow natural frequency ratios (f* = fnx/fny) is studied using a three-dimensional (3D) computational fluid dynamics (CFD) approach. Numerical simulation is carried out for a constant mass ratio of 2 at a fixed Reynolds number Re = 500. The reduced velocity ranges from 2 to 12. Three natural frequency ratios are considered, i.e., f* = 1, 1.5 and 2. The structural damping is set to zero to maximise the response of the cylinder. The main objective of this study is to investigate the effect of f* on the 2DOF VIV responses and the 3D characteristics of the flow. It is discovered that there is a significant increase in the vibration amplitude, and the peak amplitude shifts to a higher reduced velocity when f* increases from 1 to 2. A single-peak cross-flow response is observed for the identical in-line and cross-flow mass ratios when f* = 2. Dual resonance is found to exist over the range of f* studied. The preferable trajectories of the cylinder in the lock-in range are counterclockwise figure-eight orbits. Oblique figure-eight trajectories appear at Vr = 6, 7 and 8 when f* = 1. The third harmonic component which is observed in the lift fluctuation increases with f*. The correlation decreases in the lock-in range and reaches its minimum value around the transition region between the lock-in and post-lock-in ranges. Three vortex shedding modes (2S, P + S and 2P) appear in the present simulation. A dominant P + S mode is associated with the oblique figure-eight trajectories. Variation of vortex shedding flows along the cylinder is observed leading to the poor correlation of the sectional lift forces

The effect of base column on vortex-induced vibration of a circular cylinder with low aspect ratio

The end condition of a cylinder is known to influence its vortex-induced vibration (VIV) response. The design of a short circular cylinder with a concentric base column attached to its bottom is adopted by some floating structures for improved stability, but how the base column would affect the VIV of the cylinder has been rarely studied. In this paper, the VIV of an elastically mounted rigid cylinder with a base column and a low aspect ratio of 2 is investigated numerically by solving the Navier-Stokes equations. The numerical methods are validated against two existing VIV studies, including a 2D cylinder and a cylinder with a finite length. The impacts of the base column on the cylinder are analysed. It is found that although the free end effects associated with fluid flowing around the cylinder end are still present, attaching the base column leads to the expansion of the lock-in regime of the cylinder response. A relationship between non-dimensional response amplitude and lift coefficient is established, which takes into consideration the geometrical properties of the base column. By analysing the energy transfer from fluid to structure, the base column is also found to have significant damping effects on the cylinder response

The dynamic response of floating offshore wind turbine platform in wave-current condition

In this paper, the fluid–structure interaction of floating offshore wind turbine (FOWT) platforms under complex ocean conditions is investigated using OpenFOAM and in-house developed models. Two types of FOWT platform, i.e., a semi-submersible platform and a barge platform, are studied for their dynamic responses to either wave or current. The results reveal that a semi-submersible platform exhibits larger cross-flow motion and lock-in phenomenon, while a barge platform experiences smaller motion with no significant lock-in within the velocity range examined. The combined wave–current conditions are further studied for the semi-submersible platform, with different angles between wave and current, the current speeds, and wave parameters. Unlike other investigations focusing on colinear wave–current interaction, in which the waves usually mitigate vortex-induced motion (VIM); here, we find that waves might lead to an enhanced VIM with a large angle between current and wave. The evaluation on the interaction effect factor shows that the largest wave height in the lock-in region does not lead to the most dangerous scenario, herein, the largest platform motion. Instead, a smaller wave height with a large wave period can induce even larger motion

The effect of cubic stiffness nonlinearity on the vortex-induced vibration of a circular cylinder at low Reynolds numbers

The vortex-induced vibration (VIV) of a circular cylinder elastically supported by linear and cubic springs is investigated numerically at low Reynolds numbers. The cylinder has a low mass ratio and zero structural damping. Nine dimensionless cubic stiffness nonlinearity strength values are considered. It is found that within the parameter space examined, the VIV response for the linear and softening springs can be divided into four regimes, namely the initial, upper, lower and desynchronised regimes. When the softening spring nonlinearity gets stronger, there exist a reduction in the peak amplitude and shifts in the initial-upper branch and upper-lower branch transitions to lower Reynolds number ranges. In contrast, as the hardening spring nonlinearity increases, the response envelope moves to a higher Reynolds number range and the profile of the initial and upper branches becomes smoother with the lower branch gradually disappearing. In the hardening spring case, the beating response is observed near the low end of the initial branch up to the high end of the initial branch. The modulations in the vibration amplitude gradually diminish with increasing Reynolds number. The cubic spring results coincide with those of the linear spring when they are presented with the equivalent reduced velocity. Due to the low Reynolds number range considered, the majority of the vortex shedding is in the 2S mode. The wake in the cases with smaller vibration amplitudes exhibits a single-row configuration. Whereas, a double-row vortex street is mainly observed in the upper branch. Disorders in the wake are found to be associated with beating responses which have larger vibration amplitudes