Vortex induced vibrations of long flexible cylinders with and without wake interference

L'objectiu d'aquesta tesi ha estat la caracterització de la resposta de les bandes en els números de manera d'alta. Tres experiments separats van ser executats per crear un conjunt de dades de resposta model per a matiners relació de massa realista baixes, i la determinació de les característiques deixant de bandes flexibles sotmesos a vibracions multimode. Es van utilitzar un conjunt de models verticals. Dues bandes llargues 3m van ser fabricats per a aquesta finalitat. Les bandes es classifiquen d'acord a la relació de massa, un model tenia una relació de massa de 2,7, i l'altre tenia relació de massa menor de 1,1. Els models van ser provats en un flux uniforme, amb una velocitat de fins a 2 m / s. Respostes tant en línia i de flux creuat es van mesurar utilitzant calibradors de tensió, i les dades es converteixen en desplaçaments. L'alta relació de pujada massiva indica oscil·lacions fins a quart manera. La resposta de baixa proporció de pujada massiva es va limitar a segon mode de flux creuat. Les contribucions modals es van determinar i es va observar la presència simultània de múltiples maneres en múltiples freqüències. Coeficients de resistència obtinguts van indicar similitud amb l'oscil·lació de flux transversal, amb valors d'arribar a 2,5 per al cilindre de relació de massa baixa. La resposta d'un tub vertical flexible en el deixant d'un cilindre rígid es va determinar utilitzant diferent distància de la cilindros.El objetivo de esta tesis ha sido la caracterización de la respuesta de las bandas en los números de modo de alta. Tres experimentos separados fueron ejecutados para crear un conjunto de datos de respuesta modelo para madrugadores relación de masa realista bajas, y la determinación de las características estela de bandas flexibles sometidos a vibraciones multimodo. Se utilizaron un conjunto de modelos verticales. Dos bandas largas 3m fueron fabricados. Las bandas se clasifican de acuerdo a la relación de masa, un modelo tenía una relación de masa de 2,7, y el otro tenía relación de masa menor de 1,1. Los modelos fueron probados en un flujo uniforme, con una velocidad de hasta 2 m / s. Respuestas tanto en línea y de flujo cruzado se midieron usando calibradores de tensión, y los datos se convierten en desplazamientos. La alta relación de subida masiva indica oscilaciones hasta cuarto modo. La respuesta de baja proporción de subida masiva se limitó a segundo modo de flujo cruzado. Las contribuciones modales se determinaron y se observó la presencia simultánea de múltiples modos en múltiples frecuencias. Coeficientes de resistencia obtenidos indicaron similitud con la oscilación de flujo transversal, con valores de llegar a 2,5 para el cilindro de relación de masa baja. La respuesta de un tubo vertical flexible en la estela de un cilindro rígido se determinó usando diferente distancias.The objective of this thesis was the characterization of the response of risers at high mode numbers. Three separate experiments were executed to create a set of model response data for realistically low mass ratio risers, and the determination of wake characteristics of flexible risers undergoing multi-mode vibrations. A set of riser models were used. Two 3m long risers were fabricated for this purpose. The risers were categorized according to the mass ratio, one model had a mass ratio of 2.7, and the other had lower mass ratio of 1.1. The models were tested in a uniform flow, with speeds up to 2m/s. Both in-line and cross-flow responses were measured using strain gages, and data was converted to displacements. The high mass ratio riser indicated oscillations up to 4th mode. The low mass ratio riser response was limited to 2nd mode in cross-flow. The modal contributions were determined and simultaneous presence of multiple modes at multiple frequencies were observed. Drag coefficients obtained indicated similarity with the cross-flow oscillation, with values reaching 2.5 for the low mass ratio cylinder. The response of a flexible riser in the wake of a rigid cylinder was determined using different separation distance from the leading cylinder. Significant similarities to those of an isolated flexible cylinder were observed, in form of response regimes at each structural mode of oscillation, with contribution from adjacent modes

High-order methods with LES model for incompressible flows

Peer ReviewedPostprint (published version

A numerical study of fin and jet propulsions involving fluid-structure interactions

Fish swimming is elegant and efficient, which inspires humans to learn from them to design high-performance artificial underwater vehicles. Research on aquatic locomotion has made extensive progress towards a better understanding of how aquatic animals control their flexible body and fin for propulsion. Although the structural flexibility and deformation of the body and fin are believed to be important features to achieve optimal swimming performance, studies on high-fidelity deformable body and fin with complex material behavior, such as non-uniform stiffness distributions, are rare. In this thesis, a fully coupled three-dimensional high-fidelity fluid-structure interaction (FSI) solver is developed to investigate the flow field evolution and propulsion performance of caudal fin and jet propulsion involving body and/or fin deformation. Within this FSI solver, the fluid is resolved by solving unsteady and viscous Navier-Stokes equations based on the finite volume method with a multi-block grid system. The solid dynamics are solved by a nonlinear finite element method. The coupling between the two solvers is achieved in a partitioned approach in which convergence check and sub-iteration are implemented to ensure numerical stability and accuracy. Validations are conducted by comparing the simulation results of classical benchmarks with previous data in the literature, and good agreements between them are obtained. The developed FSI solver is then applied to study the bio-inspired fin and jet propulsion involving body deformation. Specifically, the effect of non-uniform stiffness distributions of fish body and/or fin, key features of fish swimming which have been excluded in most previous studies, on the propulsive performance is first investigated. Simulation results of a sunfish-like caudal fin model and a tuna-inspired swimmer model both show that larger thrust and propulsion efficiency can be achieved by a non-uniform stiffness distribution (e.g., increased by 11.2% and 9.9%, respectively, for the sunfish-like model) compared with a uniform stiffness profile. Despite the improved propulsive e performance, a bionic variable fish body stiffness does not yield fish-like midline kinematics observed in real fish, suggesting that fish movement involves significant active control that cannot be replicated purely by passive deformations. Subsequent studies focus on the jet propulsion inspired by squid locomotion using the developed numerical solver. Simulation results of a two-dimensional inflation-deflation jet propulsion system, whose inflation is actuated by an added external force that mimics the muscle constriction of the mantle and deflation is caused by the release of elastic energy of the structure, suggest larger mean thrust production and higher efficiency in high Reynolds number scenarios compared with the cases in laminar flow. A unique symmetry-breaking instability in turbulent flow is found to stem from irregular internal body vortices, which cause symmetry breaking in the wake. Besides, a three-dimensional squid-like jet propulsion system in the presence of background flow is studied by prescribing the body deformation and jet velocity profiles. The effect of the background flow on the leading vortex ring formation and jet propulsion is investigated, and the thrust sources of the overall pulsed jet are revealed as well. Finally, FSI analysis on motion control of a self-propelled flexible swimmer in front of a cylinder utilizing proportional-derivative (PD) control is conducted. The amplitude of the actuation force, which is applied to the swimmer to bend it to produce thrust, is dynamically tuned by a feedback PD controller to instruct the swimmer to swim the desired distance from an initial position to a target location and then hold the station there. Despite the same swimming distance, a swimmer whose departure location is closer to the cylinder requires less energy consumption to reach the target and hold the position there.Fish swimming is elegant and efficient, which inspires humans to learn from them to design high-performance artificial underwater vehicles. Research on aquatic locomotion has made extensive progress towards a better understanding of how aquatic animals control their flexible body and fin for propulsion. Although the structural flexibility and deformation of the body and fin are believed to be important features to achieve optimal swimming performance, studies on high-fidelity deformable body and fin with complex material behavior, such as non-uniform stiffness distributions, are rare. In this thesis, a fully coupled three-dimensional high-fidelity fluid-structure interaction (FSI) solver is developed to investigate the flow field evolution and propulsion performance of caudal fin and jet propulsion involving body and/or fin deformation. Within this FSI solver, the fluid is resolved by solving unsteady and viscous Navier-Stokes equations based on the finite volume method with a multi-block grid system. The solid dynamics are solved by a nonlinear finite element method. The coupling between the two solvers is achieved in a partitioned approach in which convergence check and sub-iteration are implemented to ensure numerical stability and accuracy. Validations are conducted by comparing the simulation results of classical benchmarks with previous data in the literature, and good agreements between them are obtained. The developed FSI solver is then applied to study the bio-inspired fin and jet propulsion involving body deformation. Specifically, the effect of non-uniform stiffness distributions of fish body and/or fin, key features of fish swimming which have been excluded in most previous studies, on the propulsive performance is first investigated. Simulation results of a sunfish-like caudal fin model and a tuna-inspired swimmer model both show that larger thrust and propulsion efficiency can be achieved by a non-uniform stiffness distribution (e.g., increased by 11.2% and 9.9%, respectively, for the sunfish-like model) compared with a uniform stiffness profile. Despite the improved propulsive e performance, a bionic variable fish body stiffness does not yield fish-like midline kinematics observed in real fish, suggesting that fish movement involves significant active control that cannot be replicated purely by passive deformations. Subsequent studies focus on the jet propulsion inspired by squid locomotion using the developed numerical solver. Simulation results of a two-dimensional inflation-deflation jet propulsion system, whose inflation is actuated by an added external force that mimics the muscle constriction of the mantle and deflation is caused by the release of elastic energy of the structure, suggest larger mean thrust production and higher efficiency in high Reynolds number scenarios compared with the cases in laminar flow. A unique symmetry-breaking instability in turbulent flow is found to stem from irregular internal body vortices, which cause symmetry breaking in the wake. Besides, a three-dimensional squid-like jet propulsion system in the presence of background flow is studied by prescribing the body deformation and jet velocity profiles. The effect of the background flow on the leading vortex ring formation and jet propulsion is investigated, and the thrust sources of the overall pulsed jet are revealed as well. Finally, FSI analysis on motion control of a self-propelled flexible swimmer in front of a cylinder utilizing proportional-derivative (PD) control is conducted. The amplitude of the actuation force, which is applied to the swimmer to bend it to produce thrust, is dynamically tuned by a feedback PD controller to instruct the swimmer to swim the desired distance from an initial position to a target location and then hold the station there. Despite the same swimming distance, a swimmer whose departure location is closer to the cylinder requires less energy consumption to reach the target and hold the position there

UNSTEADY AERODYNAMICS OF FLAPPING WING: GROUND EFFECT AND ROTATIONAL LIFT

Ph.DDOCTOR OF PHILOSOPH

Fluid-Structure Interaction of Flexible Whisker-Type Beams and Its Implication for Flow Sensing by Pair-Wise Correlation

(1) Background: Sensing of critical events or flow signatures in nature often presents itself as a coupled interaction between a fluid and arrays of slender flexible beams, such a wind-hairs or whiskers. It is hypothesized that important information is gained in highly noisy environments by the inter-correlation within the array. (2) Methods: The present study uses a model sea lion head with artificial whiskers in the form of slender beams (optical fibres), which are subjected to a mean flow with overlaid turbulent structures generated in the wake of a cylinder. Motion tracking of the array of fibres is used to analyse the correlation of the bending deformations of pairs of fibres. (3) Results: Cross-correlation of the bending signal from tandem pairs of whiskers proves that the detection of vortices and their passage along the animals head is possible even in noisy environments. The underlying pattern, during passage of a vortex core, is a jerk-like response of the whiskers, which can be found at later arrival-times in similar form in the downstream whisker's response. (4) Conclusion: Coherent vortical structures can be detected from cross-correlation of pairs of cantilever-beam like sensors even in highly turbulent flows. Such vortices carry important information within the environment, e.g. the underlying convection velocity. More importantly in nature, these vortices are characteristic elementary signals left by prey and predators. The present work can help to further develop flow, or critical event, sensory systems which can overcome high noise levels due to the proposed correlation principle

Vortex-induced vibrations of circular cylinders in steady and oscillatory flow

Vortex-induced vibration (VIV) of cylindrical structures in fluid flow is of interest to many fields of engineering. For example, it influences the dynamics of offshore riser tubes bringing oil from the seabed to the surface and is a key issue in deep water riser design. Water depths up to 3000 m are found in typical oil extraction areas. Understanding VIV is also important in many other offshore engineering applications, such as mooring lines of floating offshore wind turbines, undersea pipelines and flexible slender pipes. These slender structures can experience VIV when exposed to marine current, because the dynamic vortex shedding flow leads to oscillatory forces. This study focuses on the Vortex-induced vibration (VIV) of circular cylinders in steady and oscillatory flows. As the first step, a two-dimensional (2D) numerical model is used to study VIV of a single circular cylinder in combined steady and oscillatory flow. The numerical model is based on the Reynolds-Averaged Navier-Stokes equations. Special focus is to investigate the effects of flow ratio (the percentage of the steady current velocity in the total fluid velocity) on the response of the cylinder. The simulations are carried out for a constant Keulegan–Carpenter (KC) number. The second step is to investigate the vortex-induced vibration (VIV) of multiple circular cylinders elastically connected together in a side-by-side arrangement subject to steady flow at a low Reynolds number of 150 and a low mass ratio of 2. Simulations are conducted for two-, five- and ten-cylinder systems over a wide range of reduced velocity that covers the whole lock-in regime for each of the cases. The differences between the responses of a multiple-cylinder system and a single cylinder are discussed. The shedding of vortices and flow interference between multiple circular cylinders in side-by-side arrangement in steady flow are examined. Then, a numerical study of vortex-induced vibration of four rigidly connected and four separately mounted circular cylinders in an inline square configuration at a Reynolds number of 150, a low mass ratio of 2 and a range of spacing ratio L from 1.5 to 4 is carried out. For a rigidly connected four-cylinder array, the maximum and minimum response amplitudes occur at L=1.5 and L=2.0, respectively, for the range of spacing ratio covered in this study and the maximum respond amplitude at L=1.5 is accompanied by a wider lock-in range than a single isolated cylinder case. For spacing ratios L ≥ 2.5, the lock-in regime of four rigidly connected cylinders is similar to that of a single cylinder and the response amplitudes in the lock-in regime are slightly higher than that of a single cylinder. The biased vortex street leads to a shift of the mean position of the cylinder array with the largest mean position shift being observed at L=3. Four response modes are identified for four separately mounted cylinders. These are the in-phase mode, the anti-phase mode, the correlated out-of-phase mode and the uncorrelated mode. The response mode for a cylinder in the four cylinder system is dependent not only on the spacing ratio, but also on the initial condition of the flow. The response amplitude under the in-phase mode is generally higher than that under the anti-phase mode at identical spacing ratios. This is attributed to the interaction of vortices in the wake of the cylinders. Two-dimensional studies have been popularly used to investigate the fundamental mechanisms of VIV due to its efficiency. Because the flow in the wake of a circular cylinder is in a three-dimensional fashion when the Reynolds number is in the turbulent regime, even when the cylinder is rigid and the free-stream flow is uniform, three-dimensional simulation of VIV is necessary and has been carried out in this study. Vortex-induced vibration (VIV) of tapered and uniform cylinders is investigated numerically at a constant Reynolds number of 500 using three-dimensional numerical simulations. The objectives of the study are to identify the difference between the responses of tapered and uniform cylinders. Simulations are conducted using parameters as close to the experimental condition as possible. Two cylinders are considered: one with a length to diameter ratio of 4.3 and a mass ratio of 2.27 and another one with a length to diameter ratio of 12.3 and a mass ratio of 6.1. While the simulation of the shorter cylinder is mainly for validating the numerical model, detailed analysis of the vibration amplitude and frequency, the vortex shedding flow mode and the lift coefficient are performed for the longer cylinder. For some reduced velocities, it is found that vortex shedding is in 2P mode at the small-diameter part of the cylinder and 2S mode at the larger-diameter part, forming a hybrid flow mode

On Wake Interference in the Flow around Two Circular Cylinders: Direct Stability Analysis and Flow-Induced Vibrations

The flow around two identical circular cylinders, arranged in configurations where one of the cylinders is immersed in the wake of the other, is studied using numerical simulations. Two aspects of such flows were considered. The first is the stability of nominally two-dimensional time-periodic wakes to three-dimensional perturbations. We investigated flows around tandem and staggered arrangements with diverse centre-to-centre distances. Direct stability analysis and numerical simulations were employed, and the results are compared to those obtained for the flow around an isolated cylinder. The onsets of the three-dimensional instabilities were calculated and the unstable modes are fully described. In addition, we assess the nonlinear character of the bifurcations and physical mechanisms are proposed to explain the instabilities. The second aspect considered in this thesis is the flowinduced vibration experienced by a rigid cylinder when it is mounted on an elastic base and immersed in the wake of a fixed identical cylinder. Tandem arrangements with centre-to-centre distances varying from 1.5 to 8 cylinder diameters were tested. The flow was simulated using an Arbitrary Lagrangian-Eulerian approach that coupled the solution of the structure equations with that of the flow. Two- and three- dimensional simulations were performed to assess the mutual influence between the three-dimensional flow structures in the wake and the motion of the cylinder. The response of the downstream cylinder is compared to that of an elastically-mounted rigid isolated cylinder. Based on the simulation results we propose physical mechanisms to explain the origin of the excitation

Harnessing Hydrokinetic Energy from Vortex-Induced Vibration (VIV)

In this dissertation, the application of Vortex-Induced Vibration (VIV) and Wake-Induced Vibration (WIV) of a bluff body for harnessing the kinetic energy of a fluid flow is presented. The application of induced vibration due to vortices in harnessing hydrokinetic energy of the fluid is relatively immature and this research work, which is written as a compilation of journal articles, attempts to address major scientific and technological gaps in this field. The project spans both VIV and WIV, with a particular attention to the development of a better understanding of the wake behaviour in a tandem configuration and the effect of boundary layers for harnessing the kinetic energy of the flow. Accordingly, two separate coupled test cases of tandem bodies comprising Coupled Circular-Cylinder (CCC) and Coupled Cylinder-Airfoil (CCA) configurations were proposed and investigated. In the first series of tests on the CCC, two circular cylinders were employed to investigate the unsteady wake interactions on the energy yield. The upstream cylinder was fixed, while the downstream one was mounted on a virtual elastic base with one degree of freedom. The virtual elastic system consisted of a motor and a controller, a belt-pulley transmission and a carriage. In the CCC, the influence of the Reynolds number, gap between cylinders and boundary layers on the dynamic response of the downstream cylinder were numerically and experimentally investigated. In a numerical analyse of the system, a dynamic mesh technique within the ANSYS Fluent package was utilized to simulate the dynamic response of the cylinder. The experimental tests confirmed the numerical outcomes and demonstrated that in the WIV mechanisms, a positive kinetic energy transfer from fluid flow to the cylinder was achieved. It is also observed that the dynamic response of the cylinder under the WIV mechanism differs from the dynamic response of VIV. In addition, both numerical and experimental results indicated that a staggered arrangement with 3.5 ≤ x₀/D ≤ 4.5 and 1 ≤ y₀/D ≤ 2 (here, D is the diameter of the cylinder, and x₀ and y₀ are the horizontal and vertical offsets, respectively) is the optimum arrangement among all test cases to harness the energy of vortices, resulting in a power coefficient of 28%. This was achieved due to the favourable phase lag between the velocity of the cylinder and force imposed by the fluid. The results revealed that for the staggered arrangement of the cylinders, the WIV responses can occur at frequencies outside the range in which VIV is observed. In the second series of tests utilizing a CCA, the downstream circular cylinder was replaced by a symmetric airfoil with two degrees of freedom; heave and pitch. The heave degree of freedom employed the same virtual elastic base used for the CCC experiments. The pitch angle of the foil was actively controlled, as opposed to using passive mechanical impedance, since this enables full control over the foil behaviour, thereby facilitating the adjustment of the angle of attack accurately and rapidly. The results of CCA show that both longitudinal and lateral distances play an important role in the Strouhal number, power density and, consequently, the heave response of the airfoil. In addition, it was shown that the circulation of the vortices was influenced by the gap spacing between the cylinder and the airfoil. Furthermore, it was found that an optimum angle of attack of α = 10° is the most efficient for harnessing the energy of vortices with a maximum power coefficient of 30% for cases with 3.5 ≤ x₀/D ≤ 4.5 and 1 ≤ y₀/D ≤ 1.5 arrangements. Such a range is narrower laterally when compared with the optimum arrangement of the CCC. This work provided the foundation for further work to utilize the potential of this technology and further explore the opportunity to harness the vortical power in shallow water and ocean currentsThesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 201

Numerical Simulations of Vortex-Induced Vibrations in Marine Riser Pipes and Circular Cylinders

This thesis presents and discusses the results of two distinct investigations. The first is a Direct Numerical Simulation investigation of prescribed transverse oscillations of a two-dimensional circular cylinder in a fluid flow of Reynolds number 100. The second involves the numerical simulation of the Vortex-Induced Vibrations of long riser pipes in the sub-critical Reynolds number regime, using a strip theory code that employed a Large Eddy Simulation model. Before commencing the long riser investigation the code was thoroughly benchmarked against data from appropriate prescribed cross-stream oscillation experiments; the results of that benchmarking work are also presented in this thesis. The principal objectives of the low Reynolds number Direct Numerical Simulations were to use prescribed oscillations to explain phenomena that have been observed in free oscillation experiments, and also to investigate the different levels and types of synchronisation that exist between the cylinder and its wake in a given amplitude-frequency domain. It was found that the contour of zero hydrodynamic excitation closely matches the response envelopes reported from experimental and numerical investigations of the transverse Vortex-Induced Vibrations of lightly damped cylinders. Furthermore, the zero contour inferred that the maximum amplitude of free cross-stream vibration is 0.56 cylinder diameters in Reynolds number 100 flow, and the shape of the contour confirmed the existence of hystereses at low and high reduced velocities in free vibration. The present study also revealed two new coalesced shedding modes, here labelled C∗(2S) and C∗(P+S), that differ in their formation mechanism from the known C(2S) mode. In the benchmarking of the Large Eddy Simulation code at sub-critical Reynolds numbers a clear trend was observed in which the prediction of the flow physics was altered by changing the level of sub-grid scale turbulence dissipation in the code’s Smagorinsky turbulence dissipation model. It was found that by carefully tuning the level of turbulent dissipation the code could deliver very good predictions of the key physical quantities important in Vortex-Induced Vibrations; namely the component of the lift coefficient at the oscillation frequency and the phase angle by which this lift coefficient leads the cylinder displacement. Regarding the simulations of the Vortex-Induced Vibrations of a long model riser, it has been shown that responses in high modes of vibration at harmonics of the displacement-dominant response frequency (at 3 and 5 times the cross-stream displacement dominant frequency in the cross-stream direction and at 2 and 3 times the in-line displacement dominant frequency in the in-line direction) can be important with regard to the curvature variation along the riser, and can therefore contribute very significantly to the overall fatigue damage rate experienced by a riser undergoing VIV. Comparisons with experimental data in terms of maximum and mean displacements and modes and frequencies of vibration, were generally good for both uniform and linearly sheared flow profiles. Furthermore, it was observed that the majority of the responses involved travelling waves, even when the flow profile was uniform