Flow induced vibration

A new numerical approach for modeling a class of flow-structure interaction problems typically encountered in biological systems is presented. In this approach, a previously developed, sharpinterface, immersed-boundary method for incompressible flows is used to model the fluid flow and a new, sharp-interface Cartesian grid, immersed boundary method is devised to solve the equations of linear viscoelasticity that governs the solid. The two solvers are coupled to model flow-structure interaction. This coupled solver has the advantage of simple grid generation and efficient computation on simple, single-block structured grids. The accuracy of the solid-mechanics solver is examined by applying it to a canonical problem. The solution methodology is then applied to the problem of laryngeal aerodynamics and vocal fold vibration during human phonation. This includes a threedimensional eigen analysis for a multi-layered vocal fold prototype as well as two-dimensional, flowinduced vocal fold vibration in a modeled larynx. Several salient features of the aerodynamics as well as vocal-fold dynamics are presented.

This paper presents an electromechanical dynamic modelling of the partially smart pipe structure subject to the vibration responses from fluid flow and input base excitation for generating the electrical energy. We believe that this work shows the first attempt to formulate a unified analytical approach of flow-induced vibrational smart pipe energy harvester in application to the smart sensor-based structural health monitoring systems including those to detect flutter instability. The arbitrary topology of the thin electrode segments located at the surface of the circumference region of the smart pipe has been used so that the electric charge cancellation can be avoided. The analytical techniques of the smart pipe conveying fluid with discontinuous piezoelectric segments and proof mass offset, connected with the standard AC–DC circuit interface, have been developed using the extended charge-type Hamiltonian mechanics. The coupled field equations reduced from the Ritz method-based weak form analytical approach have been further developed to formulate the orthonormalised dynamic equations. The reduced equations show combinations of the mechanical system of the elastic pipe and fluid flow, electromechanical system of the piezoelectric component, and electrical system of the circuit interface. The electromechanical multi-mode frequency and time signal waveform response equations have also been formulated to demonstrate the power harvesting behaviours. Initially, the optimal power output due to optimal load resistance without the fluid effect is discussed to compare with previous studies. For potential application, further parametric analytical studies of varying partially piezoelectric pipe segments have been explored to analyse the dynamic stability/instability of the smart pipe energy harvester due to the effect of fluid and input base excitation. Further proof between case studies also includes the effect of variable flow velocity for optimal power output, 3-D frequency response, the dynamic evolution of the smart pipe system based on the absolute velocity-time waveform signals, and DC power output-time waveform signals.

Experiments have been carried out on a pair of circular cylinders to investigate the effectiveness of pivoting parallel plates as wake-induced vibration suppressors. Measurements of amplitude of vibration and average drag are presented for a circular cylinder, free to respond in the cross-flow direction, with mass ratio 2 and a damping level of 0.7% of critical damping. Reduced velocities were up to nearly 30, with associated Reynolds numbers up to 2.3 Â 10 4 and the results presented are for a centre-to-centre separation of cylinders of 4 diameters. It is shown how vortex-induced vibration and wake-induced vibration of the downstream cylinder of a tandem pair can be practically eliminated by using free to rotate parallel plates. The device achieves vibration suppression with a substantial drag reduction when compared to a pair of fixed tandem cylinders at the same Reynolds number. Results for a single splitter plate and helical strakes are also presented for comparison and were found not to be effective in suppressing wake-induced vibration.

The wake-induced vibration (WIV) of two staggered cylinder with two degrees of freedom (2-dof) has been investigated by experiments in a water channel for Reynolds number between 2000 and 25 000. The streamwise separation was fixed to 4 diameters and the lateral separation varied between 0 and 3 diameters for tandem and staggered configurations. Results are presented in the form of trajectories of motion and dynamic response curves of displacements, frequencies and force coefficients. Excitation caused by the WIV mechanism is found to get weaker as the initial position of the downstream cylinder is increased from the centreline of the wake (tandem arrangement) towards the sides. For a lateral separation of 3 diameters wake interference was already found to be negligible. Evidence of a type of wake-stiffness concept is also observed to occur for 2-dof WIV in tandem arrangement, especially for higher reduced velocities. A similar mechanism may also be occurring for staggered arrangements around the centreline.

The effect of base suction on flapping and energy harvesting through the piezoelectric membrane in the wake of a cylindrical bluff body is studied in this paper by carrying out a series of wind tunnel experiments. The effect of S/D (ratio of the distance between the cylinder and membrane to the cylinder diameter), L/D (ratio of the membrane length to the cylinder diameter), and flow speed on the flapping dynamics and energy generation from the polyvinylidene fluoride membrane placed behind the bluff body was studied. Different flapping modes were found from optimal coupling to fully deformed. The flapping motion of the membrane was determined by using a high-speed camera, and an oscilloscope was used for the measurement of the generated voltages. Also, the flapping dynamics and output voltages were extensively studied in the sub and post-critical regions by varying the S/D ratio, L/D ratio, and flow velocity. An increase of 38% in energy harvesting is observed when the piezomembrane was placed at L/D ¼ 1 and S/D ¼ 2 with free stream velocity U ¼ 10 m/s. An increase or decrease in voltages is attributed to the observation that the amount of harvested energy changes by varying the stated parameters. Optimum energy can be harvested by fine-tuning of flow and geometrical parameters and adjusting the piezoelectric flag in the specified range. Published under license by AIP Publishing. https://doi.

The galloping response of a circular cylinder fitted with three different splitter plates and free to oscillate transverse to a free stream has been investigated considering variations in plate length and plate porosity. Models were mounted in a low mass and damping elastic system and experiments have been carried out in a recirculating water channel in the Reynolds number range of 1500 to 16 000. Solid splitter plates of 0.5 and 1.0 diameter in length are shown to produce severe galloping responses, reaching displacements of 1.8 diameters in amplitude at a reduced velocity of around 8. Fitting a slotted plate with a porosity ratio of 30% also caused considerable vibration, but with a reduced rate of increase with flow speed. All results are compared with the typical vortex-induced vibration response of a plain cylinder. Force decomposition in relation to the body velocity and acceleration indicates that a galloping mechanism is responsible for extracting energy from the flow and driving the oscillations. Visualisation of the flow field around the devices performed with PIV reveal that the reattachment of the free shear layers on the tip of the plates is the hydrodynamic mechanism driving the excitation.

Experiments in the Reynolds number range of 1000 to 12,000 have been carried out on a free-to-rotate short-tail fairing fitted to a rigid length of circular cylinder to investigate the effect of rotational friction on the stability of this type of VIV suppressor. Measurements of the dynamic response are presented for models with low mass and damping which are free to respond in the cross-flow and streamwise directions. It is shown how vortex-induced vibration can be reduced if the rotational friction between the cylinder and the short-tail fairing exceeds a critical limit. In this configuration the fairing finds a stable position deflected from the flow direction and a steady lift force appears towards the side to which the fairing has deflected. The fluid-dynamic mechanism is very similar to that observed for a free-to-rotate splitter plate of equivalent length. A non-rotating fairing as well as splitter plates is shown to develop severe galloping instabilities in 1-dof experiments.

A three-dimensional computational fluid dynamics (CFD) boiling model is proposed in this paper to investigate the impacts of changing operating conditions on the thermal-hydraulic characteristics in the secondary side of steam generator (SG). This model encompasses the mass, momentum, energy equations, turbulent models, and property variation model for the two-phase mixture. The changes of SG operating conditions selected in current works mainly include the decrease of inlet temperature and the increase of inlet flowrate, respectively. Based on the simulation results, the local velocity, temperature, enthalpy, and void fraction distributions, etc in the SG secondary side can be reasonably captured. With these localized thermal-hydraulic distributions, the fluid energies across the SG tubes of U-bend region can be also obtained, which are strongly associated with the fluid-induced vibration (FIV) damage. Comparisons of these key parameters between different conditions can clearly demonstrate the impacts of changing SG operation conditions. Therefore, this CFD boiling model can provide the useful information for the plant staff to take the appropriate actions for the alleviation of SG tube degradation or elevation of the operating power.

Flow induced vibration of tube bank structures, could has the harmful effects on a system. In the current work vibration of cantilever tube in the present of fluid flow is investigated. For this purpose the cantilever tube modeled as a simple mass-spring system. The numerical results are compared with Blevins and Coughran works (J. Fluids eng., vol.131, 2009), for reduced velocity between 5 and 8, the mass ratio of 5.02 and the structural damping ratio of 0.05. Numerical results predict the range of reduced velocity, which the amplitude of vibrations raises, but, the upper branch of amplitude is in lower values than in experimental. Finally variation of amplitude, L C and D C versus time, and vortex shedding are plotted.

This paper deals with both linear and nonlinear analyses of sheet flutter in a narrow passage caused by fluid-structure interaction. The sheet is considered as a combination of massless beam elements, springs and discrete mass particles, in which the mass of each particle and spring coefficients are calculated based on the beam model. In the linear analysis, equations of motion of the system are obtained by formulating fluid dynamic forces and moments acting on each sheet element, yielding an eigenvalue problem. To show the validity of the proposed method, calculated results were compared with previously reported experimental ones. In the nonlinear analysis, nonlinear fluid dynamic forces are introduced to simulate the behavior of the sheet, showing the appearance of limit-cycle vibration in high flow speed. r

The wake-induced vibration (WIV) of the downstream cylinder of a tandem pair is investigated for different diameter ratios of D 1 =D 2 ¼ 1=1, 1/2 and 1/3, where D 1 and D 2 refer to the upstream and downstream cylinders, respectively. The streamwise separation between the cylinders was L=D 1 ¼ 3:5, 7.0 and 6.5, respectively, measured from the centre of the upstream cylinder to the forward stagnation point of the downstream cylinder. Experiments with low mass-damping cylinders have been conducted in a water channel at around Re¼ 25 000. The dynamic response showed that the downstream cylinder experienced WIV for all diameter ratios investigated, with displacement amplitudes reaching more than 1.5 diameters for higher reduced velocities beyond the vortex resonance range. The frequency response showed a similar behaviour for all three configurations, giving hints that a type of wake-stiffness mechanism might be governing the frequency of oscillation for all diameter ratios. The response was found to be dependent on both D 1 =D 2 and L=D 1. In all cases, the static upstream cylinder was found to shed vortices as an isolated cylinder, not influenced by the presence or movement of the downstream body. Lift and drag coefficients as well as measurements of velocity fluctuations in both wakes are presented for all cases.

The aerodynamic response of a circular cylinder to nonharmonic forcing of the inflow velocity is studied by numerically solving the two-dimensional Navier-Stokes equations on an orthogonal curvilinear mesh. The effect of varying the inflow velocity waveform while maintaining other forcing parameters constant is considered in this paper. A Reynolds number of 180 based on a reference velocity is employed in the simulations. The forcing frequency is 0.84 times the natural vortex shedding frequency in the unforced wake while the peak-to-peak amplitude of velocity oscillation is set at 65% of the reference velocity. The results show that the wake response is locked-on at the forcing frequency for all cases tested. The aerodynamic response is systematically modified by the imposed changes in the velocity waveform. The magnitude and the phase of the fluctuating drag and lift forces and the mean drag force are affected. These effects are induced by different modes of vortex formation and shedding in the wake; it is found that the rolling up of the individual shear layers on both sides can be manipulated to produce single vortices or vortex pairs.

A streamwise bending vibration mode of a Tainter gate skinplate involving rotational vibration about a horizontal rotational axis along the weir-plate surface was identified through post-failure modal analysis on one of the remaining Folsom dam gates. This modal deflection of the skinplate of a gate pushes and draws the water in the reservoir inducing hydrodynamic pressure fluctuations (termed the push-and-draw pressure). To analyze this push-and-draw pressure, a simplified analytical model was formulated. The model consisted of a rigid, vertical, flat weir-plate undergoing rotational vibration with its horizontal rotational axis along the weir-plate surface above a horizontal bed with a vertical step down at the gate position. Results from the theoretical analysis (based on dissipative wave radiation theory) are presented in terms of non-dimensional parameters, such as the Froude number and a reduced height of the skinplate rotation center. The validity of the analysis was confirmed by forced vibration experiments using a rigid, vertical, flat weir-plate.

Previous wind tunnel experiments by the present authors showed that two kinds of longitudinal vortex induce cross-flow vibrations (LVIVs) on a circular cylinder accompanied by the vortex-synchronization (lock-in) by setting another cylinder downstream with proper gaps in cruciform arrangement, like the well-known Karman vortex induced vibration (KVIV) of a single cylinder. In this work, experiments on the three types of VIVs are carried out using a water tunnel to investigate influences of the mass ratio M R and the damping ratio z on the oscillation response. It is shown that the two LVIVs, i.e. the trailing-and the necklace-VIVs, occur in water flow over respective ranges of flow velocity U in spite of large differences in values of M R and z as compared with experiments in air flow. However, their influences on oscillation responses of the two LVIVs are largely different from each other, and also from the KVIV. They are compared by referring to the de-synchronization prediction based on the dimensional analysis and the equation on cylinder motion. The variation of behaviors among the three types of VIV are attributed to the differences among the three vortices in their threshold functions for synchronization and the vortex-synchronization functions which express the feedback effect of the cylinder oscillation on the alternating lift generated by the vortex.

Experiments were carried out in a towing tank regarding the flow around a stationary array of circular cylinders with low aspect ratio, H/D=1.5, piercing the water free surface. One isolated cylinder and one array of 4 cylinders were tested; this number of cylinders corresponds to the number of the columns of the typical multi-column offshore floater system, as semi-submersibles and TLP platforms. The aims are to create a reliable database for CFD studies and to better understand the influence of the relative position of the cylinders on the flow around this type of structure and afterward to use this research in VIM calculations for free multi-columns systems. Two parameters were investigated: distance between column centers, S/D = 2, 3 and 4; and current incidence angle, 0, 22.5 and 45 degrees. The Reynolds number range covers 50,000 up to 250,000. Forces were measured in each column using a three degree-of-freedom load cell. Results of lift and drag forces are presented for each column separately and for the whole system.

The utilization of biomass for heat and power generation has aroused the interest of
most researchers especially those of energy .In converting solid fuel to a usable form of energy,
pyrolysis plays an integral role. Understanding this very important phenomenon in the
thermochemical conversion processes and representing it with appropriate mathematical models
is vital in the design of pyrolysis reactors and biomass gasifiers. Therefore, this study presents
analytical solutions to the kinetic and the heat transfer equations that describe the slow pyrolysis
of a biomass particle. The effects of Biot number, temperature and residence time on biomass
particle decomposition were studied. The results from the proposed analytical models are in good
agreement with the reported experimental results. The developed analytical solutions to the heat
transfer equations which have been stated to be “analytically involved” showed average percentage
error and standard deviations 0.439 and 0.103 from the experimental results respectively as
compared with previous model in literature which gives average percentage error and standard
deviations 0.75 and 0.106 from the experimental results respectively. This work is of great
importance in the design of some pyrolysis reactors/units and in the optimal design of the biomass
gasifiers.

In order to study cross ow induced vibration of heat exchanger tube bundles, a new uid-structure interaction model based on surface vorticity method is proposed. With this model, the vibration of a exible cylinder is simulated at Re = 2:67 × 10 4 , the computational results of the cylinder response, the uid force, the vibration frequency, and the vorticity map are presented. The numerical results reproduce the amplitude-limiting and non-linear (lock-in) characteristics of ow-induced vibration. The maximum vibration amplitude as well as its corresponding lock-in frequency is in good agreement with experimental results. The amplitude of vibration can be as high as 0:88D for the case investigated. As vibration amplitude increases, the amplitude of the lift force also increases. With enhancement of vibration amplitude, the vortex pattern in the near wake changes signiÿcantly. This uid-structure interaction model is further applied to simulate ow-induced vibration of two tandem cylinders and two side-by-side cylinders at similar Reynolds number. Promising and reasonable results and predictions are obtained. It is hopeful that with this relatively simple and computer time saving method, ow induced vibration of a large number of exible tube bundles can be successfully simulated. Copyright ? 2004 John Wiley & Sons, Ltd.

Experiments regarding flow-induced vibration on floating rounded squared section cylinders with low aspect ratio were carried out in an ocean basin equipped with a rotating-arm apparatus. Floating squared section cylinders with rounded edges and aspect ratios of L/D = 2.0 were elastically supported by a set of linear springs in order to provide low structural damping to the system. Two different incidence angles were tested, namely 0 and 45 degrees. The Reynolds numbers covered the range from 2, 000 to 30, 000. The aim was to understand the flow-induced vibrations around single columns, gathering information for further understanding the causes for the Vortex-Induced Motions in semi-submersible and TLP platforms. Experiments on circular and squared sections cylinders (without rounded edges) were also carried out to compare the results with the rounded square section cylinders (with rounded edges). The amplitude results for in-line, transverse and yaw amplitude for 0-degree models showed to be higher for squared section cylinders compared to those for the rounded square section cylinders. No significant difference between the 45-degree models was observed. The results of ratio between frequency of motion in the transverse direction and natural frequency in still water confirmed the vortex-induced vibration behavior for the squared and rounded square section cylinders for 45-degree incidence; and also the galloping characteristics for 0-degree incidence cases. The rounded effect on the square section cylinders showed to be

Measurements are presented of response and drag for a flexibly mounted circular cylinder with low mass and damping. In one set of experiments it is free to respond in only the cross-flow direction and in a second it is free to respond in two degrees of freedom. It is shown how vortex-induced vibration can be practically eliminated by using free-to-rotate, two-dimensional control plates. Further it is shown that these devices achieve VIV suppression with drag reduction. The device producing the largest drag reduction was found to have a drag coefficient equal to about 60% of that for a plain, fixed cylinder over the Reynolds number range of the experiments, up to 30 000. The importance of torsional resistance of the devices is discussed and it is shown that if it is too low large oscillations of the device and cylinder will develop and if it is too high galloping is initiated.

The objective of this paper is to numerically investigate the effects of parametric excitation on the energy harvesting from transverse galloping of a square prism. The rigid square prism is elastically supported and is free to oscillate in the transverse direction with respect to the free-stream velocity. A polynomial fit is used for mod-eling the fluid force. Energy can be harvested from the system by means of a piezoelectric coupling and a linear dashpot. Curves of standard-deviations of electric tension , prism displacement and time-averaged powers harvested at the electric resistance and at the dashpot are presented as functions of the non-dimensional free-stream velocity. The analysis reveals that the parametric exci-tation in the principal Mathieu's instability may enhance the electric tension and the power that can be harvested from the system. Additionally, the paper presents maps of dissipated power as functions of the parameters that govern the parametric excitation for a given non-dimensional free-stream velocity, showing how the energy harvesting from both the dashpot and the electric resistance can be enhanced.

Flow induced instability has been observed in numerous fields, including the aerospace industry, power generation/transmission, and nuclear reactor components. This paper considers the stability analysis of a cantilever pipe excited by the flow of fluid. The developed mathematical model can be used to determine a number of critical flow velocities where the instability may occur. The pipe is modeled as an Euler-Bernoulli beam by considering the effect of fluid mass and the Coriolis acceleration caused by the flow. The required mathematical procedure is presented and it is accompanied by a set of design guidelines for determining the stable and unstable condition for these structures. Parameters to be considered are velocity of the flow, density of the flow, and properties of the pipe as well as its geometry.

The problem in question permeates the fluid induced acoustic levitation, which consists of the introduction of a large amount of pressure waves, resulting from the sound vibration of a high frequency exciter creating a pressure field from a compression film, as a consequence of an opposing force to gravity. It is possible for objects to be suspended, even if they are large or heavy. Understanding the behavior of this type of model is of paramount importance due to the numerous applications and its versatility. The approach used in the present work is new, as it seeks to describe the pressure field by means of a system of ordinary differential equations solved simultaneously using the method of progressive finite differences. The proposed model consists of a receiving disc and an exciting surface, where both are surrounded by a cylinder so that the system is sealed and that there is no mass flow. In addition, it is necessary to analyze the reliability of the proposed model. For this, at first, the problem is modeled, so that the exciter is static, that is, it is not active. Thus, the focus of the analysis is only on the receiver and its movement from rest. With this simplification, and the introduction of the parameter beta , which depends on the mass and cross-sectional area of the disc, it is possible to compare the numerical model with an analytically predicted response, which validates the hypothesis. Finally, the last case analyzed will be one with a movement imposed on the exciter with known speed, position and frequency and obviously the implications of this for the receiver and the system such as the variation of the specific mass of the film, the internal temperature and the position of the disc over time.

The concept of Aero-elastic Wind Energy Generator (AeWEG) is applied for producing power at low wind speeds. A straight rectangular rigid wing subjected to controlled torsional flutter is employed to do suitable work for producing power. A mathematical model of an AeWEG is developed and performance under various conditions is predicted. Matlab code is used to predict the influencing parameters for the design of AeWEG. Damping oscillation, Practical Resonance and Transient Oscillation solutions are also developed. Numerical analysis has been carried out to optimize the influencing design parameters. The simulation results evidenced that low velocity flutter phenomenon can be used positively for the small-scale power production purpose.