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.

Isotactic polypropylene (iPP) is one of the widely used commercial thermoplastics. Physical properties of iPP can be tailored to the requirements with respect to structure, microstructure and processing, thus research continues in the development and modification of the polymer. With the advancement of chemistry, as our understanding in tailoring of the molecular structure has enhanced, iPP has become more of a generic name.

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.

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.

A thermodynamical framework is presented that describes the solidification of molten polymers to an amorphous as well as to a semicrystalline solid-like state. This framework fits into a general structure developed for materials undergoing a large class of entropy producing processes. The molten polymers are usually isotropic in nature and certain polymers crystallize, with the exception of largely atactic polymers, which solidify to an amorphous solid, to an anisotropic solid. The symmetry of the crystalline structures in the semicrystalline polymers is dependent upon the thermomechanical process to which the polymer is subjected to. The framework presented takes into account that the natural configurations associated with the polymer melt (associated with the breaking and reforming of the polymer network) and the solid evolve in addition to the evolving material symmetry associated with these natural configurations. The functional form of the various primitives such as how the material stores, dissipates energy and produces entropy are prescribed. Entropy may be produced by a variety of mechanisms such as conduction, dissipation, solidification, rearragement of crystalline structures due to annealing and so forth. The manner in which the natural configurations evolve is dictated by the maximization of the rate of dissipation. Similarly, the crystallization and glass transition kinetics may be obtained by maximization of their corresponding entropy productions. The restrictions placed by the second law of thermodynamics, frame indiference, material symmetry and incompressibility allows for a class of constitutive equations and the maximization of the rate of entropy production is invoked to select a constitutive equation from an allowable class of constitutive equations. Using such an unified thermodynamic approach, the popular crystallization equations such as Avrami equation and its various modifications such as Nakamura and Hillier and Price equations are obtained. The predictions of the model obtained using this framework are compared with the spinline data for amorphous and semicrystalline polymers.

In the study, vibrations of small elastic cylinders mounted in the shear flow of an air jet are investigated experimentally. In such cases, the amplitude of the cylinder oscillation changed along with the variation of the jet velocity gradient is due to the influence of fluid elastic instability. The experiment is based on the method of the magnetic field induction to measure the motion of the small cylinder, and it involves measurements of the varying velocity in a jet through the hot- wire anemometer. We focus on the fluid-elastic instability of a circular cylinder in shear flow. The vibration behaviors of the cylinder above the critical condition are be examined with different velocity gradients, mass ratios and damping factors. The vibration amplitude of the cylinder is also larger as velocity gradient is larger. With lower mass ratios and damping factor, moreover, the orbit of cylinder is larger. When the velocity gradient is increasing, the frequency of cylinder vibration becomes higher.

Yes / The present work deals with the flow-induced multiple orientations and crystallization structure of polymer melts under a complex flow field. This complex flow field is characteristic of the consistent coupling of extensional “pulse” and closely followed shear flow in a narrow channel. Utilizing an ingenious combination of an advanced micro-injection device and long chain aliphatic polyamides (LCPA), the flow-induced crystallization morphology was well preserved for ex-situ synchrotron micro-focused wide angle X-ray scattering (μWAXS) as well as small angle X-ray scattering (SAXS). An inverted anisotropic crystallization structure was observed in two directions: perpendicular and parallel to the flow direction (FD). The novel anisotropic morphology implies the occurrence of wall slip and “global” fountain flow under the complex flow field. The mechanism of structure formation is elucidated in detail. The experimental results clearly indicate that the effect of extensional pulse on the polymer melt is restrained and further diminished due to either the transverse tumble of fountain flow or the rapid retraction of stretched high molecular weight tails. However, the residual shish-kebab structures in the core layer of the far-end of channel suggest that the effect of extensional pulse should be considered in the small-scaled geometries or under the high strain rate condition.

Erosion corrosion of plumbing materials in domestic water systems is a complex phenomenon driven by water quality, hydrodynamic and electrochemical factors. Erosion corrosion accounts for over a third of copper hot water system failures in the U.S., hundreds of millions in damage, and may be expected to increase with newer Legionella control strategies including increased use of water recirculation and high temperatures. Additionally, some nonleaded alloys introduced after the passage of a new federal law restricting lead content in plumbing, have been anecdotally implicated as failing prematurely from erosion corrosion compared to traditional alloys. This dissertation includes 1) a critical review of the literature, 2) investigation of a recent rapid erosion corrosion failure in a large building plumbing system, 3) replication of this phenomena in copper and nonleaded brass in laboratory studies, and 4) evaluation of 12 nonleaded alloys against conventional leaded brass. Current plumbing codes and guidelines to prevent erosion corrosion were found to be widely inconsistent and lacking scientific evidence. Large-scale recirculating hot water pipe-loop experiments demonstrated that an aggressive hard water with entrained aragonite (CaCO3) particles could cause fully penetrative failures (i.e., leaks) in brand new copper pipe and nonleaded brass fittings in just 3-49 days. This represents the first time rapid erosion corrosion failures have ever been replicated in the laboratory under conditions similar to those encountered in practice. The entrained particulates dramatically accelerated attack on metals, especially at pipe bends. In general, lowering pH, increasing flow velocity, increasing temperatures, entrainment of particles (of bigger sizes), and addition of chlorine disinfectant increased erosion corrosion rates. These results scientifically proved that hard waters are not inherently less aggressive than soft water, and in fact if CaCO3 solids form they can be much more aggressive. Finally, cavitation and erosion corrosion resistance of 12 nonleaded alloys was evaluated against leaded brass; stainless steels demonstrated superior performance, silicon brass had the greatest susceptibility and remaining alloys were in the middle. This performance data can aid decision making regarding choice of alloys for various water applications. Our work over the years, including involvement in the Flint Water Crisis, demonstrated that practicing trustworthy science as a public good requires commitment to scientific rigor, truth-seeking, managing conflicts of interest, and comprehensible evidence-based science communication. Critical problems in 21st century public science were highlighted including perverse incentives, misconduct, postmodernist "science anarchist" thought, and ineffectiveness of U.S. water utilities in communicating tap water safety to the American public. / Ph. D.