bluuebunny/crossref_metadata_2025 huggingface dataset:
Vibration Tests of a Cut-off System Using Water and Functional Fluids
Authors: Sunakoda Katsuaki , Matsuoka Taichi
Published: July 2008

In the previous paper, the authors proposed a new Cut-off system for vibration control using water and/or functional fluid which acts as a series inertia mass. In this paper, frequency response vibration tests of the Cut-off system installed in one degree of freedom vibration system are carried out using a shaking table. Transmissibility between main mass and base is measured and evaluated. The Cut-off system consists of a piston cylinder, by-pass pipe, and the fluid. It is clarified that the series mass has a large inertia effect which is proportional to the forth power of the inner diameter of the piston cylinder. The experimental results are compared with the numerical analysis, and the effects of vibration isolation are confirmed.
Study on Vibration Control Device Using Power Generator
Authors: Ohtake Takafumi , Sunakoda Katsuaki , Matsuoka Taichi
Published: July 2008

In this paper, the authors propose a vibration control device using power generator in order to develop a small damper which is suitable for structural vibration control in space. The vibration control device consists of a ball screw, piston, ball nut, gear, power generator, and rod ends. The linear motion of the piston is converted into a rotary motion by the ball screw, and then the electric power is generated as dissipation of energy. A very low sinusoidal input displacement is applied to the device, and the characteristics of the damping force are examined with open circuit and short circuit of the generator. Experimental results are compared with theoretical results. Numerical simulation is applied to a flexible structure, by using sinusoidal input motion, and random motion.
Governing Equations of Motion of Walking Behavior of Unanchored Flat-Bottom Cylindrical Shell Tanks Subjected to Horizontal Sinusoidal Ground Motion
Authors: Taniguchi Tomoyo , Imai Koji
Published: July 2008

The governing equations of motion of walking phenomena of unanchored flat-bottom cylindrical shell tanks subjected to horizontal sinusoidal ground motion are examined. The equations of motion are derived through variational approach. The physical quantities related to the walking phenomena are the mass of tank itself, tank content, the effective mass of liquid for bulging motion, that for rocking motion, that for rocking-bulging interaction motion, and friction force including self-weight reduction effects. The roles of each physical quantity during the walking motion are clearly identified. Comparison of the time history of experimental results and that of analytical ones corroborates accuracy of the proposed equations of motion.
Wind-Induced Failures of Highway Light Poles During Winter Storms
Authors: Caracoglia Luca , Jones Nicholas P.
Published: July 2008

Wind-induced failures of aluminum-alloy highway light poles during a winter storm are investigated. Wind-induced phenomena including along-wind buffeting and galloping-type vibration, initiated by the presence of freezing precipitation on the surface of the units, are analyzed. The dynamic performance of these systems is evaluated through numerical statistical simulations. The representation of the aerodynamic coefficients in the presence of freezing rain is based upon existing literature. Statistical methods were preferred due to the absence of field data and the inherent uncertainty associated with this particular event.
Development of a New Added Mass Model and Dynamic Analysis for Cylindrical Tank
Authors: Zhuang Z. , Liu H. Z. , Li Q. , Yamaguchi S. , Toyoda M.
Published: July 2008

In order to investigate dynamic response under seismic loading for liquid tank, a new added mass formula and finite element model has been developed in this paper, which is implemented into ABAQUS as the user element subroutine. The simulation results for elastic-plastic Elephant Foot Bulging (EFB) and Diamond Shape Bulging (DSB) are validated the reliability for the added mass formula. Some shortcoming has been appointed out for the classical added mass equation, which is only suited for the liquid tank with rigid wall. Used the new added mass model, the dynamic nonlinear analysis for the tank has been carried out under dynamic loading. Through the response history and frequency analysis, it indicates that the external work on the tank model by earthquake loading is a key factor to result in plastic deformation of EFB and DSB.
Seismic Analysis of Reinforced Concrete Walls With Granular Infill
Authors: Mertz Greg , Houston Thomas
Published: July 2008

Reinforced concrete walls sandwiching granular infill may be used to enhance missile protection of selected facilities. Two behaviors complicate the seismic response of the assemblage of granular material contained by the two concrete wall elements. First, the granular material tends to settle when the walls pull apart in a breathing mode. This settling increases the lateral pressure acting on each wall, generating a set of forces that acts to spread the walls apart. Settling of the granular material combined with spreading of the walls results in breathing mode deformations that can occur in a ratcheting behavior, with the walls moving progressively further apart with each cycle of strong ground motion. Second, friction forces develop between the two walls and granular material. These forces may cause partial flexural coupling of the two walls (i.e., partial composite action). Soil mechanics solutions for lateral soil pressure acting in trenches are adapted to predict the lateral pressure of granular infill. The granular material is represented by a bilinear lateral response representing the active flow regime. The seismic response of granular infill concrete walls is studied using nonlinear finite element analysis. Simple structural models appropriate for routine seismic analysis that capture important aspects of the seismic response are proposed.
Active Feedback Control of Leakage-Flow-Induced Sheet Flutter by Injection and Suction of Fluid at Inlet or Outlet of Passage
Authors: Watanabe Masahiro , Koyama Yutaka
Published: July 2008

This paper proposes and develops a new non-contact active feedback control of leakage-flow-induced sheet flutter in a narrow passage, by injection and suction of fluid at inlet or outlet of the passage. The strategy of this active control technique is that the active feedback, which is by the activation of the injection/suction of the fluid at the inlet or outlet of the passage, suppresses the original source of the self-exciting fluid force acting on the structure, i.e., cancels the self-excited feedback mechanism. In this paper, the effective suppression of the leakage-flow-induced sheet flutter by the active feedback control technique is demonstrated experimentally. A flexible sheet, as a controlled object, is subjected to fluid flow in a narrow passage. The leakage-flow-induced flutter occurs to the flexible sheet in the translational motion over the critical flow velocity. The leakage-flow-induced sheet flutter is actively controlled and suppressed by the activation of injection/suction of the fluid at the inlet or outlet of the passage. The critical flow velocity under the controlled condition is examined with varying the controller gain and phase-shift between the injection/suction of the fluid and the sensor (vibration displacement) signal of the flexible sheet. As a result, it is indicated experimentally that the active feedback control technique increases the critical flow velocity, and suppress the leakage-flow-induced sheet flutter effectively. Moreover, the control performance is examined experimentally, and stabilization mechanism by the active feedback control is discussed.
Nonlinear Equations of Motion for the Maneuvering Flexible Aircraft Wings
Authors: Fazelzadeh S. Ahmad , Mazidi Abbas
Published: July 2008

In this paper, the complete dynamical equations for the general maneuvering flexible wings with sweep and dihedral angles are formulated. These equations are valid for an isotropic non-uniform wing; include transverse shear and warping effects. The equations of motion and boundary conditions are derived using Hamilton’s variational principle. Interaction between rigid-body motion caused by the angular velocities of the general maneuver, and elastic deformations of the wing, results in nonlinear terms, form an important contribution to the final equations. For model validation, the simplified partial differential equations of pull-up maneuver are transformed into a set of differential equations through a Galerkin approach and finally the results of numerical simulation are presented. The combination of flexible structural motion and maneuver parameters are very effective on natural frequencies and instability boundaries.
Aerothermoelastic Behavior of Supersonic Rotating Thin-Walled Beams Made of Functionally Graded Materials
Authors: Fazelzadeh S. Ahmad , Hosseini Mohammad
Published: July 2008

In this study a thin walled-beam made of functionally graded material (FGM) which is used as rotating turbomachinery blade under the aerothermoelastic loading is considered. The governing equations, based on small deflection with transverse shear and rotary inertias beam model, are considered with the presetting angle included. The effects of constant angular velocity and wall temperature due to flow motion also have been included. Quasi-steady aerodynamic pressure loadings are determined using first order piston theory as well as steady wall temperature through gas dynamic theory. Due to aerothermoelastic terms in loading, the gyroscopic matrix was generated in equations of motion. The blade partial differential equations are transformed into a set of ordinary differential equation through the Galerkin approach. The resulting system of equations is solved through the numerical integration. The effects of the variation of Mach number and geometric parameters on the natural frequencies are simulated. Results are indicative of the important influence of aerothermoelastic parameters on vibration behavior.
Suppression Characteristics of Sloshing in Vessels by a Bulkhead
Authors: Kobayashi Nobuyuki , Watanabe Masahiro , Honda Tomokazu , Ohno Katsunori , Zhang Yajun
Published: July 2008

This paper deals with suppression characteristics of sloshing in the vessel by a bulkhead, which divides the vessel vertically into two sections but communicates with two sections at the lower part of the vessel. The sloshing mode is separated into the U-tube mode sloshing and the sloshing mode in the separated tank. The effect of aperture height beneath the bulkhead and exciting amplitude on the suppression characteristics of U-tube mode sloshing is investigated experimentally. As a result, following phenomena are clarified. Three types of flow pattern are observed; the flow pattern A with unsteady vortexes, the flow pattern B with single swirl and a vortex, and the flow pattern C with twin swirls. The flow pattern changes from A to B or C with increasing the exciting amplitude. The damping ratio with the bulkhead is much larger than that without the bulkhead. The damping generated by the bulkhead depends on the exciting amplitude. The flow pattern plays an important role for dissipation of the energy of the sloshing. In particular, the vortexes and swirls increase the energy dissipation of the sloshing.
Annular Axial Flow-Induced Vibration of Cylindrical Shells by Flu¨gge’s Shell Theory
Authors: Fujita Katsuhisa , Katou Makoto
Published: July 2008

The unstable phenomena of thin cylindrical shells subjected to annular axial flow are investigated. In this paper, the analytical model is composed of an elastic axisymmetric shell and a rigid one which are arranged co-axially. Considering the fluid structure interaction between shells and fluid flowing through an annular narrow passage, the coupled equation of motion is derived using Flu¨gge’s shell theory and Navier-Stokes equations. The unstable phenomena of thin cylindrical shells are clarified by using the root locus based on the complex eigenvalue analysis. The numerical parameter studies on the shells with a freely supported end and a rigid one, and with both simply supported ends, are performed taking the dimensins of shells, the characteristics of flowing fluid so on as parameters. The influence of these parameters on the threshold of instability of the coupled vibration between thin cylindrical shells and annular axial flowing fluid are investigated and discussed.
Theoretical Analysis on Radial Gate Vibration
Authors: Ogihara Kunihiro
Published: July 2008

The instability analysis on vibration of radial gate has been done in 1999 under the research project In JPGA (Japan penstock and water gate association) and also the model test of gate vibration has been preceded. The almost theoretical results have been satisfied of the model test results, but there is one point not to be able to explain the relation of natural frequency of gate support system. The instable condition is given by the forth power of frequency of gate system in model test, and the fact that the region of instable becomes smaller as the gate opening larger, is same as the results of theoretical analysis. In this paper, author develop the new approach which is considering the pressure change by stopping the water flow same as water hummer in pipe line. And the unstable analysis on vibration equation of radial gate motion has been done and gotten good agreement on model test results.
A Mesoscopic Model Approach to Passively Control Vortex Wakes Using Single/Multiple Bodies
Authors: Frandsen Jannette B.
Published: July 2008

In this paper, vortex patterns are studied when single or several objects are located in the bluff-body wakes. The suitability of a mesoscopic approach involving a single phase Lattice Boltzmann (LB) model is examined. The central idea behind proposing the present formulation is to capture smaller scales naturally, postponing the need of applying empirical turbulence models. In contrast, to continuum mechanics based numerical models, where only space and time are discrete, the discrete variables of the LB model are space, time and particle velocity. With reference to the Boltzmann equation of classical kinetic theory, the distribution of fluid molecules is represented by particle distribution functions. It is notable that the formulation avoids the need to include the Poisson equation. An elastic-collision scheme with no-slip walls is prescribed. Although the long term goal is to predict bluff-body high Reynolds number flows, the present study is limited to laminar flow simulations. The case studies include sharp edge bodies embedded in Re flows in the order of 100–250. The 2-D uniform grid solutions are compared with findings reported in the literature and promising agreements have been found. This study is important to a variety of applications, in particular, the wind, ocean and coastal engineering communities. From a mitigation point of view, the model presents an easy means of re-arranging bluff bodies to study optimum solutions for VIV suppression. It is notable that the CPUs are favorable for the multiple bluff body solutions compared to current published continuum mechanics models.
On Treatment of Nonlinear Damping for Occurrence of Damping-Controlled Fluid-Elastic Instability
Authors: Nakamura Tomomichi
Published: July 2008

An estimated damping ratio is used to evaluate the occurrence of the fluid-elastic instability. However, many structures contain some nonlinearity, especially in their damping ratios. Then, it is a great problem what damping value should be used for the estimation of the critical flow velocity. As the nonlinear damping usually shows an increasing trend with the vibration amplitude of tubes, vibrations of a tube with the negative damping due to the fluid flow are calculated for some parametric cases, where the structural damping component has a linear relation to the tube-amplitude. The results show the tube plays as a kind of limit cycle in a few seconds, and the final amplitude depends on the nonlinear trend of the structural damping. It is suggested that this nonlinear damping trend should be used for the design estimation of the fluid-elastic instability.
Fundamental Study on the Efficiency of Power Generation System by Use of the Flutter Instability
Authors: Matsumoto Masaru , Mizuno Keisuke , Okubo Kazumasa , Ito Yasuaki
Published: July 2008

Recently, Isogai et al.[1] proposed a new idea on power generation by use of flutter instability of thin airfoil. The heaving steady oscillation, which produces the electrical power from wind energy, can be excited by the forced torsional oscillation initially given to the heaving and torsional 2DOF system at any wind velocity ranges in his study. On the other hand, the authors have investigated the coupled flutter mechanism of various bluff bodies from the point of two different torsional and heaving branches by use of the “Step-by-Step flutter analysis method” (abbreviated by the SBS method).[2,3,4] Isogai’s idea is identical to the heaving branch flutter in the SBS method. Thus, the improvement of the power generation performance is investigated in this paper basing upon the flutter mechanism, and the efficiency of power generation by use of the heaving branch flutter was compared with that by the torsional branch flutter. It has been clarified that the obtained energy by the heaving oscillation would be larger than the supplied energy for giving initial torsional oscillation to the system and loss-energy caused by the structural damping and the aerodynamic damping over certain wind velocity range.
A Complete Model for Damping in Linear Systems
Authors: Goyder H. G. D.
Published: July 2008

Damping causes vibration to decay and prevents a resonant structure from having infinite vibration amplitudes. Damping is therefore an essential component in any vibration model. It is shown that the traditional model of a mass spring and viscous damper is incomplete. In general, an additional term must be added to the commonly used vibration models to account for damping effects. Examples of these additional terms are generated by using damping models that are mathematically complete and do not rely on experimental data. This approach provides a novel and purely theoretical method for tackling damping issues. Analysis of several simple systems enables a generic damping model to be proposed. Despite the increased complexity of the proposed damping model it emerges that viscous dampers, combined with springs are a widely applicable and correct damping model.
The Behaviour of Fluid-Conveying Pipes, Supported at Both Ends, by the Complete Extensible Nonlinear Equations of Motion
Authors: Modarres-Sadeghi Yahya , Pai¨doussis Michael P. , Camargo Alexandra
Published: July 2008

In this paper, the post-divergence behaviour of fluid-conveying pipes supported at both ends is studied using the complete extensible nonlinear equations of motion. The two coupled nonlinear partial differential equations are discretized via Galerkin’s method and the resulting set of ordinary differential equations is solved by Houbolt’s finite difference method and also using AUTO. Typically, the pipe is stable and retains its original static equilibrium position up to where it loses stability by a supercritical pitchfork bifurcation. By increasing the flow velocity, the amplitude of the buckled position increases, but no secondary instability can be observed thereafter, in agreement with Holmes’ results for his simplified model. The effect of different parameters on the behaviour of the pipe has been studied. By increasing the externally applied tension, or by increasing the gravity parameter, the critical flow velocity for the pitchfork bifurcation increases. The pitchfork bifurcation is subcritical if the nondimensional externally imposed tension, is greater than the nondimensional axial rigidity. The solution in the vicinity of the critical point for this case is confirmed to be subcritical, although the fold and the stable non-trivial solution thereafter could not be seen — perhaps because the model is correct to only third-order of magnitude. Dynamic instabilities may be possible for a pipe hinged at both ends but free to slide axially at the downstream end, according to preliminary results.
Correlation Between Vibration Excitation Forces and the Dynamic Characteristics of Two-Phase Flow in a Rotated Triangular Tube Bundle
Authors: Zhang C. , Pettigrew M. J. , Mureithi N. W.
Published: July 2008

Two-phase cross flow exists in many shell-and-tube heat exchangers. Flow-induced vibration excitation forces can cause tube motion that will result in long-term fretting-wear or fatigue. Detailed flow and vibration excitation force measurements in tube bundles subjected to two-phase cross flow are required to understand the underlying vibration excitation mechanisms. Some of this work has already been done. The distributions of both void fraction and bubble velocity in rotated-triangular tube bundles were obtained. Somewhat unexpected but significant quasi-periodic forces in both the drag and lift directions were measured. The present work aims at understanding the nature of such unexpected drag and lift quasi-periodic forces. An experimental program was undertaken with a rotated-triangular array of cylinders subjected to air/water flow to simulate two-phase mixtures. Fiber-optic probes were developed to measure local void fraction. Both the dynamic lift and drag forces were measured with a strain gage instrumented cylinder. The investigation showed that the quasi-periodic drag and lift forces are generated by different mechanisms that have not been observed so far. The quasi-periodic drag forces appear related to the momentum flux fluctuations in the main flow path between the cylinders. The quasi-periodic lift forces, on the other hand, are mostly correlated to oscillations in the wake of the cylinders. The relationships between the lift or drag forces and the dynamic characteristics of two-phase flow are established through fluid mechanics momentum equations. The quasi-periodic lift forces are related to local void fraction measurements in the unsteady wake area between upstream and downstream cylinders. The quasi-periodic drag forces correlate well with similar measurements in the main flow stream between cylinders.
Random Forces Resulting From Internal Two-Phase Flow on Bends and Tees
Authors: de Langre E. , Riverin J. L. , Pettigrew M. J.
Published: July 2008

The time dependent forces resulting from a two-phase air-water mixture flowing in an elbow and a tee are measured. Their magnitudes as well as their spectral contents are analyzed. Comparison is made with previous experimental results on similar systems. For practical applications a dimensionless form is proposed to relate the characteristics of these forces to the parameters defining the flow and the geometry of the piping.
Assessment of CFD Techniques for Wind Turbine Aeroelasticity
Authors: Liauzun Ce´dric
Published: July 2008

Two CFD techniques are assessed aiming at wind turbine aeroelasticity: a classic finite volume formulation to solve the Navier-Stokes equations, and a viscous-inviscid interaction formulation. Those 2 methods are tested with a NACA 634 – 421 airfoil, and both steady and unsteady numerical results are compared with experiments performed in the IAT wind tunnel. The Naviers-Stokes solver associated with the Wilcox k–ω turbulence model with SST corrections gives pretty good predictions for incidence in the range 0–15°, even for the static stall at 9°. The viscous-inviscid interaction solver provides results very close to the experiments for the whole range of incidence (0–30°). As far as unsteady simulations are concerned, both solvers give similar results. The computed lift evolutions versus incidence and excitation frequencies reproduce fairly well the experimental ones.
Flutter of Long Flexible Cylinders in Axial Flow
Authors: de Langre E. , Paidoussis M. P. , Modarres-Sadeghi Y. , Doare´ O.
Published: July 2008

We consider the stability of a thin flexible cylinder considered as a beam, when subjected to axial flow and fixed at the up-stream end only. A linear stability analysis of transverse motion aims at determining the risk of flutter as a function of the governing control parameters such as the flow velocity or the length of the cylinder. Stability is analysed applying a finite difference scheme in space to the equation of motion expressed in the frequency domain. It is found that, contrary to previous predictions based on simplified theories, flutter may exist for very long cylinders, provided that the free downstream end of the cylinder is well-streamlined. More generally, a limit regime is found where the length of the cylinder does not affect the characteristics of the instability, and the deformation is confined to a finite region close to the downstream end. These results are found complementary to solutions derived for shorter cylinders and are confirmed by linear computations using a Galerkin method. A link is established to similar results on long hanging cantilevered systems with internal or external flow. The limit case of vanishing bending stiffness, where the cylinder is modelled as a string, is analysed and related to previous results. A simple model for the behaviour of long cylinders is proposed.
A Numerical Investigation on the Dynamics of Two-Dimensional Cantilevered Flexible Plates in Axial Flow
Authors: Tang Liaosha , Pai¨doussis Michael P.
Published: July 2008

A cantilevered plate immersed in an otherwise uniform flow may lose stability at a high enough flow velocity; flutter takes place when the flow velocity exceeds a critical value. In the current research, a nonlinear equation of motion of the plate is developed using the inextensibility condition. Also, an unsteady lumped vortex model is used to calculate the pressure difference across the plate. The pressure difference is then decomposed into a lift force and an inviscid drag force. The fluid loads are coupled with the plate equation of motion and a numerical model of the fluid-structure system is developed. Analysis of the system dynamics is carried out in the time-domain. Both the stability and the post-critical behaviour of the system are studied. The flutter boundary and the vibration modes predicted by the current theory are found to be in good agreement with published experimental data.
Scaling the Vibrations of Water Pipes: Application to Industrial Case Studies
Authors: Moussou P. , Papaconstantinou Th. , Cohen J.
Published: July 2008

A scaling approach for the vibrations of water pipes was proposed in the ASME PVP 2005 Conference. Using a single hole orifice in non-cavitating conditions as a reference noise source, the vibrations of a piping system were estimated on the basis of plane wave propagation for the acoustics, and on the basis of vibrating beams for the structure. The scaling of the velocity Power Spectral Density (PSD) of a pipe is then a function of the pressure drop of the device considered, of the inner and outer diameters of the pipe, of the flow velocity, of the support spacing and of the densities of steel and water. The present paper describes the application of the scaling approach to industrial case studies: velocity PSD were measured on a sample of piping systems with pressures varying from 3 to 100 bars, with flows varying from 30 to 1000 m3/h and with hydraulic powers varying from 10 to 200 kW. The sources of vibrations were orifices, globe valves and butterfly valves in turbulent or cavitating regimes, and centrifugal pumps operating at nominal or partial flow regimes. In most cases, the measured data match the scaling law based on the single hole orifice noise, as an order of magnitude. A closer look reveals that different sources of vibrations exhibit recognizable patterns; the partial flow regime of a centrifugal pump generates a velocity PSD different from the nominal regime, and the cavitation of a globe valve generates a velocity PSD very different from the one generated by the cavitation of a single hole orifice. Typical non-dimensional spectra are proposed for each type of noise generating device.
Interference Between Two Tandem Cylinders of Different Diameters
Authors: Mahbub Alam Md. , Zhou Y.
Published: July 2008

This paper presents a detailed investigation of Strouhal frequencies, forces and flow structures resulting from the interference between two tandem cylinders of different diameters. The upstream cylinder diameter (d) was varied from 25 mm to 6 mm, while the downstream cylinder diameter (D) of 25 mm was unchanged, the corresponding diameter ratio, d/D, being 1.0 ∼ 0.24. The spacing between the cylinders was 5.5d. At this spacing, the shear layers separated from the upstream cylinder roll up alternately, forming a vortex street in the gap between and behind the cylinders. Two dominant frequencies are detected behind the downstream cylinder at d/D = 1.0 ∼ 0.4. One is the same as detected between the cylinders, and the other is of relatively low frequency and is probably generated by the downstream cylinder. Whilst the former remains unchanged, the latter increases with decreasing d/D, due to an increase in the incident flow velocity of the downstream cylinder. The time-averaged drag on the downstream cylinder also increases with decreasing d/D, though the fluctuating forces drop because vortices impinging upon the downstream cylinder are impaired by decreasing d/D.
Flutter of a Rectangular Cantilevered Plate
Authors: Eloy Christophe , Souilliez Claire , Schouveiler Lionel
Published: July 2008

We address theoretically the stability of a cantilevered rectangular plate in an uniform and incompressible axial flow. We assume that the fluid viscosity and the plate viscoelastic damping are negligible. In this limit, a flutter instability arises from a competition between the destabilising fluid pressure and the stabilising flexural rigidity of the plate. The flutter modes are assumed to be two-dimensional but the potential flow is calculated in three dimensions in the asymptotic limit of large plate span. Using a Galerkin method and Fourier transforms, we are able to predict the flutter modes, their frequencies and growth rates. The critical flow velocity is calculated as a function of the mass ratio and the plate aspect ratio. We demonstrate a new result: a plate of finite span is more stable than a plate of infinite span.
Vortex and Wake Effects on Closely Spaced Marine Risers
Authors: Germain G. , Gaurier B. , Le Boulluec M. , Fontaine E. , Capul J.
Published: July 2008

The present paper describes some ongoing research performed for a better understanding of the hydrodynamic loads acting on a riser placed in the wake of an upstream one. Experiments on model tests scaled with real configurations for dual riser interaction in uniform and steady current are presented. A particular attention is paid on how fluid interaction between two cylinders of equal diameter in tandem configuration can significantly modify their structural response in term of amplitude and frequency, compared to that of a single cylinder. The circulation water channel allows to reach Reynolds numbers from 5.5 103 to 5 104. Both in-line and cross-flow responses have been studied and are presented as functions of the reduced velocity. Observations demonstrate that wake effects can be relatively strong. If the dynamic of the upstream cylinder becomes to be well understood, the dynamic of the downstream one is hence much more complex and difficult to predict. When risers become in close proximity due to wake induced oscillations, collisions between cylinders can be observed. Comparisons between experimental and numerical results of the dynamics of a single riser in a flow are also presented.
Feedback Control of the Flutter of a Cantilevered Plate in an Axial Flow
Authors: Doare´ Olivier , Paes Thijs , Ferre´ Michel
Published: July 2008

We investigate the experimental control of the instabilities of a plate in an axial fluid flow. In absence of control, the plate is subjected to a flutter instability once a critical flow velocity is reached. In the present work, the objective of the feedback control is to increase the critical velocity and reduce the vibration amplitude once the flutter has appeared. Initially, the plate vibration and the action of the piezoelectric sensors is modelled in order to obtain a discrete state-space model of the controlled system. A Galerkin method is used, so that the discrete coordinates are the modal amplitudes of a beam when the flow velocity is zero. The action of the actuator is classically modeled as a momentum acting on the plate. To estimate the validity of the model, frequency response measurements are performed on the system. A good correspondence is found between the model and experiments. Dissipation coefficients are experimentally evaluated. Next, the feedback control loop design is investigated. As a first approach, a PI controller system is implemented. The controllability and stability limits of the closed loop system are investigated. We choose to implement experimentally this control, as it does not require an overly precise modelisation of the disturbances acting on the plate. Impulse response of the system without flow is performed to investigate the optimal control gain. Other tests are performed to show how the controller works against disturbances from a fluid flow. Despite the strong limitations that have been previously mentionned, some encouraging results have been found. The critical velocity is increased and the amplitude of vibration is lowered.
Cylinder Wake Dynamics in the Presence of Stream-Wise Harmonic Forcing
Authors: Rodriguez M. , Mureithi N. W.
Published: July 2008

The vortex wake flow dynamics downstream of a cylinder undergoing streamwise harmonic (fe/fs=1) forced oscillations has been investigated numerically using a CFD code for Re=1000. The steady-state of the wake flow has been analysed considering the amplitude of oscillations as a perturbation parameter. The resulting dynamics of the fluid lift and drag forces acting on the cylinder have been linked to the different vortex wake modes observed downstream of the cylinder. Forced oscillations lead to periodic, quasi-periodic and chaotic responses depending on the amplitude of oscillation of the cylinder. Different vortex wake patterns or modes (including 2S, P+S and S modes) have also been identified and described. Symmetry related bifurcations both in the computed fluid force dynamics as well as in the vortex wake patterns were identified. The key role played by spatio-temporal symmetry in the interaction between the wake flow and the oscillating cylinder has been elucidated by a Proper Orthogonal Decomposition (POD) of the wake velocity field. Symmetric and antisymmetric spatio-temporal modes were identified and bifurcations in the wake flow were explained in terms of mode interactions in the wake.
Phase Angle in Transient Regimes of Vortex-Induced Vibration: An Experimental Determination
Authors: Laneville Andre´ , Dallaire Pierre-Olivier
Published: July 2008

This article deals with an experimental determination of the relative phase angle between the shedding of vortices and motion in the case of the Aeolian vibrations of a flexible circular tube. The objective is to compare and determine if the phase value of a transient regime differs from that measured in a steady vibrating state. For each mode of vortex shedding (Von Ka´rma´n, 2S and 2P), the results show that the value of the phase angle in the unsteady and steady regimes is relatively similar if an appropriate dependency is selected: in the cases of the 2P and 2S modes of vortex shedding, this dependency is either the dimensionless amplitude or the ratio of the velocity of the structure motion to the oncoming flow velocity; in the case of the Von Ka´rma´n regime, at the onset of the instability, the usual reduced velocity is a better dependent variable. The analysis of jumps occurring in the instruments output reveals a boundary between the Von Ka´rma´n and the 2P modes of vortex shedding.
Characteristics of Two-Phase Flows in Vertical Pipe
Authors: Sim W. G. , Mureithi N. K. , Bae B. M. , Pettigrew M. J.
Published: July 2008

The characteristics of two-phase flow in a vertical pipe are investigated to gain a better understanding of vibration excitation mechanisms. An analytical model for two-phase flow in a pipe was developed by Sim et al. (2005), based on a power law for the distributions of flow parameters across the pipe diameter, such as gas velocity, liquid velocity and void fraction. An experimental study was undertaken to verify the model. The unsteady momentum flux impinging on a ‘turning tee’ (or a ‘circular plate’) has been measured at the exit of the pipe, using a force sensor. From the measured data, especially for slug flow, the predominant frequency and the RMS value of the unsteady momentum flux have been evaluated. It is found that the analytical method, given by Sim et al. for slug flow, can be used to predict the momentum flux.
Experimental Investigation of Coaxial Side Branch Resonators
Authors: Oshkai Peter , Yan Ting
Published: July 2008

Digital particle image velocimetry is employed to investigate acoustically-coupled flow past a coaxial deep cavity (side branch) resonator mounted in a duct. The emphasis is on the effect of the separation between the coaxial side branches on the interaction between separated shear layers that form across the side branch openings. Various resonator geometries are characterized in terms of patterns of instantaneous and time-averaged flow velocity, vorticity, and streamline topology at several phases of the acoustic cycle. In addition, phase-averaged images of the flow in conjunction with unsteady pressure measurements are evaluated in order to provide insight into the mechanisms of acoustic power generation. Generally speaking, the acoustic source undergoes a significant transformation as the distance between the coaxial side branches changes. When the side branches are located relatively far away from each other, each of them forms an independent acoustic source. As the distance between the side branches decreases, interaction between the associated oscillating shear layers results in formation of a single acoustic source of complex spatial structure.
Flow-Induced In-Line Oscillation of Two Circular Cylinders in Tandem Arrangement
Authors: Okajima Atsushi , Yasui Satoru , Kiwata Takahiro , Kimura Shigeo
Published: July 2008

Flow-induced in-line oscillation of two tandem circular cylinders has been experimentally studied by free-oscillation tests in a wind tunnel. Only one of the two cylinders was elastically supported easily to move in the in-line direction for reduced mass-damping parameter Cn ≈ 1; the other of the tandem cylinders was fixed to the tunnel sidewalls. The gap between upstream and downstream cylinders was changed from s = 0.3 to 3. We measured the response amplitudes of the oscillatory cylinder in the in-line direction and the vortex-shedding frequency in the wake. The flow around the tandem cylinders was visualized by the smoke-wire method. As a result of the upstream cylinder, there was a wide excitation region of reduced velocity, Vr = 1.5 to 2.5 in all ranges of gap ratios of s = 0.3 to 3. This excited oscillation was mainly induced by symmetric vortex shedding; the other excitation of Vr = 3 to 3.5 at wide gap ratios of s = 2 to 3, was induced by alternate Karman vortex shedding, as with the in-line oscillation of a single cylinder. The downstream cylinder had an excitation region in a wide range of Vr = 2 to 4 for the narrow gap distance of s = 0.3 to 0.75, which seems to be induced by alternate Karman vortex shedding. The other excitation regions due to symmetrical vortices were limited to the regions of gap distances of s = 0.75 to 2. Furthermore, the downstream cylinder oscillated as a buffeting phenomenon influenced by wake-fluctuation of the upstream cylinder, when gap distance was greater than s = 2.5.
Force Evolution Model for Vortex-Induced Vibration of an Elastic Cylinder in a Cross Flow
Authors: Wang X. Q. , Xie W.-C. , So R. M. C.
Published: July 2008

Vortex-induced vibration of a single circular cylinder is a fundamental and significant case of flow-induced vibrations in both engineering practice and academic research. A force evolution model is developed for vortex-induced vibration of an elastic cylinder in a cross flow. In this model, the elastic cylinder is represented by the Euler-Bernoulli beam theory, and the vortex-induced force acting on the stationary cylinder is modeled by a bounded-noise process. Fluid-structure interaction is represented by an evolution process based on the concept of quasi-steady flow. A numerical iterative approach is used to simulate the evolution process and obtain time histories of cylinder vibration and vortex-induced force. The model is validated against available measurements. It is shown that the proposed model can reproduce the salient features observed in experiments. Furthermore, quantitative agreement with experimental measurements is obtained in general. However, when the vibration amplitude is very large due to intense fluid-structure interaction, the proposed model prediction is not quite satisfactory. This suggests that the concept of quasi-steady flow is only applicable for relatively weak fluid-structure interaction.
A Time-Domain Method for the Vibration of Mistunued Bladed Disk Assemblies
Authors: Crouzet Fabien
Published: July 2008

A chained method to compute the single mode vibration of mistuned bladed disk assemblies is proposed. Aerodynamic coupling effects are first evaluated by computing unsteady aerodynamic coefficients for a quasi-3D geometry. The code is parallelized with a decomposition domain method and validated with the 4th Aeroelastic Standard Configuration. Aerodynamic coefficients are then included in a mass-spring model of the bladed disk assembly. The chained method is applied to the last row blades of a low pressure steam turbine in which localized high levels of vibrations have been observed. The phenomenon of vibration localization found by experiments is predicted with a reasonably good accuracy. Numerical results indicate that aerodynamic coupling is predominant and highly influences mistuning sensitivity. It is also shown that the mistuned row is very sensitive to structural damping. Intentional mistuning is used as a mean to reduce the amplitude magnification on the mistuned assembly. Modifying the frequency of every two blades of the assembly has proved to efficiently reduce localization. A very similar result is obtained by locally modifying the characteristics of a few blades located in high magnification zones.
Damping of Tubes With Internal Two-Phase Flow
Authors: Gravelle Alexandre , Ross Annie , Pettigrew Michel J. , Mureithi Njuki W.
Published: July 2008

Two-phase internal flow is present in many piping system components. Although two-phase damping is known to be a significant constituent of the total damping, the energy dissipation mechanisms that govern two-phase damping are not well understood. In this paper, damping of vertical clamped-clamped tubes subjected to two-phase air-water internal flow is investigated. Experimental data is reported, showing no dependence of two-phase damping on tube natural frequency, and a strong dependence on void fraction, flow velocity and flow regime. Two-phase damping increases with void fraction, reaches a maximum, and decreases beyond that point. The maximum damping ratio is roughly 3% for all flow velocities. It is reached at around 50% void fraction for high velocities, and 25% void fraction for low velocities. Data points plotted on two-phase flow pattern maps indicate that damping is greater in a bubbly flow regime than it is in a slug or churn regime. The maximum two-phase damping is reached at the highest void fraction before the transition to a slug or churn flow regime. It appears that two-phase damping may depend on the interface surface area between phases.
Flow Induced Pulsations Caused by Split Flow in a T-Branch Connection
Authors: Belfroid Stefan P. C. , Schiferli Wouter , Peters Rene M. C. A. M. , Buffing Joeri
Published: July 2008

In process installations with closed side branches, flow induced pulsations (FIP) are a well known phenomenon, which has been extensively studied. The cause of this phenomenon is vortex shedding in the T-connection between the main pipe and the side branch. In literature, only cases with zero flow into the side branch (‘cross flow’) have been studied and identified as a source of pulsations. In this paper, it is shown that with a small cross flow — either leaking into the main pipe or leaking out of the main pipe — also flow-induced pulsations can be excited by vortex shedding in the T-connection. By using a vortex blob method it has been demonstrated that cross flows even up to 25% of the main flow lead to significant FIP source strength. With increasing cross flow merging from the side branch into the main flow, the maximum source strength shifts to slightly lower Strouhal number due to the increased convective velocity of the vortex structures. The source strength decreases rapidly for increasing cross flow. As a rule of thumb, the source strength becomes negligible for cross flows larger than 25% of the total flow. The results of this analysis can be used in the design of process installations, in particular flow metering and mixing stations, where combinations of open and closed side branches occur, as well as merging flows.
Closed-Loop Control Vortex-Airfoil Interaction Noise
Authors: Zhang M. M. , Cheng L. , Zhou Y.
Published: July 2008

Closed-loop controlled interactions between an airfoil and impinging vortices were experimentally investigated. This work aims to minimize the fluctuating flow pressure (p) at the leading edge of the airfoil, which is a major source of the blade-vortex interaction noises commonly seen in rotorcrafts. Piezo-ceramic actuators were used to create a local surface perturbation near the leading edge of the airfoil in order to alter the airfoil-vortex interaction. Two closed-loop control schemes were investigated, which deployed p and the streamwise fluctuating flow velocity (u) as the feedback signal, respectively. While the control effect on p was measured using a fast response pressure transducer, the oncoming vortical flow was monitored using a particle image velocimetry and a hot wire. It was found that the control scheme based on the feedback signal u led to a pronounced impairment in the strength of oncoming vortices and meanwhile a maximum reduction in p by 39%, outperforming the control scheme based on the feedback signal p. Physics behind the observations is discussed.
Two-Side-by-Side Cantilevered Cylinders in a Cross Flow
Authors: Liu Y. , Cui Z. X.
Published: July 2008

Three dimensional flow over two side-by-side cantilevered cylinders is numerically studied for understanding the complex wake interference behind the cylinder pair. The two cylinders stand on the ground. The lattice Boltzmann method with multiple relaxation time is used for the solution of three dimensional unsteady flow. The simulations are carried out with Reynolds number Re = 200, aspect ratio L/D = 10 and four transverse pitch ratios: T/D = 1.2, 1.5, 2.0 and 3.0, where Re is defined by incoming velocity U, cylinder diameter D and kinetic viscosity v, L is cylinder span length and T is center to center spacing between the cylinder pair. Numerical results show that the wake patterns depend on not only T/D, but also the end flow conditions. Both the wake street and statistics of fluid force vary along the cylinder span considerably, indicating strong three dimensionality of the wake flows and their interaction.