Bluff bodies immersed in a fluid stream are susceptible to flow-induced vibrations. Depending on the body dynamic characteristics and flow conditions, different types of flow-induced vibrations may occur. The failure of a blade in a large mixing vessel in a chemical plant raised the question of the response of a parabolic cross-section bluff body to the flow excitation. Experiments were conducted in a wind tunnel using two- dimensional “sectional” models. Models with parabolic, semi-elliptic and semi-circular cross-section were investigated. In the dynamic experiments, flow velocity was increased from 0 to 22 m\s, and the oscillating amplitude and wake response were monitored. Vortex-induced vibrations were observed with Strouhal numbers for parabolic and semi-circular cross-sections of 0.13 and 0.12, respectively. Steady lift force and fluid moment for different angles of attack were monitored in the static experiments. From these results, lift and moment coefficients were calculated. For the closed semi-circular cross-section, Reynolds number had a strong influence on the lift coefficient. With an increase in Reynolds number, the lift coefficient decreased. The largest difference was noted at an angle of attack a = 45°. In contrast, the open semi-circular model lift coefficient was independent of Reynolds number. In the experiments where the elastic axis of the model coincided with the model centre of gravity, galloping was not observed in the plunge mode. When the model elastic axis was moved to a position 90 mm behind the test model centre of gravity, galloping was observed for the semi-elliptic and parabolic models. The onset of galloping coincided with the vortex-induced resonance. Changing the model elastic axis position introduced a combination of plunge and torsional motion, and latter is believed to be responsible for the existence of galloping. The parabolic model was modified in an attempt to eliminate galloping instability. Fins were added at the separation points to widen the wake and prevent the reattachment of the flow to the afterbody. With these changes, galloping was not observed, although the oscillation amplitudes remained unacceptably high. The present investigation revealed previously unknown characteristics of semi-elliptical and parabolic cross-section bluff body behaviour in fluid flow. At the same time, it laid a foundation for the solution to the practical problem encountered when a parabolic cross-section bluff body was used as a mixing blade. / Thesis / Master of Engineering (ME)

The wake of a bluff body is a representative issue in vortex dynamics that plays a central role in civil engineering, ocean engineering and thermal engineering. In this work, a flowing soap film was used to investigate the wakes of multiple stationary circular cylinders and of a single oscillating cylinder. Corresponding computer simulations were also conducted. Vortex formation of a stationary circular cylinder was analyzed by proper orthogonal decomposition (POD). The POD analysis was used to define an unsteady vortex formation length, which suggests a relationship between the vortex formation length of a single cylinder and the critical spacing of two cylinders in a tandem arrangement. A systematic parametric study of the wake structure was conducted for a controlled transversely oscillating cylinder. Neural network and support vector machine codes assisted the wake classification procedure and the identification of boundaries between different wake regimes. The phase map of the vortex shedding regimes for the (quasi) two-dimensional experiment qualitatively agrees with previous three-dimensional experiments. The critical spacings of two identical tandem circular cylinders in a flowing soap film system were determined using visual inspections of the wake patterns and calculations of the Strouhal frequencies. The dimensionless spacing was both increased and decreased quasi-statically. Hysteresis was observed in the flow patterns and Strouhal numbers. This study appears to provide the first experimental evidence of critical spacing values that agree with published computational results. The wake interaction between a stationary upstream circular disk and a free downstream circular disk was also investigated. With the ability to tie together the wake structure and the object motion, the relationship between energy generation and flow structure in the simplified reduced order model system was studied. The research results find the optimal efficiency of the energy harvesting system by a parametric study. / PHD / The wake of a bluff body is a classic issue in vortex dynamics that has been the subject of much research in civil engineering, ocean engineering and thermal engineering. Bluff bodies, especially circular cylinders, can be found extensively in heat exchangers, cooling systems and offshore structures. Flow-induced vibration of a bluff body due to the formation of a wake is an important problem in many fields of engineering. Flow-induced vibration determines the oscillation of flexible pipes that transfer oil from the seabed to the surface of the ocean, for example [71]. In civil engineering, flow-induced vibration affects the design of bluff structures in wind such as bridges, chimneys and buildings [62]. Flow-induced vibration caused by vortices being shed from a bluff body is also a promising way to extract energy from geophysical flows [10]. FIV energy harvesting systems are especially suitable for slow flow speeds in the range 0.5-1.5m/s which cannot be efficiently harvested by traditional hydroelectric power technologies. When a pair of tandem cylinders is immersed in a flow, the downstream cylinder can be excited into wake-induced vibrations (WIV) due to the interaction with vortices coming from the upstream cylinder. In this work, a flowing soap film was used to investigate the flow-induced vibration of the downstream cylinder of a tandem pair. With the ability to tie together the wake structure and the object motion, we investigate the relationship between energy generation and flow structure in the reduced order model system. The research results find the optimal efficiency of the energy harvesting system by a parametric study. To get deep physical understanding of the flow-induced vibration, wake structures of a circular cylinder undergoing controlled motion and the critical spacing of two identical tandem circular cylinders were also investigated in this research. These research results can help not only the optimization of energy harvesting systems based on flow-induced vibration of the circular-cylinder system, but also will benefit the understanding of wake interactions between multiple bluff bodies such as schooling fish, natural draft cooling towers and wind turbine farms.

The present study aims to explore dynamical behavior of the fluid-elastic instability of a circular cylinder in shear flow by numerical simulations. The theoretical model comprises two groups of transient conservation equations of mass and momentum and the governing equations are solved numerically with Fluent software to determine the flow property. The analysis presented that there exist both vortex-induced vibration and flow-elastic vibration for single cylinder in sear flow. The numerical results with a Harmonic Model built from Gambit indicate that there is a transverse force acting from high velocity side toward the low velocity side in shear flow. The transverse force make cylinder move periodically and thus go to a vibration. Furthermore, this study appraises the amplitude and orbit of fluid elastic vibration of a circular cylinder in shear flow and shows the effects of the shear velocity slope and damping factor on fluid elastic vibration of the cylinder. Here in the thesis, as the function applied with Fluent of displaying dynamic mesh on-time, the movement and re-mesh of cylinder could be observed. A vibration expansion diagram was presented and the pictures of flow velocity and flow pressure were retrieved from Fluent.

In the recent decades the Fluid-Structure Interaction (FSI) problem has been of great interest to many researchers and a variety of methods have been proposed for its numerical simulation. As FSI simulation is a multi-discipline and a multi-physics problem, its full simulation consists of many details and sub-procedures. On the other hand, reliable FSI simulations are required in various applications ranging from hemo-dynamics and structural engineering to aero-elasticity. In hemo-dynamics an incompressible fluid is coupled with a flexible structure with similar density (e.g. blood in arteries). In aero-elasticity a compressible fluid interacts with a stiff structure (e.g. aircraft wing) or an incompressible flow is coupled with a very light structure (e.g. Parachute or sail), whereas in some other engineering applications an incompressible flow interacts with a flexible structure with large displacement (e.g. oil risers in offshore industries). Therefore, various FSI models are employed to simulate a variety of different applications. An initial vital step to conduct an accurate FSI simulation is to perform a study of the physics of the problem which would be the main criterion on which the full FSI simulation procedure will then be based. In this thesis, interaction of an incompressible fluid flow at low Reynolds number with a flexible circular cylinder in two dimensions has been studied in detail using some of the latest published methods in the literature. The elements of procedures have been chosen in a way to allow further development to simulate the interaction of an incompressible fluid flow with a flexible oil riser with large displacement in three dimensions in future. To achieve this goal, a partitioned approach has been adopted to enable the use of existing structural codes together with an Immersed Boundary (IB) method which would allow the modelling of large displacements. A direct forcing approach, interpolation / reconstruction, type of IB is used to enforce the moving boundary condition and to create sharp interfaces with the possibility of modelling in three dimensions. This provides an advantage over the IB continuous forcing approach which creates a diffused boundary. And also is considered as a preferred method over the cut cell approach which is very complex in three dimensions with moving boundaries. Different reconstruction methods from the literature have been compared with the newly proposed method. The fluid governing equation is solved only in the fluid domain using a Cartesian grid and an Eulerian approach while the structural analysis was performed using Lagrangian methods. This method avoids the creation of secondary fluid domains inside the solid boundary which occurs in some of the IB methods. In the IB methods forces from the Eulerian flow field are transferred onto the Lagrangian marker points on the solid boundary and the displacement and velocities of the moving boundary are interpolated in the flow domain to enforce no-slip boundary conditions. Various coupling methods from the literature were selected and improved to allow modelling the interface and to transfer the data between fluid and structure. In addition, as an alternative method to simulate FSI for a single object in the fluid flow as suggested in the literature, the moving frame of reference method has been applied for the first time in this thesis to simulate Fluid-Structure interaction using an IB reconstruction approach. The flow around a cylinder in two dimensions was selected as a benchmark to validate the simulation results as there are many experimental and analytical results presented in the literature for this specific case.

Flow-induced structural vibration is experienced in many engineering applications, such as aerospace industry and civil engineering infrastructures. One of the main mechanisms of flow-induced vibration is instability which can be triggered by parametric excitations or fluid-elastic forces. Experiments show that turbulence has a significant impact on the stability of structures. The objective of this research is to bridge the gap between flow-induced vibration and stochastic stability of structures. The flow-induced vibration of a spring-supported circular cylinder is studied in this research. The equations of motion for the cylinder placed in a cross-flow are set up, in which the vortex force is modeled by a bounded noise because of its narrow-band characteristics. Since the vibration in the lift direction is more prominent in the lock-in region, the system is reduced to one degree-of-freedom, i.e., only the vibration of the cylinder in the lift direction is considered. The equation of motion for the cylinder can be generalized as a two-dimensional system excited by a bounded noise. Stochastic analysis is used to determine the moment Lyapunov exponents and Lyapunov exponents for the generalized system. The results are then applied to study the parametric instability of a cylinder in the lock-in region. Fluidelastic instability can occur when the cylinder is placed in a shear flow. The equations of motion are established by using the quasi-steady theory to model the fluid-elastic forces. To study the turbulence effect on the stability of the cylinder, a real noise or an Ornstein-Uhlenbeck process is used to model the grid-generated turbulence. The equations of motion are randomized resulting in a four-dimensional system excited by a real noise. The stability of the stochastic system is studied by determining the moment Lyapunov exponents and Lyapunov exponents. Parameters of the system and the noise are varied to investigate their effects on the stability. It is found that the grid-generated turbulence can stabilize the system when the parameters take certain values, which agrees with the experimental observations. Many flow-induced vibration problems can be modeled by a two degrees-of-freedom system parametically excited by a narrow-band process modeled by a bounded noise. The system can be in subharmonic resonance, combination (additive or differential) resonance, or both if the central frequency of the bounded noise takes an appropriate value. The method for a single degree-of-freedom system is extended to study the stochastic stability of the two degrees-of-freedom system. The moment Lyapunov exponents and Lyapunov exponents for the three cases are obtained using a perturbation method. The effect of noise on various types of parametric resonance, such as subharmonic resonance, combination additive resonance, and combined subharmonic and combination additive resonance, is investigated. The main contributions of this thesis are stochastic stability analysis of one-degree-of-freedom systems and two-degree-of-freedom systems. Stability analysis for systems under the excitation of real noise and bounded noise is carried out by determining the moment Lyapunov exponents and Lyapunov exponents. Good agreement is obtained between analytical results and those obtained from Monte Carlo simulations. In the two degrees-of-freedom case, the effect of free stream turbulence on cylinder vibration and its stability is examined.

Flow-induced structural vibration is experienced in many engineering applications, such as aerospace industry and civil engineering infrastructures. One of the main mechanisms of flow-induced vibration is instability which can be triggered by parametric excitations or fluid-elastic forces. Experiments show that turbulence has a significant impact on the stability of structures. The objective of this research is to bridge the gap between flow-induced vibration and stochastic stability of structures. The flow-induced vibration of a spring-supported circular cylinder is studied in this research. The equations of motion for the cylinder placed in a cross-flow are set up, in which the vortex force is modeled by a bounded noise because of its narrow-band characteristics. Since the vibration in the lift direction is more prominent in the lock-in region, the system is reduced to one degree-of-freedom, i.e., only the vibration of the cylinder in the lift direction is considered. The equation of motion for the cylinder can be generalized as a two-dimensional system excited by a bounded noise. Stochastic analysis is used to determine the moment Lyapunov exponents and Lyapunov exponents for the generalized system. The results are then applied to study the parametric instability of a cylinder in the lock-in region. Fluidelastic instability can occur when the cylinder is placed in a shear flow. The equations of motion are established by using the quasi-steady theory to model the fluid-elastic forces. To study the turbulence effect on the stability of the cylinder, a real noise or an Ornstein-Uhlenbeck process is used to model the grid-generated turbulence. The equations of motion are randomized resulting in a four-dimensional system excited by a real noise. The stability of the stochastic system is studied by determining the moment Lyapunov exponents and Lyapunov exponents. Parameters of the system and the noise are varied to investigate their effects on the stability. It is found that the grid-generated turbulence can stabilize the system when the parameters take certain values, which agrees with the experimental observations. Many flow-induced vibration problems can be modeled by a two degrees-of-freedom system parametically excited by a narrow-band process modeled by a bounded noise. The system can be in subharmonic resonance, combination (additive or differential) resonance, or both if the central frequency of the bounded noise takes an appropriate value. The method for a single degree-of-freedom system is extended to study the stochastic stability of the two degrees-of-freedom system. The moment Lyapunov exponents and Lyapunov exponents for the three cases are obtained using a perturbation method. The effect of noise on various types of parametric resonance, such as subharmonic resonance, combination additive resonance, and combined subharmonic and combination additive resonance, is investigated. The main contributions of this thesis are stochastic stability analysis of one-degree-of-freedom systems and two-degree-of-freedom systems. Stability analysis for systems under the excitation of real noise and bounded noise is carried out by determining the moment Lyapunov exponents and Lyapunov exponents. Good agreement is obtained between analytical results and those obtained from Monte Carlo simulations. In the two degrees-of-freedom case, the effect of free stream turbulence on cylinder vibration and its stability is examined.

Elastic cylinder vibration due to different shear parameter in the water flow is investigated experimentally in this research. The water flow ranges from 0.4 m/s to 1.06 m/s. It is found from the experiment that shear parameter has a significant influence on the amplitude of the cylinder vibration. The greater the shear parameter becomes, the later the delaying phenomenon also becomes. The delaying phenomenon will bring about resonant procrastination. Additionally, the greater shear parameter lessens the cylinder¡¦s drag force, but the lift force will be augmented, and the vibration orbit will be asymmetric. At lower flow velocity, cylinder¡¦s displacement is greater. With the enhancement of the shear parameter or the reduced velocity, the flow type and the vortex street behind the cylinder will turn more and more impalpable, and eventually become chaotic.

This study developed a microfluidic flow sensor for the detections of velocity and viscosity, especially for ultra-low viscosity detection. An etched optic fiber with the diameter of 9 £gm is embedded in a microfluidic chip to couple green laser light into the microfluidic channel. The flow induced vibration causes periodic flapping motion of the optic fiber cantilever because of the pressure difference from two sides of fiber cantilever. Through the frequency analysis, the fluidic properties including the flow rate and the viscosity can be detected and identified. Results show that this developed sensor is capable of sensing liquid samples with the flow rates from 0.17 m/s to 68.81 m/s and the viscosities from 0.306 cP to 1.200 cP. In addition, air samples (0.0183 cP) with various flow rates can also be detected using the developed sensor. Although the detectable range for flow rate sensing is not wide, the sensitivity is high of up to around 3.667 mm/(s¡EHz) in test liquid in DI water, and when detecting air the sensitivity is 6.190 mm/(s¡EHz). The developed flow sensor provides a simple and straight forward method for sensing flow characteristics in a microfluidic channel.

The present study aims to explore the dynamical behavior in the uniform flow by numerical method. The theoretical model is based on transient of continuity equation and momentum equation in CFD software: Fluent. With User Define Function¡]UDF¡^, we can simulate the Vortex-induced vibration¡]VIV¡^under the uniform flow by numerical method and plot the contour of amplitude and flow field under different Reynolds number. We will identify the accurate and capable of central difference method in UDF by comparing with the previous study. Also, we focus on whether the amplitude and flow situation will effect by uniform flow in different degrees or not. Furthermore, this study shows how the time step size and mesh effect the conclusion so that we could have the best choice on model.

The dissertation presents a CFD approach for 3D simulation of long risers. Long riser VIV simulation is at the frontier of the CFD research area due to its high demand on computational resources and techniques. It also has broad practical application potentials, especially in the oil and gas industry. In this dissertation, I used a time domain simulation program - Finite-Analytic Navier-Stokes (FANS) code to achieve the 3D simulations of riser VIV. First, I developed a riser modal motion solver and a direct integration solver to calculate riser dynamic motions when subject to external forces. The direct integration solver provides good flexibility on inclusion of riser bending stiffness and structural damping coefficients. I also developed a static catenary riser solver based on trial and error iteration technique, which allowed the motion solvers to handle catenary risers and jumpers with arbitrary mass distribution. I then integrated the riser motion solvers to the existing FANS code, and applied the CFD approach to a series of riser VIV problems including a 2D fixed/vibrating riser, a 3D vertical riser in uniform and shear currents, a 3D horizontal riser in uniform and shear current, a hypothetic 3,000 ft marine top tensioned riser in uniform current, a practical 1,100m flexible catenary riser in uniform current, and a hypothetic 265m flexible jumper partially submerged in uniform current. I developed a VIV induced fatigue calculation module based on rain flow counting technique and S-N curve method. I also developed a modal extraction module based on the least squares method. The VIV details, including flow field vorticities, rms a/D, riser motion trajectories, PSDs, modal components, VIV induced stress characteristics, and VIV induced fatigue damages were studied and compared to the published experimental data and results calculated using other commercial software tools. I concluded that the CFD approach is valid for VIV simulations in 3D. I found that the long riser VIV response shows complex behaviors, which suggests further investigation on the lock-in phenomenon, high harmonics response, and sensitivity to the lateral deflections.