FLOW-INDUCED VIBRATION OF A FLEXIBLE CIRCULAR CYLINDER

The offshore industry is currently experiencing challenges in designing flexible risers, cables etc., due to their susceptibility to FIV. Deeper understanding of the physics behind FIV is necessary in developing risers etc. This work presents two sets of experimental studies, collectively focusing on critical parameters that may greatly influence cylinder’s hydrodynamic response. A Tygon tube was towed from rest to steady speed before slowing down to rest again in still water. Axial pre-tension and mass ratio was varied for parametrically studying their effects on the cylinder’s hydrodynamic response, which was characterized mainly by vibration amplitudes and frequencies. The resulting effects of varying profile on flow-vibration amplitudes and frequencies have been quantified and expressed with respect to reduced velocity. A 2D numerical study has also been conducted to study the wake behind a circular cylinder, showing 4 types of vortex shedding modes

Modeling and Analysis of Stably Stratified Wall-Bounded Turbulent Flows

Stably stratified wall-bounded turbulent flows have been drawing a great deal of research interest. The profound driver is the underlying physics in the topic that concerns many problems in industry as well as the natural environment, e.g., stratified mixing efficiency in topographically complex boundary regions and closure parameterization in operational atmospheric models. In this dissertation, stratified turbulence in wall-bounded flows is studied and modeled via a series of numerical simulations. Under sufficiently strong stratification, fully developed wall-bounded turbulence could transition to intermittent flows in which laminar and turbulent patches coexist. Using direct numerical simulations (DNS), I explore the boundary at which such a transition occurs in the parameter space for stably stratified channel flow (SCF). A range of friction Reynolds (Reτ ) and Richardson (Riτ ) numbers, parameters that are observed to control the dynamics, are covered by the numerical simulations. For each Reτ investigated, the stratification level is varied incrementally from moderate to relatively strong, leading the fully turbulent flows to transition to intermittently turbulent. My results show that depending on Riτ and Riτ , SCF could exhibit intermittency in the near-wall and/or the channel core. At low-Reτ -high-Riτ , intermittency spans the entire channel depth, whereas at high-Reτ -low-Riτ , intermittency is confined in the channel core. Within the tested range, I identify the near-wall intermittency boundary by quantifying the volume fraction of turbulent patches in each simulation. The applicability of various dimensionless parameters for predicting the onset of near-wall intermittency is examined. My results suggest that near-wall intermittency in SCF occurs for Nusselt number, N u ≲ 3. A first-order closure model based on a K-profile type parameterization is developed for SCF. The model is shown to have good agreement with predicted mean profiles of velocity and potential temperature closely matching their DNS counterparts. The boundary at which near-wall intermittency occurs in SCF is delineated using the developed model based on the critical value N u = 3. The second topic of this dissertation concerns with the development of a computational framework for modeling stably stratified turbulent flows over flat boundaries. I examine the performance of a turbulence modeling framework consisting of residual-based variational multiscale method (RBVMS) and isogeometric analysis (IGA) applied to two canonical numerical experiments, namely stably stratified channel flows at Reτ = 180, 550, and a stable boundary layer (SBL). In the SCF cases, the framework is implemented with two augmentation companion features, namely weak imposition of Dirichlet boundary conditions (WD) and a new subgrid-scale (SGS) model. The performance of the modeling framework, as well as its interaction with the two companion features, are assessed in both weakly and strongly stratified regimes. In comparison to existing direct numerical simulation (DNS) data, my study reveals that RBVMS–IGA framework is able to faithfully capture the flow structures and one-point statistics in SCF simulation with relatively coarse grid resolution. The framework also demonstrates its capability of replicating intermittent flow dynamics under strong stratification. Such dynamics are reproduced robustly when the modeling framework is enhanced with WD and the new SGS model, features that are shown to generally improve numerical accuracy of simulations for the cases tested. My results confirm the computational efficiency as well as the robustness of RBVMS–IGA framework in modeling stratified wall-bounded flows. In addition, we develop a wall-function-based weak imposition of Dirichlet boundary condition (WFWD) for stably stratified flows. The performance of WFWD is validated with SCF at Reτ = 550 and in a stable atmospheric boundary layer, demonstrating its effectiveness and potential in mitigating the effects of under-resolved boundary layer on stratified wall-bounded flow modeling. Comparisons are made against results of the original formulation of WD, as well as direct or large-eddy simulations whenever available. My results show that WFWD with a smooth wall function offers improved accuracy over its WD counterpart in predicting one-point statistics of SCF at various degrees of stratification. Furthermore, on account of adopting a rough wall function WFWD successfully predicts the occurrence of super-geostrophic jet as well as statistics that are in good agreement with highly-resolved large-eddy simulations. My findings suggest that formulating the weak imposition of Dirichlet boundary condition based on wall functions could mitigate shortcomings of WD when factors like roughness play a significant role. The final portion of the dissertation focuses on modeling stably stratified wall-bounded turbulent flows over complex boundaries. The performance of the developed computational framework is validated against observations in a laboratory experiment on strongly stratified flow past a three-dimensional bell-shaped hill. Good agreement is observed for qualitative flow physics, with the predicted occurrences of flow separation, recirculation, and hydraulic jump closely matching those in the experiment. In addition, the dividing-streamline height and the wavelength of lee wave computed from the present framework compare well to theoretical predictions. I show that the present framework is able to tackle various degrees of stratification in wall-bounded flows. The effect of weak imposition of Dirichlet boundary condition on the performance of the framework is also examined. The dissertation is concluded with an outlook toward applying the present framework to modeling stratified flow past real-world terrains at microscale (∼10 m) by simulating stratified flow past a two-dimensional environmental terrain

Effect of mass ratio on hydrodynamic response of a flexible cylinder

The effect of the mass ratio on the flow-induced vibration (FIV) of a flexible circular cylinder is experimentally investigated in a towing tank. A Tygon tube with outer and inner diameters of 7.9 mm and 4.8 mm, respectively, was employed for the study. The tube was connected to a carriage and towed from rest to a steady speed up to 1.6 m/s before slowing down to rest again over a distance of 1.6 m in still water. Reynolds number based on the cylinder’s outer diameter was 800–13,000, and the reduced velocity (velocity normalized by the cylinder’s natural frequency and outer diameter) spanned from 2 to 25. When connected, the cylinder was elongated from 420 mm to 460 mm under an axial pre-tension of 11 N. Based on the cylinder’s elongated length, the aspect ratio (ratio of the cylinder’s length to outer diameter) was calculated as 58. Three mass ratios (ratio of the cylinder’s structural mass to displaced fluid mass, m*) of 0.7, 1.0, and 3.4 were determined by filling the cylinder’s interior with air, water, and alloy powder (nickel-chromium-boron matrix alloy), respectively. An optical method was adopted for response measurements. Multi-frequency vibrations were observed in both in-line (IL) and cross-flow (CF) responses; at high Reynolds number, vibration modes up to the 3rd one were identified in the CF response. The mode transition was found to occur at a lower reduced velocity for the highest tested mass ratio. The vibration amplitude and frequency were quantified and expressed with respect to the reduced velocity. A significant reduced vibration amplitude was found in the IL response with increasing mass ratios, and only initial and upper branches existed in the IL and CF response amplitudes. The normalized response frequencies were revealed to linearly increase with respect to the reduced velocity, and slopes for linear relations were found to be identical for the three cases tested

Mitigating flow-induced vibration of a flexible circular cylinder via pre-tension

This paper studies the effects of pre-tension on flow-induced vibration of a flexible cylinder with two degrees of freedom in a towing tank. A 0.45. m section of Tygon tubing with an outer and inner diameter of 7.9. mm and 4.8. mm respectively, was employed as a flexible test model, the mass ratio and aspect ratio of which is 0.77 and 57, respectively. It was towed from rest up to 0.8. m/s before slowing down to rest again over a distance of 1.6. m in still water with three cases of pre-tension from 0 to 8. N. The Reynolds number based on the cylinder\u27s outer diameter spanned from approximately 780-6300 while the reduced velocity ranged from 2 to 16. Emphasis was on revealing the possible relationship between pre-tension and response amplitude as well as the frequency. The vibration amplitude and frequency obtained during the brief constant towing speed were quantified and expressed in terms of reduced velocity. Narrowed lock-in bandwidth and reduced response amplitude were observed. The findings from this set of experiments are compared to the existing knowledge in the literature

Large-eddy simulation of a full-scale underwater energy storage accumulator

Underwater energy storage provides an alternative to conventional underground, tank, and floating storage. This study presents an underwater energy storage accumulator concept and investigates the hydrodynamic characteristics of a full-scale 1000 m3 accumulator under different flow conditions. Numerical simulations are carried out using an LES turbulence model. Time-averaged and transient flow structures and force characteristics are analyzed. The results show that the vortex structures are complex and scale-rich, but periodic vortex shedding can still be identified. The shedding frequency is consistent with the fluctuation of the lift force in the cross-flow direction. A dominant Strouhal number of 0.18 is found. The mean drag and lift coefficients stabilize at 0.45 and 0.60, respectively, and are insensitive to Reynolds number variation. Modal analysis shows that the natural frequency of the accumulator falls between 27 and 48 Hz and is much higher than the vortex shedding frequency. Thus, for the accumulator model investigated in this study, the risk of vortex-induced vibration (VIV) fatigue damage is very low

Underwater Compressed Gas Energy Storage (UWCGES): Current Status, Challenges, and Future Perspectives

Underwater compressed air energy storage was developed from its terrestrial counterpart. It has also evolved to underwater compressed natural gas and hydrogen energy storage in recent years. UWCGES is a promising energy storage technology for the marine environment and subsequently of recent significant interest attention. However, it is still immature. In this study, the latest progress in both academic and industrial fields is summarized. Additionally, challenges facing this emerging technology are analyzed. The pros and cons of UWCGES are provided and are differentiated from the terrestrial variant. Technical, economic, environmental, and policy challenges are examined. In particular, the critical issues for developing artificial large and ultra-large underwater gas storage accumulators and effective underwater gas transportation are comprehensively analyzed. Finally, the demand for marine energy storage technology is briefly summarized, and the potential application scenarios and application modes of underwater compressed gas energy storage technology are prospected. This study aims to highlight the current state of the UWCGES sector and provide some guidance and reference for theoretical research and industrial development

Underwater Compressed Gas Energy Storage (UWCGES): Current Status, Challenges, and Future Perspectives

Underwater compressed air energy storage was developed from its terrestrial counterpart. It has also evolved to underwater compressed natural gas and hydrogen energy storage in recent years. UWCGES is a promising energy storage technology for the marine environment and subsequently of recent significant interest attention. However, it is still immature. In this study, the latest progress in both academic and industrial fields is summarized. Additionally, challenges facing this emerging technology are analyzed. The pros and cons of UWCGES are provided and are differentiated from the terrestrial variant. Technical, economic, environmental, and policy challenges are examined. In particular, the critical issues for developing artificial large and ultra-large underwater gas storage accumulators and effective underwater gas transportation are comprehensively analyzed. Finally, the demand for marine energy storage technology is briefly summarized, and the potential application scenarios and application modes of underwater compressed gas energy storage technology are prospected. This study aims to highlight the current state of the UWCGES sector and provide some guidance and reference for theoretical research and industrial development

Effect of mass ratio on hydrodynamic response of a flexible cylinder

The effect of the mass ratio on the flow-induced vibration (FIV) of a flexible circular cylinder is experimentally investigated in a towing tank. A Tygon tube with outer and inner diameters of 7.9 mm and 4.8 mm, respectively, was employed for the study. The tube was connected to a carriage and towed from rest to a steady speed up to 1.6 m/s before slowing down to rest again over a distance of 1.6 m in still water. Reynolds number based on the cylinder’s outer diameter was 800–13,000, and the reduced velocity (velocity normalized by the cylinder’s natural frequency and outer diameter) spanned from 2 to 25. When connected, the cylinder was elongated from 420 mm to 460 mm under an axial pre-tension of 11 N. Based on the cylinder’s elongated length, the aspect ratio (ratio of the cylinder’s length to outer diameter) was calculated as 58. Three mass ratios (ratio of the cylinder’s structural mass to displaced fluid mass, m*) of 0.7, 1.0, and 3.4 were determined by filling the cylinder’s interior with air, water, and alloy powder (nickel-chromium-boron matrix alloy), respectively. An optical method was adopted for response measurements. Multi-frequency vibrations were observed in both in-line (IL) and cross-flow (CF) responses; at high Reynolds number, vibration modes up to the 3rd one were identified in the CF response. The mode transition was found to occur at a lower reduced velocity for the highest tested mass ratio. The vibration amplitude and frequency were quantified and expressed with respect to the reduced velocity. A significant reduced vibration amplitude was found in the IL response with increasing mass ratios, and only initial and upper branches existed in the IL and CF response amplitudes. The normalized response frequencies were revealed to linearly increase with respect to the reduced velocity, and slopes for linear relations were found to be identical for the three cases tested