bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of CFD Predictions-of Multi-Mode Vortex-Induced Vibrations of a Tension Riser With Laboratory Measurements
Authors: Bearman P. W. , Chaplin J. R. , Fontaine E. , Graham J. M. R. , Herfjord K. , Lima A. , Meneghini J. R. , Schulz K. W. , Willden R. H. J.
Published: July 2008

This paper compares previously unpublished laboratory measurements of the vortex-induced vibrations of a vertical tension riser with predictions of five CFD-based riser codes. The experiments were carried out with a 13m long, 28mm diameter model riser of which the upper 7m was in air, and the remainder in water with a uniform flow generated by carriage motion at speeds up to 1m/s. In-line and cross-flow responses were computed from measurements of the curvature of the riser at 32 stations over its length. Key results are compared with blind CFD predictions in which the flow is computed in two dimensions on each of a number of horizontal planes. The CFD codes used in this exercise represent a wide range of approaches, represented by (1) a vorticity-stream function method using a finite volume technique for integrating the vorticity transport equation, (2) finite elements with linear interpolation functions on a triangular grid, (3) a pressure-correction method in a finite-volume integration scheme on hybrid unstructured grids, (4) a discrete vortex formulation incorporating the growing core size or core spread method to model diffusion of vorticity, (5) a velocity-vorticity method using a hybrid Eulerian–Lagrangian vortex-in-cell method with LES, diffusion computed on a grid, and convection of vorticity by discrete vortices. In individual test cases, maximum computed cross-flow responses were between 23% and 160% of measured values, and there was a much wider range in ratios of predicted to measured curvatures. Overall, average ratios of predicted to measured cross-flow responses were between 59% and 106% for the 5 different codes, and those for maximum in-line deflections between 71% and 109%. Predicted curvatures in either direction were on average between 30% and 170% of the measured values.
Finite Element Modeling of Buried Pipelines Subjected to Seismic Loads: Soil Structure Interaction Using Contact Elements
Authors: Kokavessis Nikos K. , Anagnostidis George S.
Published: July 2008

Buried pipelines pass under lands exposed to traffic, railroads and highways, and cross through seismically active lands. The pipelines are exposed to the combined loads of soil weight, traffic load and seismic loads, and internal pressure. Several design practices exist for the combination of stresses due to soil, seismic and traffic loads, such as ASME design guide, VDTUV 1063, and DIN 2413. However, the soil structure interaction is not considered especially in the case of seismic loads. When the pipeline loses its support due to soil liquefaction then the problem can only be solved using the finite element method. Customarily, spring elements have been used to model the pipeline-soil interaction under the combined action of action of seismic, soil and pressure loads. This meshing technique, however, is tedious and time consuming. Furthermore it requires the analyst to kill spring elements that are not active. Verification of the model is thus difficult and prone to errors. In this work frictional elements are used to model the pipeline-soil interaction. This meshing technique is automatic and it does not require keeping track of the contact surfaces. The soil surface is the contact surface and the pipeline outside diameter is the target. The pipeline can be modelled as either a contact surface or a contact line depending on the dimensionality of the model.
Nonlinear Aeroelasticity Computations in Transonic Flows Using Tightly Coupling Algorithms
Authors: Feng Zhengkun , Soulai¨mani Azzeddine
Published: July 2008

A nonlinear computational aeroelasticity model based on the Euler equations of compressible flows and the linear elastodynamic equations for structures is developed. The Euler equations are solved on dynamic meshes using the ALE kinematic description. Thus, the mesh constitutes another field governed by pseudo-elatodynamic equations. The three fields are discretized using proper finite element formulations which satisfy the geometric conservation law. A matcher module is incorporated for the purpose of pairing the grids on the fluid-structure interface and for transferring the loads and displacements between the fluid and structure solvers. Two solutions strategies (Gauss Seidel and Schur-Complement) for solving the nonlinear aeroelastic system are discussed. Using second order time discretization schemes allows us to use large time steps in the computations. The numerical results on the AGARD 445.6 aeroelastic wing compare well with the experimental ones and show that the Schur-complement coupling algorithm is more robust than the Gauss-Seidel algorithm for relatively large oscillation amplitudes.
Shaking Table Tests of Critical Equipment With Simple Isolators
Authors: Tsai C. S. , Chen Wen-Shin , Yu Shih-Hsien , Yang Chen-Tsung
Published: July 2008

Because the earthquake is one kind of non-predictable calamity and happens suddenly, its disaster and consequence are larger than other calamities. Mankind must face not only the emotional effects caused by earthquakes, but also the damage to the structure and substructure systems. The fire, damaged pipeline systems cased by earthquake and the destruction of the semiconductor, equipment or microelectronics in high-tech factories will cause an enormous and a chain of economic losses. Therefore, there is a need of an economical and efficient method to protect equipments from earthquake damage. Namely, in addition to promoting the earthquake-resistant capacity of structures, it is also important to ensure the safety of the expensive equipment and facilities. In this study, it is aimed at developing a new simple isolator with appropriate damping for critical equipment. The basic principle of the simple isolator is to lengthen the natural period of equipment, and simultaneously to reduce the earthquake-induced energy and the displacement of the isolator by additional damping. A series of shaking table tests for critical equipment isolated with simple isolators were carried out in the Department of Civil Engineering, Feng Chia University, Taichung, Taiwan. From these test results, it is illustrated that the simple isolator can reduce more than 80% responses of accelerations under earthquakes with peak ground acceleration of above 0.450g. Therefore, the simple isolator can be recognized as a feasible and promising way in mitigating the seismic responses of equipment. In addition, the simple isolator possesses enough energy absorbing capacity to reduce its maximum displacement and the restoring force to bring the isolator back to the original position without significant residual displacement.
Shaking Table Tests of a Building Isolated With Trench Friction Pendulum System
Authors: Tsai C. S. , Lu Po-Ching , Chen Wen-Shin
Published: July 2008

It has been proven that the seismic isolation technology is a very promising tool to lessen damage caused by earthquakes. In order to provide a cheap and efficient base isolator for engineering practice, a new isolator named the trench friction pendulum system (TFPS), which is easy to design and manufacture, is proposed in this study. A series of shaking table tests of a scaled steel structure equipped with TFPS isolators were performed in the Department of Civil Engineering, Feng Chia University, Taichung, Taiwan. Experimental results demonstrate that the TFPS isolator can isolate most of earthquake induced energy trying to impart into the superstructure and that the device is not only cheap but also efficient for seismic mitigation.
Study on the Damping Effects of an Impact Damper Using a Elasto-Plastic Material
Authors: Ito Tomohiro , Fujita Katsuhisa , Okaya Naotoshi
Published: July 2008

Conventional impact dampers often utilize the steel balls because of its low cost and handling easiness. But the steel-ball impact dampers sometimes collapse or generate very large noise because of large shock at the impact. And as for the design of the impact damper, in the conventional approaches, the analytical modeling for the impact force is based on the contact theory proposed by H. Hertz, in which the restitution coefficient is assumed to be constant, i.e., the dependency on the relative velocity is not taken into consideration. However, some experimental results show that the restitution coefficient depends on the relative velocity at the impact. In this study, the elasto-plastic materials are employed as an impact damper material in order to suppress the large shock for the damper vessel and large impact noise. Therefore, the impact force modeling is modified so as that the elasto-plasticity of the material can be considered. This modeling can also consider the dependency of the restitution coefficient on the relative velocity. An impact damper which composed of a vessel and several particles made of elasto-plastic material such as lead is treated. The frequency response of the damper vessel and the damping effect of the damper are evaluated for the 2 kinds of impact force modeling by numerical simulations. Also, the effects of the particle number and the vessel configuration are evaluated. As a result, the effects of the above mentioned parameters are clarified.
Nonlinear Vibration Response of a Cylindrical Water Storage Tank Caused by Coupling Effect Between Beam-Type Vibration and Oval-Type Vibration: Part 2 — Simulation
Authors: Maekawa Akira , Suzuki Michiaki , Fujita Katsuhisa
Published: July 2008

The first report, “Part 1 - Vibration experiment”, has revealed that the nonlinear vibration response phenomena of beam-type vibration of a cylindrical water storage tank are caused by the coupling effect of beam-type vibration and oval-type vibration. This paper describes the modeling of the above phenomena into a nonlinear equivalent single-degree-of-freedom system and the results of the simulation analysis to investigate the mechanism of the nonlinear vibration responses of the tank when these two types of vibration are coupled. The conventional linear seismic response analysis method of the tank generally considers only beam-type vibration because the participation factor of oval-type vibration due to seismic excitations is theoretically zero. However, oval-type vibration was found to be generated and accompanied by the out-of-plane deformation of the tank sidewall when the test tank was experimentally subjected to large input acceleration excitations. So, it was considered that the geometrical nonlinearity due to oval-type vibrations was the main reason why the beam-type vibration response was nonlinear. Therefore, a static large-deformation analysis by the finite element method was conducted for the tank model whose sidewall had the oval-type vibration mode shape as the out-of-plane deformation in order to calculate the flexural rigidity of the tank. As a result, the flexural rigidity of the tank was found to decrease as the displacement of oval-type vibration increased. In other words, the geometrical nonlinearity caused by the oval-type vibration affects the flexural rigidity of the tank and generates the nonlinear vibration response. Furthermore, an equivalent single-degree-of-freedom system model was made to understand the nonlinear vibration response behavior of the tank, assuming that the flexural rigidity depended on the displacement of the oval-type vibration. Simulation of the frequency sweep test using this model showed that the resonance frequency decreased as the displacement of the oval-type vibration became larger. These results demonstrate the validity of the results mentioned in the first report, “Part 1 - Vibration experiment”.
Load Discontinuity and Its Remedy in Step-by-Step Integration
Authors: Chang Shuenn-Yih , Huang Chiu-Li
Published: July 2008

The discontinuity at the end of an impulse will lead to an extra impulse and thus an extra displacement. Consequently, an amplitude distortion is introduced in the numerical solution. The difficulty arising from the discontinuity at the end of an impulse can be overcome by using a very small time step to perform the step-by-step integration since it reduces the extra impulse and thus extra displacement. However, computational efforts might be significantly increased since the small time step is performed for a complete step-by-step integration procedure. A remedy is devised to computationally efficiently overcome this difficulty by using a very small time step immediately upon termination of the applied impulse. This is because that the extra impulse caused by the discontinuity is almost proportional to the discontinuity value at the end of the impulse and the step size. The feasibility of this proposed remedy is analytically and numerically confirmed herein.
Damping Measurements in Tube Bundles Subjected to Two-Phase Cross Flow
Authors: Moran Joaquin E. , Weaver David S.
Published: July 2008

An experimental study was conducted to investigate two-phase damping in tube arrays. The objective was to compare different measurement methodologies in order to obtain a more reliable damping estimate. This will allow for improved guidelines related to failures due to fluidelastic instability in tube bundles. The methods compared were the traditionally used half-power bandwidth, the logarithmic decrement and an exponential fitting to the tube decay response. The working fluid used was Refrigerant 11 (Freon), which better models the real steam-water problem, as it allows for phase change. The void fraction was measured using a gamma densitometer, introducing an improvement over the traditional Homogeneous Equilibrium Model (HEM) in terms of velocity and density predictions. The results obtained by using the half-power bandwidth method agree with data previously reported for two-phase flow. The experiments showed that the half-power bandwidth produces higher damping values than the other two, but only up to a certain void fraction. After that point, the results obtained from the three methods are very similar. The exponential fitting proved to be more consistent than the logarithmic decrement, and it is not as sensitive as the half-power bandwidth to the frequency shifting caused by the change in added mass around the tube. By plotting the damping ratio as a function of void fraction, pitch mass flux and flow regime, we were able to verify that damping is more dependent on void fraction and flow regime than on mass flux.
Study on the Earthquake Resistant Characteristics of High Natural Frequency Mechanical Structures From a Viewpoint of Energy Balance
Authors: Minagawa Keisuke , Fujita Satoshi
Published: July 2008

This paper describes investigation regarding earthquake-resistant characteristics of mechanical structures that have high natural frequency from a viewpoint of the energy balance. In earthquake-resistant design of a mechanical structure installed in a nuclear power plant, those mechanical structures are basically considered the rigid body and static seismic design is implemented. When vertical component of an seismic wave is considered, floor response acceleration at high natural frequency band is high, and it is expected that excessive inertia force operate against the mechanical structure in design phase. Therefore designing those mechanical structures is so difficult that accurate grasp regarding vibration characteristics, the main cause of fracture and establishment of evaluation technique are required. In this study, earthquake resistant characteristics of mechanical structures that have high natural frequency are investigated from a viewpoint of the energy balance that is one of valid methods for structural calculation. At first, forced vibration experiments were executed in order to confirm differences of input energy between low natural frequency models and high natural frequency models. Then continuous vibration experiments that induce experimental model to fatigue fracture were carried out. Results of these experiments and fracture phenomena were evaluated from the viewpoint of the energy balance.
Seismic Analysis and Design of RC Frame With New Metallic Dampers
Authors: Li Hong-Nan , Li Gang
Published: July 2008

Earthquake can make structures damaged and crumble. The traditional approach to seismic design has been based upon providing a combination of strength and ductility to resist the imposed loads. Thus, the level of the structure security cannot be achieved, because the disadvantage of the designing method is lack of adjusting capability subjected to an uncertain earthquake. The presence of some damping (energy dissipation) in buildings has been recognized and studied by professional researchers. Passive energy-dissipated system, as a category of vibration control methods, lead the inputting energy from earthquake to special element, thereby reducing energy-dissipating demand on primary structural members and minimizing possible structural damage. In this paper, a new idea of designing metallic damper is presented and realized through the improved dampers that are of a certain bearing forces in plane of plate and suitable energy-dissipating capability by making metallic dampers in different shapes. New types of metallic dampers are called as “dual functions” metallic damper (DFMD), because it not only provides certain stiffness in normal use for a building, but also are of good ability of the seismic energy-dissipation. The structural configuration and mechanical characteristics of the models and prototypes of the DFMDs are analyzed and experimented so as to verify the seismic performance of the dampers. Finally, the DFMDs applied to a new building in China are introduced and numerical results demonstrate the effectiveness of the DFMD.
Fuzzy Control of Structure for Semi-Active Friction Damper Using GA
Authors: Li Hong-Nan , Zhao Dahai
Published: July 2008

The motion of friction dampers, either passive or semi-active, involves sticking and slipping phases. The idea for increasing the performance of semi-active friction damper is to maintain its motion in the slipping phase as much as possible, since energy is dissipated during the slipping phase rather than the sticking phase. The effectiveness of semi-active friction damper depends on the control strategy used. Because of nonlinear characteristic of the friction damper, the establishment of an effective control strategy is a challenging effort. In optimal control theory, the bang-bang is a class of classical control laws. However, when applied to real structure control, it will produce some problems. One of disadvantages is that differential equation has to be solved on-line during the control process, which will lead to time delay and instability to the control system. The other is that the undesirable spikes will emerge near the origin of the state space due to high speed switching of the control force. In this paper, a new strategy based on the T-S fuzzy model and modified bang-bang algorithm is proposed. First, the theory of the T-S fuzzy model is briefly introduced. Next, the modified bang-bang control law is reviewed. Then, the implementation procedure of the proposed control method is detailed for description, and the optimal control force in the consequent part of the T-S model is achieved by the genetic algorithm. Finally, on the establishment of the semi-active control law, the approach here is applied to the vibration control of a three story building with a semi-active friction damper. Numerical simulation results indicate that the proposed control strategy not only effectively reduce the chattering effect as the responses of the structure cross through the zero points in the state space but also is adaptive to varying excitations from weak and strong earthquakes.
Numerical Investigation Into Significant Reduction in Coefficient of Restitution for Fluid-Container Combined Systems
Authors: Taniguchi Tomoyo , Nakashima Teruhiro , Ando Yoshinori
Published: July 2008

The analysis procedure of rocking motion of unanchored flat-bottom cylindrical shell tanks should include an impact problem between the tank bottom plate and tank foundation. To evaluate the rocking motion of tanks based on a simple analytical procedure developed by a senior author, adequate estimation of a coefficient of restitution is necessary. This paper numerically examines the coefficient of restitution suitable for the fluid-container combined system used in such simple analysis procedure. Employing free rocking motion, an empty container and fluid-container combined system are computed. The velocity vectors of the empty container turn their direction simultaneously just after the uplifted edge hits the ground, while those of the fluid-container combined system need a time to turn their direction. This implies that the coefficient of restitution should be evaluated with effects of fluid stored in the tank.
A Partitioned Algorithm of Dynamic Fluid-Structure Interaction for Elephant Foot Bulging of Tanks
Authors: Li Q. , Liu H. Z. , Zhuang Z. , Yamaguchi S. , Toyoda M.
Published: July 2008

A partitioned coupling algorithm is presented in this paper to solve the dynamic large-displacement fluid-structure interaction (DFSI) problems. In this algorithm, the program based on arbitrary Lagrangian Eulerian (ALE) and fractional two-step method is developed to calculate computational fluid dynamics (CFD) and computational mesh dynamics (CMD). ABAQUS is used to calculate computational structure dynamics (CSD). Some user subroutines are implemented into ABAQUS and the data are exchanged among CSD, CFD and CMD. Numerical results including elephant foot bulging (EFB) of the liquid storage tank are obtained under dynamic waveform.
Turbulent Wake of a Stack and the Influence of Velocity Ratio
Authors: Adaramola M. S. , Sumner D. , Bergstrom D. J.
Published: July 2008

The effect of the jet-to-cross-flow velocity ratio, R, on the turbulent wake of a cylindrical stack of AR = 9 was investigated with two-component thermal anemometry. The cross-flow Reynolds number was ReD = 2.3×104, the jet Reynolds number ranged from Red = 7×103 to 4.6×104, and R was varied from 0 to 3. The stack was partially immersed in a flat-plate turbulent boundary layer, with a boundary layer thickness-to-height ratio of δ/H = 0.5 at the location of the stack. The flow around the stack was broadly classified into three flow regimes depending on the value of R, which were the downwash (R < 0.5), cross-wind dominated (0.5 < R < 1.5), and jet-dominated (R > 1.5) regimes. Each flow regime had a distinct structure to the mean velocity (streamwise and wall-normal directions), turbulence intensity (streamwise and wall-normal directions), and Reynolds shear stress fields.
Finite Element Models for Computing Seismic Induced Soil Pressures on Deeply Embedded NPP Structures
Authors: Xu Jim , Costantino Carl , Hofmayer Charles , Graves Herman
Published: July 2008

This paper discusses computations of seismic induced soil pressures using finite element (FE) models for deeply embedded and/or buried (DEB) stiff structures, such as those appearing in the conceptual designs of structures for advanced reactors. For DEB structures, the soil-structure interaction (SSI) effect is expected to have a strong influence on the estimate of the seismic induced soil pressures, especially for stiff structures embedded in soft soil strata. In this paper, two FE models are developed using the SASSI and LS-DYNA computer programs, representing respectively the substructure subtracting method and explicit FE algorithm. SASSI utilizes the wave propagation theory and the principle of superposition to treat the SSI phenomenon. In the LS-DYNA analysis, an attempt is made to apply the direct approach to the SSI effect, which treats the near field soil with an explicit FE mesh that is connected to a transmitting boundary to approximate wave propagation in the half-space. The structural model used for the study is derived from the characteristics of a conceptual design for an advanced reactor. The structure is founded in a soft soil overburden underlain by a rock and the input seismic motion is specified at rock outcrop and has a zero period acceleration (ZPA) equal to 0.3 g, typical of review level earthquakes for nuclear power plant structures in the Central and Eastern United States. Various depths of burial (DOB) for the structure are considered in the analysis to afford an assessment of the DOB effect on the seismic induced soil pressure estimates determined by these methods. Comparisons and discussions of the analysis results computed by the two approaches are provided.
Study on 3-D Measurement Method in Shake Table Tests Using Image Processing Technique: 4th Report — Evaluation of Synthetic Measurement Performance
Authors: Fujita Satoshi , Furuya Osamu , Hayashi Hidetaka
Published: July 2008

In recent years, the importance of the fracturing test using the full-scale model is recognized in order to upgrade an earthquake resistant design. It is, moreover, important to develop a new measurement method which can measure a complex three-dimensional behavior in such fracturing test. This study has been doing research and development of three-dimensional measurement method using an image processing technique for a measurement of dynamic displacement in shake table test without any contact. This measurement system is a very convenient system because it can measure the three-dimensional dynamic displacement in the simple experimental condition that the several makers are only attached to the surface of an experimental structure. The system therefore is the most suitable measurement system for an evaluation of complex three-dimensional behavior of test model. The fundamental hardware and software for the measurement system has been constructed until now. The fundamental dynamic measurement accuracy and effectiveness of the measurement system has been also confirmed from several shake table tests. This paper describes the measurement results of shake table test using historical wooden structure and actual wooden house model in the largest shake table facilities at E-Defense, and evaluates synthetic measurement performance of the developed measurement system.
A Full Scale Test for Acoustic Fatigue in Pipework
Authors: Goyder H. G. D. , Armstrong K. , Billingham L. , Every M. J. , Jee T. P. , Swindell R. J.
Published: July 2008

Gas flow through a corrugated pipe can produce unacceptable levels of noise. The occurrence of such noise gave rise to concerns about vibration induced fatigue of small-bore subsea pipework in the Schiehallion oil field. In order to check that the subsea pipework was free from noise-induced vibration a full scale replica of the subsea equipment containing the small-bore pipework was built and tested. The test required the generation of acoustic pressures with a 1 bar amplitude and a frequency range of 80 to 800Hz. It was also necessary to arrange for resonant conditions within the pipework and for acoustic nodes and anti-nodes to be swept though a range of possible locations. The test was conducted with full-scale conditions of methane at a static pressure of 170bar and with a range of gas flow rates. Particular attention was given to achieving the correct acoustic and structural natural frequencies together with the correct acoustic and structural damping ratios. The subsea equipment was found to be vulnerable for one operating condition. This vulnerability was removed by retro-fitting a brace to the existing subsea pipework.
Karman Vortex Effect on the Aerodynamic Forces to Rectangular Cylinders
Authors: Matsumoto M. , Yagi T. , Lee J. H. , Hori K. , Kawashima Y.
Published: July 2008

Standing on the back-ground associated with the complicated bluff body aerodynamics affected by Karman vortex, the fundamental wind tunnel tests have been carried out to clarify the role of Karman vortex on the aerostatic forces and the aerodynamic forces of 2D rectangular cylinders with various side ratios in the range from B/D=0.3 to B/D=15. For mitigation of Karman vortex, a splitter plate with suitable size was installed in a wake center. As far as stationary drag force, the sharp peak in CD-B/D diagrams, completely disappear by a splitter plate installation. On the other hand, the value of dCL/dα is significantly sensitive to the Karman vortex, in particular, even its sign changes in the range from B/D=0.6 to B/D=0.7 and B/D=3.0 to B/D=4.0, which means the significant role of the Karman vortex on the galloping instability, from the point of quasi-steady theory. Furthermore, the flutter derivatives are also drastically affected by Karman vortex in the rather wide reduced velocity far from the Karman vortex resonant reduced velocity. In summary, it would be emphasized that the substantial bluff body flow might be characterized by the side ratio of cylinders, and these fundamental flows should be significantly or slightly affected by Karman vortex depending on their side ratios of cylinders.
Numerical Simulation of Non Newtonian Hemodynamics in Compliant Vessels
Authors: Le Gouez J. M.
Published: July 2008

The numerical simulation of hemodynamics is increasingly recognized as a valuable analysis tool for the bioengineering laboratories who design implantable vascular grafts, and it is thought to become in a not so far future a complement to the physician’s analysis for the choice of interventional methods to restore a proper irrigation in diseased arteries [1]. The detailed numerical results obtained from 3d unsteady simulations permit to verify the hypotheses formulated by physiologists concerning the evolution of arterial disease and to quantify the risks associated to medical intervention. These objectives require an accurate representation of the major physiological parameters, much more complex than that encountered in standard CFD analysis of pipe flows, which makes the computation of blood flows in compliant vessels extremely challenging to the CFD and CSM communities. A comprehensive methodology for the adaptation of existing numerical schemes to this advanced modelling issues was conducted, for the behaviour laws of the fluid and walls on the one hand, and for the unsteady boundary conditions on the other hand. The complex nature of the mathematical model formed by the coupled fluid and structural mechanics equations, completed by highly unsteady boundary conditions, led to a choice of fully time-implicit algorithms with problem matching subiterations. These algorithms are successfully applied to different 3D, unsteady haemodynamics problems in a coronary artery. The complex, patients’ dependent geometry of a stenosed artery was reconstructed in CAD format from medical imaging. The accuracy of the numerical procedure is discussed, and this methodology can open the way for the validation of physiological criteria for the design of medical apparatus and the choice of interventional methods adapted to each individual patient’s situation.
Stall Induced Vibration and Flutter in a Symmetric Airfoil
Authors: Sarkar Sunetra , Bijl Hester
Published: July 2008

In this paper the aeroelastic stability of a wind turbine rotor in the dynamic stall regime is investigated. Two dimensional stall flutter models have been chosen for the aeroelastic stability analysis. Two different aeroelastic models have been considered. First, a pitching oscillation, and second, a flap-edgewise oscillation of an airfoil. Previous nonlinear aeroelastic analysis on such systems are not exhaustive. Parameters like structural non-linearity and initial conditions have not been studied, while these have a significant influence on the overall dynamics. They have now been taken up in the present analysis. The overall study helps towards a better understanding of the stall flutter mechanism in such aeroelastic systems. The study has found a strong influence of structural nonlinearity on the bifurcation pattern. Initial condition, too, plays an important role on the stability behavior as well as on the bifurcation pattern. Other system parameters of influence are nondimensional airspeed and external forcing. Interesting dynamical behavior like period-three and also super-harmonic and quasi-harmonic response have been found in the forced system, not reported before.
Passive, Semi-Active, and Active Tuned-Liquid-Column Dampers
Authors: Chen Yung-Hsiang , Ding Ying-Jan
Published: July 2008

The dynamic characteristics of the passive, semi-active, and active tuned-liquid-column dampers (or TLCD’s) are studied in this paper. The design of the latter two are based on the first one. The water-head difference of a passive TLCD is pre-set to form the so-called semi-active one in this paper. The water-head difference is released at a proper time instant during an earthquake excitation to enhance the vibration reduction of a structure. Two propellers are installed along a shaft inside and at the center of a passive TLCD to form an active one. These two propellers are driven by a servomotor controlled by a computer to provide the control force. The seismic responses of a five-story shear building with a passive, semi-active, or active TLCD are computed for demonstration and discussion. The results of this building with a tuned mass damper (or TMD) are also included for comparison.
A Simplified Model of Flow-Induced Oscillations of Collapsible Tubes Conveying Viscous Fluids
Authors: Wang X. Sheldon
Published: July 2008

In this paper, a simple mathematical model is formulated to study the static and dynamic instabilities of collapsible tubes conveying viscous fluids. The governing equations of motion for the pipe are derived under small deformation assumptions with the consideration of fluid frictional forces and inertia effects. In the case of steady flow, both buckling and flutter instabilities can be predicted based on the real and imaginary parts of the eigenvalues. In the case of pulsatile flow, the Bolotin method is employed to identify the dynamic stability regions.
An Experimental Study of Flag Flutter
Authors: Souilliez Claire , Eloy Christophe , Schouveiler Lionel
Published: July 2008

We present an experimental study of the flutter instability of a flag. Our experiments are performed in a low turbulence horizontal wind tunnel. The flags tested are made of paper or plastic of different flexural rigidities. One end of the flag is clamped to a profiled and vertical mast, the other end is left free. At low velocities, the flag is parallel to the flow and motionless. Above a critical wind velocity, the flag exhibits periodic flutter. The bifurcation is subcritical and the system shows a strong hysteresis cycle. We particulary focus on the variation of the instability threshold with respect to two non-dimensional parameters: the mass ratio and the aspect ratio. When the mass ratio increases, different flutter modes appear. We have studied systematically the effect of aspect ratio on the instability threshold. In agreement with a recent theoretical study [1], the critical velocity is a decreasing function of aspect ratio as long as three-dimensional effects are negligible (when the flag span is not too large).
Bluff-Body Flow Control by Aerodynamic Tripping
Authors: Sengupta T. K. , Kumar Gaurav
Published: July 2008

Drag reduction and lift enhancement for a circular cylinder is achieved here simultaneously by steady rotation and rotary oscillation of the cylinder. Rotary oscillation provides the necessary aerodynamic tripping of the laminar boundary layer initiated on the windward side of the cylinder and thus reducing the drag. While the steady rotation provides the high lift via Magnus-Robins effect. Results are presented here for a Reynold’s number (based on cylinder diameter and oncoming flow velocity) of 1000. Proper orthogonal decomposition (POD) of the vorticity data obtained from the solution of Navier-Stokes equation helps in identifying the physical mechanism(s) involved in the flow.
Wavelet Analysis of Flow-Induced Forces on Two Side-by-Side Stationary Cylinders: Reynolds Number Effect
Authors: Wang X. Q. , So R. M. C. , Xie W.-C.
Published: July 2008

In the present study, the effect of Reynolds number (Re) on flow interference between two side-by-side stationary cylinders and the associated flow-induced forces are investigated using a finite element method. The pitch ratio chosen is T/D = 1.7, and Re is varied within the range of laminar flow regime, i.e., 60 < Re < 200. The method of continuous wavelet transform is used to analyze flow-induced forces, especially their time-variant features. Flow patterns in the form of vorticity plot are presented to demonstrate the underlying physics. It is found that flow interference initially occurs in the inner vortices shed from the two cylinders, and extends to the outer vortices with increment of Re. The flow behind two cylinders undergoes three regimes: Regime I - unbiased gap flow, Regime II - stable biased gap flow, and Regime III - unstable gap flow. Flow-induced forces show significant variations when the flow transits from one regime to another. In particular, during the transition from Regime II to III, the forces not only increase by amplitude, but also change their nature from deterministic to random, and show some non-stationary features. This is shown to be caused by the amalgamation of inner and outer vortices behind the two cylinders when the flow interference extends from inner vortices to outer vortices. Whenever possible, the present results are compared with experimental measurements and theoretical predictions. The numerical simulations are consistent with these other results.
Coupling Between Acoustic Resonance and Fluidelastic Instability in Normal Triangular Tube Arrays
Authors: Mahon John , Meskell Craig
Published: July 2008

This paper reports on the interaction between fluidelastic instability (FEI) and acoustic resonance. In order to examine the interaction, the duct acoustics were excited with speakers placed adjacent to the tube array to artificially replicate flow-induced acoustic resonance. While the current study has clearly captured the phenomenon of interaction between the fluidelastic motion at ∼ 10 Hz and the acoustic field at ∼ 1kHz, it is not apparent what the physical mechanism at work might be. The paper details the effect on RMS level of tube vibration for three independent parameters: flow velocity, structural damping and acoustic power. The results presented show that there is a corresponding fall in the FEI vibration amplitude with increasing sound pressure level in the tube array. In addition, the effects of flow velocity and structural damping in conjunction with forced acoustics on the RMS of tube displacement are discussed.
Effects of Ventricle Motion on Chorded Mitral Prostheses
Authors: Watton P. N. , Luo X. Y. , Yin M. , Bernacca G. M. , Molloy P. , Wheatley D. J.
Published: July 2008

We assess the dynamic performance of a novel chorded mitral prosthesis based on the physiology of the human mitral valve using an immersed boundary code. In order to analyse the effect of the ventricle motion on the chorded valve in dynamic opening, in vivo MRI data is used to determine the relative motion of the mitral annulus and the papillary muscles regions of the ventricle. The dynamic fluid-structure interaction model has successfully identified the influences of the ventricle motion on the valve’s mechanical behaviour, and revealed the strengths and weaknesses of the current mitral design. Potential improvements for future designs are also discussed.
Two-Phase Damping and Interface Surface Area in Tubes With Internal Flow
Authors: Anscutter Franc¸ois , Be´guin Ce´dric , Ross Annie , Pettigrew Michel J. , Mureithi Njuki W.
Published: July 2008

Two-phase flow is common in the nuclear industry. It is a potential source of vibration in piping systems. In this paper, two-phase damping in the bubbly flow regime is related to the interface surface area between phases and, therefore, to flow configuration. Two sets of experiments were performed with a vertical tube clamped at both ends. First, gas bubbles of controlled geometry were simulated with glass spheres let to settle in stagnant water. Second, air was injected in stagnant alcohol to generate a uniform and measurable bubble flow. In both cases, the two-phase damping ratio is correlated to the number of bubbles (or spheres). Two-phase damping is directly related to the interface surface area, based on a spherical bubble model. Further experiments were carried out on tubes with internal two-phase air-water flows. A strong dependence of two-phase damping on flow configuration in bubbly flow regime is observed. A series of photographs attests to the fact that two-phase damping increases for a larger number of bubbles, and for smaller bubbles. It is highest immediately prior to the transition from bubbly flow to slug or churn flow regimes. Beyond the transition, damping decreases. An analytical model is proposed to predict two-phase flow damping in bubbly flow, based on a spherical bubble model. The results also reveal that the transition between bubbly flow and slug/churn flow depends on tube diameter. Consequently, the tube diameter also has an effect on two-phase damping. The above results could lead to some modifications of existing flow regime maps for small diameter tubes.
Effects on Pipe Vibrations of Cavitation in an Orifice and in Globe-Style Valves
Authors: Caillaud Se´bastien , Gibert Rene´-Jean , Moussou Pierre , Cohen Joe¨l , Millet Fabien
Published: July 2008

A piping system of French nuclear power plants displays large amplitude vibrations in particular flow regimes. These troubles are attributed to cavitation generated by single-hole orifices in depressurized flow regimes. Real scale experiments on high pressure test rigs and on-site tests are then conducted to explain the observed phenomenon and to find a solution to reduce pipe vibrations. The first objective of the present paper is to analyze cavitation-induced vibrations in the single-hole orifice. It is then shown that the orifice operates in choked flow with supercavitation, which is characterized by a large unstable vapor pocket. One way to reduce pipe vibrations consists in suppressing the orifices and in modifying the control valves. Three technologies involving a standard trim and anti-cavitation trims are tested. The second objective of the paper is to analyze cavitation-induced vibrations in globe-style valves. Cavitating valves operate in choked flow as the orifice. Nevertheless, no vapor pocket appears inside the pipe and no unstable phenomenon is observed. The comparison with an anti-cavitation solution shows that cavitation reduction has no impact on low frequency excitation. The effect of cavitation reduction on pipe vibrations, which involve essentially low frequencies, is then limited and the first solution, which is the standard globe-style valve installed on-site, leads to acceptable pipe vibrations. Finally, this case study may have consequences on the design of piping systems. First, cavitation in orifices must be limited. Choked flow in orifices may lead to supercavitation, which is here a damaging and unstable phenomenon. The second conclusion is that the reduction of cavitation in globe-style valve in choked flow does not reduce pipe vibrations. The issue is then to limit cavitation erosion of valve trims.
The Kinetic Energy Increment for an Unstable Vibration Induced by Sliding
Authors: Yu Chau-Cho , Tung Shih-Hen
Published: July 2008

The Mechanics of the bearing vibration induced by sliding is studied in the present paper. The phenomenon of this kind of sliding instability is carefully caught in a numerical simulation. Due to the complex contact conditions and the finite boundary, and because the size of the bearing is relatively very small compared to the travel length in a unit time for the stress waves, a spatial particle within a bearing pad moves in a rather random way. Nevertheless, the energy required to intensify the vibration is accumulated in a positive rate. It is found that the key factors are material properties, relative sliding velocity, friction coefficient, and normal pressure as well. In the present analysis, the energy accumulation is estimated and the characteristics of the threshold of such instability can be evaluated more accurately upon the result.
The Hydrodynamic Forces Acting on a Long Flexible Circular Cylinder Responding to VIV
Authors: Bearman P. W. , Huera Huarte F. J. , Chaplin J. R.
Published: July 2008

Distributions of the fluid forces acting along a long flexible circular cylinder free to respond in-line and transverse to a stepped current are presented. Forces are calculated using a finite element model of the cylinder with measured responses providing the input. The length to diameter ratio of the model used was 469, the mass ratio was 3 and the Reynolds number could be varied up to maximum value of approximately 2.6 · 104. Fluid force coefficients for two cases are presented: in the first, the dominant modes are the 2nd cross-flow and the 4th in line. For the second case the leading modes are the 7th and 12th respectively. In general, transverse force coefficients and in-line drag coefficients are found to be larger than those measured for short sections of cylinder undergoing free and forced one and two-dimensional motions. It is anticipated that the results will be of value to developers of vortex-induced vibration prediction methods.
Response of a Strip With Circular Cavity to Harmonic Forces
Authors: Yeh Chau-Shioung , Teng Tsung-Jen , Lin Bo-Yu , Shih Po-Jen
Published: July 2008

In this paper, the response of a strip with a circular cavity to a pair of harmonic forces is investigated by applying elastodynamic theory. The field is decomposed into two parts: the one is the free wave field in the strip without cavity and the scattered wave field generated by the cavity. The scattered field is contributed by the series of the source terms which consist of the line sources of P-wave and SV-wave with different orders along y-direction, and their corresponding reflective waves from the plane boundaries. Both source and reflective waves are expressed in terms of integrals which can be evaluated numerically by applying the modified steepest descend method proposed by Yeh et al. (1998). The coefficients of the series are determined by choosing the collocation points around the boundary of the cavity and applying the least square method. Then the displacements and the stresses around the boundary are calculated numerically. The numerical results are shown in diagrams for different frequencies. The dynamic stress concentration is defined and discussed in detail.
Computational Prediction of Hemolysis by Blade Flow Field of Micro-Axial Blood Pump
Authors: Chen Xin , Tan Jianping
Published: July 2008

By analyzing fluid dynamics of blood in an artificial blood pump and simulating the flow field structure and the flow performance of blood, the blood flow and the damages in the designed blood pump would be better understood. This paper describes computational fluid dynamic (CFD) used in predicting numerically the hemolysis of blade in micro-axial blood pumps. A numerical hydrodynamical model, based on the Navier-Stokes equation, was used to obtain the flow in a micro-axial blood pump. A time-dependent stress acting on blood particle is solved in this paper to explore the blood flow and damages in the micro-axial blood pump. An initial attempt is also made to predict the blood damage from these simulations.