Flow-induced Vibration Mechanisms Research Articles

Numerical analysis on heat transfer performance of spiral elastic copper tube heat exchanger with helical defectors

Based on the flow-induced vibration mechanism, three kinds of spiral elastic copper tube heat exchangers were proposed to improve the comprehensive heat transfer performance, involving the spiral elastic copper tube heat exchanger with no defector, the spiral elastic copper tube heat exchanger with forward helical deflector and the spiral elastic copper tube heat exchanger with reverse helical deflector. A both-way fluid–structure coupling method was utilized to investigate the effects of helical defectors and their arrangements on the vibration-enhanced heat transfer performance of the spiral elastic copper tubes at different inlet velocities by numerical simulation. The results indicate that: when the inlet velocity ranges from 0.1 m/s to 0.7 m/s, the heat exchanger with reverse helical deflector possesses better heat transfer performance than the other two heat exchangers. For the heat exchangers with no defector and forward helical deflector, the flow-induced vibration always promotes the comprehensive heat transfer performance. Compared with the heat exchanger with no defector, the comprehensive heat transfer performance of the heat exchanger with reverse helical deflector is higher 4.02 % averagely, while that of the heat exchanger with forward helical deflector is lower 29.34 % averagely. Furthermore, a structural optimization is presented to improve the existing disadvantages of the heat exchanger with reverse helical deflector, after structural optimization, the average amplitude of the heat exchanger with forward helical deflector is decreased by 3.26 % averagely and the heat transfer performance is raised to a maximum of 26.24 %, which are of great significance to extend the service life and improve the energy efficiency of heat exchanger.

Research on the Mechanism of Flow-Induced Vibration in the Cooling System of a Double Crystal Monochromator

To investigate the mechanism of flow-induced vibrations in the cooling system of a double crystal monochromator (DCM), this paper utilizes a multi-physics numerical simulation approach, employing ANSYS and FLUENT platforms to simulate the flow state of liquid nitrogen in the cooling system and explore the amplitude response of the DCM. Initially, simulations were conducted to examine the flow state of liquid nitrogen with varying frequency and amplitude pulsations. Subsequently, modal analysis was employed to investigate the amplitude response of the DCM in the pitch direction vibrations under pulsating excitation. Finally, this research investigated the influence of high heat load-induced liquid nitrogen boiling on a DCM. The results indicate that pipe resistance is the fundamental cause of vibration induced by pulsating excitation. Low-frequency excitation enhances the amplification factor of DCM vibration. In contrast, due to the rapid conversion of fluid kinetic energy to pressure potential energy, high-frequency excitation increases the pulsation amplitude in the pipe. Additionally, there is a linear relationship between the amplitude of liquid nitrogen velocity fluctuations and the response amplitude of a DCM. The slug flow formed after liquid nitrogen boiling generates low-frequency pulse signals, and intermittent fluid impacts cause significant vibrations in the DCM. These research findings provide a reference for the analysis and design of ultra-high-stability DCM cooling systems.

The influence of different downstream plate length towards the flow-induced vibration on a square cylinder

The investigations of flow-induced vibration have been around for decades to solve many engineering problems related to structural element. In a hindsight of advancing technology of microelectronics devices, the implementation of flow-induced vibration for energy harvesting is intrigued. The influence of downstream flat plate to flow-induced vibration experienced by a square cylinder is discussed in this study to surpass the limitation of wind energy due to geographical constraints and climate change. The mechanism of flow-induced vibration experienced by a square cylinder with downstream flat plate is numerically simulated based on the unsteady Reynolds Navier–Stokes (URANS) flow field. The Reynolds number, Re assigned in this study is ranging between 4.2 times 10^3–10.7 times 10^3 and the mass damping ratio designated for the square cylinder is m^*zeta = 2.48. The influence of three different flat plate lengths w/D = 0.5, 1 and 3 is examined. Each case of different flat plate is explored for gap separation between the square cylinder and the plate in the range 0.5 leqslant G/D leqslant 3. Based on the numerical findings, the configuration of cylinder-flat plate with length w/D = 1 has shown the highest potential to harvest high energy at comparatively low reduced velocity.

Fluidelastic instability of a rod bundle subjected to jet cross-flow

Jet flow emanating from loss of coolant accident (LOCA) holes in the baffle plates of some pressurized water reactors (PWRs) was found responsible for flow-induced vibration in nuclear fuel bundles. This could cause fretting wear of the fuel rods at the spacer grid location. Experimental tests on prototypical rod bundles subjected to jet cross-flow showed a strong vibration response akin to fluidelastic instability of tube arrays in uniform cross-flow. Vibrations similar to flow-periodicity resonance response have also been reported.Fluidelastic instability (FEI) is the most critical flow-induced vibration mechanism. For fuel rods subject to jet-cross flow, it is important to identify the specific mechanism underlying the rod vibrations and in particular, confirm possible fluidelastic instability. In previous work, a few stability boundary correlations based on the Connors equation were developed for the baffle jetting (i.e. 2D jet) problem in PWRs. With the exception of these correlations, no theoretical model accounting for fluid–structure interaction under jet cross-flow has been reported.To investigate possible fluidelastic instability, the quasi-steady model, initially developed for tube arrays in uniform cross-flow, is extended to the problem of a rod bundle subjected to a jet cross-flow. In the extended quasi-steady model, a new formulation of the fluidelastic forces as functions of the projected rod area derivative is proposed to account for the variation of the projected area through the LOCA hole (i.e. circular jet).A significant limitation of the ‘classical’ quasi-steady model is the poor representation of the time delay effect, known to be critical for correct fluidelastic instability prediction. This shortcoming is addressed in the present work. Furthermore, unsteady flow effects are partly taken into account experimentally measuring the time delay parameter for this jet-cross-flow configuration.For the present highly compact rod array, a new test apparatus is designed and built to directly measure the time delay parameter. The new results show that the time delay to be non-constant, strongly dependent on Reynolds number while relative independent of reduced velocity for Low Re. For High Re, the time delay depends on both Re and reduced flow velocity. Based on the new measurements, a formulation for the time delay parameter, dependent on Reynolds number and reduced velocity is proposed for the highly packed fuel rod bundle.The new extended quasi-steady model conclusively confirms that the vibration phenomenon observed in experimental tests is clear fluidelastic instability induced by the jet cross-flow. Moreover, the model predicts the critical jet velocity for fluidelastic instability with an error of 9% when the new Reynolds number dependent time delay formulation is employed.

A three-dimensional refined numerical simulation of cross-flow induced vibration mechanism in the tube bundle

Cross-flow induced vibration causes many accidents in the nuclear steam generator tube bundle. An accurate and refined cross-flow induced vibration numerical simulation can help avoid accidents in the designed bundle for different geometries, and provide insights into flow-induced vibration mechanism. In this paper, a three-dimensional refined numerical simulation model considering the fluid viscous damping was established and validated. First, we validated the simulation of the viscous damping that structure suffered in the fluid, and studied the relationship between the fluid viscous damping and the structural spring damping in tube bundle coupling with fluid. The results showed that fluid viscous damping coefficient and the structural spring damping coefficient can be directly added to equal the total damping coefficient in the coupling system between the tube bundle and fluid. Then, a three-dimensional simulation based on SST-SAS (the Transition SST combined with the Scale Adaptive Simulations (SAS) turbulence model) was used to study flow-induced vibration for the literature experiment tube bundle. Simulation of flow-induced vibration had the same mass-damping parameters with the literature experiment. The vibration amplitude trend and spectra of the simulation were basically consistent with that of the experiment. Simulation accurately captured the onset of the vortex-induced resonance and the transition from the vortex-induced resonance to the fluidelastic instability. And the different mechanisms and their transition were analyzed. Specially, the flow field and structure analysis showed that the local vortex-induced resonance of the front-row tubes caused the synchronous resonance of the whole tube bundle.

Fluid–structure-coupled Koopman mode analysis of free oscillating twin-cylinders

Flow-induced vibration (FIV) of twin square cylinders in a tandem arrangement was numerically investigated at Reynolds numbers 200 and gap L/D = 2.0, 4.0, and 6.0 (D is the side length of the cylinders). Fluid-structure-coupled Koopman mode analysis method was developed to synchronously identify the coherence flow and structural modes. Then, the energy transfer between cylinders and Koopman modes was analyzed to uncover the underlying mechanism of FIV. The results showed that at L/D = 2.0 and 4.0, only soft lock-in vortex-induced vibration (VIV) was observed. The oscillating amplitude for L/D = 4.0 was much higher than that of L/D = 2.0, due to the interference effects induced by fully developed gap vortices. As L/D = 6.0, VIV and galloping coexisted. For the coherence mode, the primary flow mode induced by the vortex shedding dominated the flow field at L/D = 2.0 and 4.0. The direct mode energy dominated the energy transfer process. The upstream cylinder (UC) contributed to the negative work done and thus tended to stabilize the vibration; in contrast, the downstream cylinder (DC) exhibited the opposite behavior. In the galloping branch at L/D = 6.0, both the flow field and structural response contained three main modes: one vortex-shedding-induced mode and two vibration-induced modes. For the direct mode energy, owing to the interference effects, DC contributed to more positive work done than UC by the vibration-induced modes. The vortex-induced mode was governed by DC and afforded negative work done. Moreover, all the coupled mode energy was almost equal to zero.

Fluid-structure interaction of a flexible cantilever cylinder at low Reynolds numbers

Mammals such as rats and seals can sense their surroundings with their whiskers. This work examines the coupled dynamics of a flexible cantilever cylinder in the laminar subcritical Reynolds regime to pinpoint the underlying mechanism of flow-induced vibration in sensory whiskers. We find that when certain conditions are satisfied, the cylinder undergoes sustained oscillations due to the frequency lock-in phenomenon in this Reynolds regime. The knowledge generated through this work helps clarify the role of flow-induced vibrations in sensing via whiskers and has relevance to the development of innovative bio-inspired flow-sensing devices for next-generation robotic vehicles.

A novel methodology for design optimization of heat recovery steam generators with flow-induced vibration analysis

Design of compact, lightweight, and robust heat recovery steam generators (HRSGs) is important for the implementation of offshore bottoming cycles. In this work, a novel methodology integrating flow-induced vibration analysis is proposed for the design optimization of HRSG. The vibration analysis considers four main flow-induced vibration mechanisms, namely turbulent buffeting, vortex shedding, fluidelastic instability, and acoustic resonance. The corresponding sub-models are selected from literature for estimating vibrations of finned tube bundles. The design criteria for vibration are included in the optimization problem as constraints, which affect the decision of the solver simultaneously. The methodology is demonstrated by three cases, in which not only geometric parameters but also operating parameters are set as design variables. With activating vibration constraints, the solver finds appropriate geometric and operating parameters to meet the design requirements. The comparison between the optimizations with and without the vibration constraints shows a trade-off between compactness, lightweight, efficient heat transfer, and the possibility of flow-induced vibration problems. This indicates the importance of consideration of vibration problems when designing such HRSGs.

Dynamic responses and flow-induced vibration mechanism of three tandem circular cylinders in planar shear flow

Numerical study is conducted on flow-induced vibrations (FIVs) of three circular cylinders arranged in tandem with spacing ratio L/D = 5.5 in planar shear flow. The cylinders are free to vibration in both in-line and transverse directions. A four-step semi-implicit Characteristic-based split (4-SICBS) finite element method is adopted to solve the governing Navier-Stokes equations. The mass ratio of each cylinder is set as mr = 2.0, and the structural damping ratio as zero to maximize the flow-induced responses of the cylinders. The effects of three main parameters, such as the Reynolds number (Re = 80, 120, 160), the shear ratio (k = 0.0, 0.05, 0.1), and the reduced velocity (Ur = 3–21), are considered. The changes of vibration amplitudes, frequency characteristics, motion trajectories and flow characteristics are analyzed. The results show that, the dynamic responses of the upstream cylinder are similar to those of an isolated cylinder. The Re, k and Ur play a more important role in the FIVs of the midstream and downstream cylinders. As Re ≤ 120, the dynamic responses of the cylinders change slightly with the increasing of Re, while they present a dramatic increase because of the unstable flow characteristic at Re = 160. Additionally, with the increasing of Re, the vibration response mechanism of the cylinders in the in-line direction will change from resonance phenomenon into the vortex interference. For the motion orbits of the circular cylinders, in additional to the eight-figure, raindrop, oval and irregular shape, the dual-raindrop, dual-eight and dual-oval sharp are observed. Finally, regarding the flow field, it is found that the interference of the wake vortex is highly affected by the shear ratio of the incoming flow, resulting in great changes on the wake pattern and dynamic responses. Especially, the “2T”wake pattern is observed.

Two-phase flow induced vibration of piping structure with flow restricting orifices

In this paper, two-phase flow-induced vibration in a horizontal piping structure with flow restricting orifices has been experimentally investigated. The effect of flow patterns, void fraction, mass quality and orifice area ratio on vibration response is evaluated in detail. The two-phase flow measurements and vibration signals were analyzed simultaneously in order to understand the effect of two-phase flow behaviour on the governing factors responsible for flow-induced vibration mechanisms. The results show that the dynamic vibration response of the structure increased with the increase in the liquid mass flux and upstream void fraction in the case of intermittent flow patterns. The flow pattern change across the orifice is found to strongly affect the magnitude of the resulting vibrations. The maximum response was found in the region where a transition to slug flow occurred. Also, the RMS amplitude of vibration increased with an increase in the gas flux for both stratified and annular flows. Flow pattern maps were therefore created for both horizontal and vertical dynamic responses. These maps can be used in evaluating the dynamic response of piping structures with flow restrictions which are commonly found in the power generation industry. This will allow for a better design and safe operation of systems suffering from multiphase flow-induced vibration.

Numerical and Experimental Comparative Study on the Flow-Induced Vibration of a Plane Gate

A numerical method is applied here to simulate the unstable flow and the vibration of a plane gate. A combination of the large eddy simulation (LES) method and the volume of fluid (VOF) model is used to predict the three-dimensional flow field in the vicinity of a plane gate with submerged discharge. The water surface profile, the streamline diagrams, the distribution of turbulent kinetic energy, the power spectrum density curve of the fluctuating pressure coefficient at typical points underneath the gate, and the complete vortex distribution around the gate are obtained by LES-VOF numerical calculation. The vibration parameters of the gate are calculated by the fluid-structure coupling interface transferring the hydrodynamic load. A simultaneous sampling experiment is performed to verify the validity of the algorithm. The calculated results are then compared with experimental data. The difference between the two is acceptable and the conclusions are consistent. In addition, the influence of the vortex in the slot on the flow field and the vibration of the gate are investigated. It is feasible to replace the experiment with the fluid-structure coupling computational method, which is useful for studying the flow-induced vibration mechanism of plane gates.

Investigation of the Vibration Behavior of Fluidelastic Instability in Closely Packed Square Tube Arrays

Flow-induced vibrations in heat exchanger tubes have led to numerous accidents and economic losses in the past. Fluidelastic instability is the most critical flow-induced vibration mechanism in heat exchangers. Both experimental and computational studies conducted to determine fluidelastic instability were presented in this paper. In the experiment, a water channel was built, and a closely packed normal square tube array with a pitch-to-diameter ratio of 1.28 was tested, and significant fluidelastic instability was observed. A numerical model adopting large-eddy simulation and moving mesh was established using ANSYS CFX, and results showed good agreement with the experimental findings. The vibration behaviors of fluidelastic instability were discussed, and results showed that the dominant vibration direction of the tubes changed from streamwise to transverse beyond a critical velocity. A 180° phase lag between adjacent tubes was observed in both the experiment and simulations. Normal and rotated square array cases with pitch-to-diameter ratios of 1.28 and 1.5 were also simulated. The results of this study provide better insights into the vibration characteristics of a square tube array and will help improve the fundamental research and safety design of heat exchangers.

Flow-induced vibration on a circular cylinder in planar shear flow

In this paper, the flow-induced vibrations of an elastically mounted circular cylinder subjected to the planar shear flow with the 1-DOF (only transverse direction) and 2-DOF (in-line and cross-flow directions) movements are studied numerically in the laminar flow (Re=150). Based on a characteristic-based-split (CBS) finite element method, the numerical simulation is conducted, and is verified through the benchmark problem of the uniform flow past an elastically mounted circular cylinder. The computation is carried out for lower reduced mass of Mr=2.0 and the structural damping ratio is set to zero to maximize the vortex-induced response of the cylinder. The effects of some key parameters, such as shear rate (k=0.0–0.1), reduced velocity (Ur=3.0–12.0) and natural frequency ratio (r=1.0–2.0), on the characteristics of vortex-induced vibration (VIV) responses are studied. The results show that, in the 1-DOF system, the frequency synchronization region extends with the increasing of k. The shear rate greatly affects the phase portraits, which shift from the double-valued type to the single-valued one. On the other hand, in the 2-DOF system, the increasing of k causes the extension of the single-resonant region and dual-resonant one at the lower natural frequency ratios. While at the higher natural frequency ratios, the change of k only expands the single-resonant region in the transverse direction. The predominant vortex shedding patterns are 2S and P+S modes. Finally, the interaction between vortex and cylinder as well as the mechanism of flow-induced vibration in planar shear flow are revealed. The phase between the force and its corresponding displacement changes from out-of-phase to in-phase and the higher harmonic forces appear with the increasing of shear rate, resulting in the energy transferring from the fluid to the structure and then the dynamic response of the cylinder intensifying.

Modeling of the phase lag causing fluidelastic instability in a parallel triangular tube array

Fluidelastic instability is considered a critical flow induced vibration mechanism in tube and shell heat exchangers. It is believed that a finite time lag between tube vibration and fluid response is essential to predict the phenomenon. However, the physical nature of this time lag is not fully understood. This paper presents a fundamental study of this time delay using a parallel triangular tube array with a pitch ratio of 1.54. A computational fluid dynamics (CFD) model was developed and validated experimentally in an attempt to investigate the interaction between tube vibrations and flow perturbations at lower reduced velocities Ur=1–6 and Reynolds numbers Re=2000–12000. The numerical predictions of the phase lag are in reasonable agreement with the experimental measurements for the range of reduced velocities Ug/fd=6–7. It was found that there are two propagation mechanisms; the first is associated with the acoustic wave propagation at low reduced velocities, Ur<2, and the second mechanism for higher reduced velocities is associated with the vorticity shedding and convection. An empirical model of the two mechanisms is developed and the phase lag predictions are in reasonable agreement with the experimental and numerical measurements. The developed phase lag model is then coupled with the semi-analytical model of Lever and Weaver to predict the fluidelastic stability threshold. Improved predictions of the stability boundaries for the parallel triangular array were achieved. In addition, the present study has explained why fluidelastic instability does not occur below some threshold reduced velocity.

Fluidelastic Instability Modeling of Loosely Supported Multispan U-Tubes in Nuclear Steam Generators

Steam generators in nuclear power plants have experienced tube failures caused by flow-induced vibrations. Two excitation mechanisms are responsible for such failures; random turbulence excitation and fluidelastic instability. The random turbulence excitation mechanism results in long-term failures due to fretting-wear damage at the tube supports, while fluidelastic instability results in short-term failures due to excessive vibration of the tubes. Such failures may require shutdowns, which result in production losses, and pose potential threats to human safety and the environment. Therefore, it is imperative to predict the nonlinear tube response and the associated fretting-wear damage to tubes due to fluid excitation. In this paper, a numerical model is developed to predict the nonlinear dynamic response of a steam generator with multispan U-tubes and anti-vibration bar supports, and the associated fretting wear due to fluid excitation. Both the crossflow turbulence and fluidelastic instability forces are considered in this model. The finite element method is utilized to model the vibrations and impact dynamics. The tube bundle geometry is similar to the geometry used in CANDU steam generators. Eight sets of flat-bar supports are considered. Moreover, the effect of clearances between the tubes and their supports, and axial offset between the supports are investigated. The results are presented and comparisons are made for the parameters influencing the fretting-wear damage, such as contact ratio, impact forces, and normal work rate. It is clear that tubes in loose flat-bar supports have complex dynamics due to a combination of geometry, tube-to-support clearance, offset, and misalignment. However, the results of the numerical simulation along with the developed model provide new insight into the flow-induced vibration mechanism and fretting wear of multispan U-tubes that can be incorporated into future design guidelines for steam generators and large heat exchangers.

A REVIEW OF NUMERICAL RESEARCH ON AEROELASTIC DYNAMICAL RESPONSE OF AERO-ENGINE BLADES

The high cycle fatigue failure of aero-engine blades occurs frequently. Aeroelastic dynamic response is a major concern in engineer applications. A review of numerical researches on blade aeroelastic dynamic response is presented in the paper, including numerical simulation, test study and possible research trends. The flow-induced vibration mechanism of blade dynamic response and the numerical method about fluid-structure interaction will be the focus in further mechanism study. For engineer applications, it is valuable to develop a prediction model for dynamic response and to evaluate its potential risks with resonance in the design process. And more attention should be paid on the aerodynamic damping technology based on flow control.

The interaction between flow-induced vibration mechanisms of a square cylinder with varying angles of attack

AbstractThis study examines the influence of angle of attack of a square section cylinder on the cylinder’s flow-induced vibration, where the direction of the vibration is transverse to the oncoming flow. Our experiments, which traversed the velocity–angle of attack parameter space in considerable breadth and depth, show that a low-mass ratio body can undergo combinations of both vortex-induced vibration and galloping. When the body has an angle of attack that makes it symmetric to the flow, such as when it assumes the square or diamond orientation, the two mechanisms remain independent. However, when symmetry is lost we find a mixed mode response with a new branch of vortex-induced oscillations that exceeds the amplitudes resulting from the two phenomena independently. The oscillations of this higher branch have amplitudes larger than the ‘upper branch’ of vortex-induced vibrations and at half the frequency. For velocities above this resonant region, the frequency splits into two diverging branches. Analysis of the amplitude response reveals that the transition between galloping and vortex-induced vibrations occurs over a narrow range of angle of incidence. Despite the rich set of states found in the parameter space the vortex shedding modes remain very similar to those found previously in vortex-induced vibration.

A single flexible tube in a rigid array as a model for fluidelastic instability in tube bundles

Fluidelastic instability is considered the most critical flow induced vibration mechanism in tube and shell heat exchangers, and as such has received the most attention. The present study examines the concept of using a single flexible tube in a rigid array for predicting fluidelastic instability. The experimental work published in the open literature involving the use of a single flexible tube in a rigid array is critically reviewed. Based on this, an experiment is designed to facilitate precise control of the system parameters and to study tube response at different locations in the array. Experiments were conducted using a fully flexible array as well as a single flexible tube in the same rigid array. It is found that a single flexible tube located in the third row of a rigid parallel triangular array becomes fluidelastically unstable at essentially the same threshold as for the fully flexible array. However, when the single flexible tube is located in the first, second, fourth, or fifth rows, no instability behavior is detected. Thus, tube location inside the array significantly affects its fluidelastic stability behavior when tested as a single flexible tube in a rigid array. It is concluded that, in general, fluidelastic instability in tube arrays is caused by a combination of the damping and stiffness mechanisms. In certain cases, a single flexible tube in a rigid array will become fluidelastically unstable and provide a useful model for fundamental research and developing physical insights. However, it must be cautioned that this behavior is a special case and not generally useful for determining the stability limit of tube arrays.

Flow induced vibration studies on PFBR control plug components

The construction of Prototype Fast Breeder Reactor (PFBR), a 500MWe liquid sodium cooled reactor, is in progress at Kalpakkam in India. Control plug (CP) is located right above the core subassemblies in the hot pool. Control plug is an important component as many of the critical reactor parameters are sensed and controlled by the components housed in the control plug assembly. In PFBR primary circuit, components are basically thin walled, slender shells with diameter to thickness ratio ranging from 100 to 650. These components are prone to flow induced vibrations. The existence of free liquid (sodium) surfaces, which is the source of sloshing phenomenon and the operation of primary sodium pump in the primary pool are other potential sources of vibration of reactor components.Control plug is a hollow cylindrical shell structure and provides passages and support for 12 absorber rod drive mechanisms (ARDM) which consists of 9 control and safety rods and 3 diverse safety rods, 210 thermo wells to measure the sodium temperature at the exit of various fuel subassemblies, three failed fuel localization modules (FFLM) and acoustic detectors. It consists of a core cover plate (CCP), which forms the bottom end, two intermediate supports plate, i.e. lower stay plate (LSP) and upper stay plate (USP) and an outer shell. The CCP is located at a distance of 1.3m from the core top. With such a gap, there will be long free hanging length of the thermocouple sleeves, Delayed neutron detector (DND) sampling tubes and ARDM shroud tubes and hence they are susceptible to flow induced vibrations. Since control plug is partially immersed in hot sodium, the reactor transients are felt by the components, hence it is very much essential to understand the vibration response of the control plug components. The main vibration excitation mechanisms are vortex shedding, turbulence buffeting, fluid-elastic instability, etc.In order to assess the susceptibility of CP internals against flow induced vibrations (FIVs), to measure structural response and to validate the analytical codes developed for FIV analysis of CP along with the assumptions of supports for various CP internals therein, a flow induced vibration testing program was formulated in 1:4 scale model of PFBR CP. As the first phase of this program, experimental modal analysis of CP internals was carried out in air to estimate the modal parameters. Subsequently, flow induced vibration studies were conducted in the 1:4 scale model of CP in water. In this model, size of the tubes, shell and plates are reduced to ¼ size of the PFBR CP. The stiffness of CP parts is reduced by 4 times and mass decreased by 64 times which results in scaling up of modal frequencies by 4 times. The CP internals in the model were instrumented with accelerometers and strain gages. The studies were conducted in water with flows derived on the basis of velocity similitude. The output signals from the sensors were acquired and analyzed to obtain frequency spectra, overall vibration amplitude and strain values at various locations inside CP. The study carried out confirms the absence of resonance due to flow induced vibration mechanisms for the entire operating range. This paper elucidates the modeling details, similitude criteria, instrumentation employed and experimental results obtained with discussion on results.

A Study on Characteristics of Flow-Induced Vibrations of Two Circular Cylinders in Staggered Arrangement

This study investigated on the characteristics of flow-induced vibrations of two circular cylinders in staggered arrangement. The response characteristics on elastically supported two circular cylinders were examined by a free vibration test. Measurements were performed for stagger angle α=5°, 10°, 15°, 25°, 45°and 60°, and the spacing between two cylinders L/D=0.1-3.2, where α is the angle between the free-stream flow and the line connecting the centers of two cylinders, L is the gap width between two cylinders, and D is the diameter of two cylinders. The new findings for characteristics of flow-induced vibrations of two cylinders in staggered arrangement are as follows. (i) There were seven patterns of flow-induced vibration with changing stagger angle a and spacing ratio L/D, (ii) the generation mechanism of flow-induced vibration was clarified by doing flow visualization test and generation regions of flow-induced vibration at each stagger angle were arranged, (iii) when the cylinder was perturbed at stagger angle 10 and 15 degrees, flow-induced vibration genetated two cylinders was a divergent vibration, (iv) there were two patterns of flow-induced vibration at critical spacing L/D in which exist two flow patterns.