Flow induced vibration

We address theoretically the linear stability of a variable aspect ratio, rectangular plate in an uniform and incompressible axial flow. The flutter modes are assumed to be two-dimensional but the potential flow is calculated in three dimensions. For different values of aspect ratio, two boundary conditions are studied: a clampedfree plate and a pinned-free plate. We assume that the fluid viscosity and the plate viscoelastic damping are negligible. In this limit, the flutter instability arises from a competition between the destabilising fluid pressure and the stabilising flexural rigidity of the plate. 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 aspect ratio of the plate. A new result is demonstrated: a plate of finite span is more stable than a plate of infinite span.

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.

This paper presents experimental results concerning flow-induced oscillations of circular cylinders arranged in tandem. New measurements on the dynamic response oscillations of an isolated cylinder and flow interference of two cylinders are shown. Preliminary flow visualization employing a PIV system is also shown. The models are mounted on an elastic base fitted with flexor blades and instrumented with strain gauges. The base is fixed on the test-section of a water channel facility. The flexor blades possess a low-damping characteristic (z ' 0:00820:0109) and they are free to oscillate only in the cross-flow direction. The Reynolds number of the experiments is from 3000 to 13 000, and reduced velocities, based on natural frequency in still water, vary up to 12. The interference phenomenon on VIV is investigated by conducting experiments in which the upstream cylinder is maintained fixed and the downstream one is mounted on the elastic base. The results for an isolated cylinder are in accordance with other measurements found in the literature for m à ' 2 and 8. For the tandem arrangement and m à ' 2, the trailing cylinder oscillation presents what previous researchers have termed interference galloping behaviour for a centre-to-centre gap spacing ranging from 2D to 5:6D. These initial results validate the experimental setup and lead the way for future work, including tandem, staggered and side-by-side arrangements with the two cylinders free to move.

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.

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.

The problem of critical velocity of fluid-conveying pipes resting on a Winkler foundation with elastically supported of the pipe is studied in this paper. The aim of this work is deriving a new analytical model to perform a general study to investigate the dynamic behavior of a pipe under general boundary conditions by considering the supports as compliant material with linear and rotational springs, and study the effect of foundation and stiffness values on the critical velocity. This model describes both the classical (simply support, free, built, guide) and the restrained boundary condition and it is not required to derive a new critical velocity equation if the boundary conditions is changed ,also the result will be near to reality by knowing the physical parameters for the compliant material and the pipe. The cases studied in this work are flexible support of the ends pipe with or without intermediate restrain support, and pipe resting on Winkler foundation. . The general theoretical conclusions are that the supports and foundation values have significant effects on the dynamic characteristics of the pipe.

The problem of vibrations cause the collapse of the systems and cause significant economic losses if not avoided. For this reason, the researchers have dealt with this issue in all years, but the problem is not over. In this paper will highlight the problem of controlling the vibrations resulting from fluid flow inside the pipes, in addition to the study of reducing the vibrations of these pipes. The ideal control is designed based on, the state space theory and according to the linear behavior of time, because the stability of the pipes are found from linear change. Different factors were studied on the controller's performance and then the optimal factors were obtained to obtain the best performance. The research included the study of the response and the natural frequency and the study of the dynamic behavior of different types of stabilization of the pipes in the presence with no hydraulic damping (active control) and monitoring the response and stability of each case of stabilization. Where, the control investigation include using the analytical solution of general equation of motion for pipe by using state space technique. There, the parameters studied for active control pipe vibration included the effect of hydraulic damper position, base width of hydraulic damper, damping, and the flow Pressure, on the pipe vibration response, with various pipe boundary conditions supported.

Environment air pollution and global warming are alarming concern worldwide. Increasing air pollution, rapid growth of industrialization and the global trend of urbanization have totally disturbed the eco balance of resources on earth. The present study was done to visualize the potential of ethanol and biodiesel as an alternative fuel in C.I. engine. The salient features of the investigation include: (i) The study of basic fuel properties of ethanol, biodiesel and their blends with diesel for C.I. engine. (ii) To evaluate the potential of using biodiesel, ethanol and their blends with diesel for C.I, engine. (iii) Study of the engine performance and exhaust emission characteristics for various blends. Experiment set up was developed to carryout engine performance and emission characteristic studies on selected fuel blends at different load conditions. The present work has resulted in giving a good insight into the performance and emission characteristics of the C.I. engine using ethanol, biodiesel, diesel fuel blends. As fuel property point of view density and pour point of all the fuel blends are under the standard limits for diesel fuel.

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.

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 regarding flow-induced vibration on floating squared section cylinders with low aspect ratio were carried out in an ocean basin with rotating-arm apparatus. The floating squared section cylinders were elastically supported by a set of linear springs to provide low structural damping to the system. Three different aspect ratios were tested, namely í µí°¿/í µí°· = 1.0, 2.0 and 3.0, and two different incidence angles, namely 0 and 45 degrees. The aims were to understanding the flow-induced vibration around single columns of multi-column platforms, such as semi-submersible and TLP. VIV on circular cylinders were also carried out to compare the results. The range of Reynolds number covered was 2,000 < Re < 27,000. The in-line and transverse amplitude results showed to be higher for 45-degree incidence compared with 0-degree, but the maximum amplitudes for squared section cylinders were lower compared with the circular ones. The double frequency in the in-line motion was not verified as in circular cylinders. The yaw amplitudes cannot be neglected for squared section cylinders, maximum yaw amplitudes around 10 degrees were observed for reduced velocities up to 15. Keywords: flow-induced vibration (FIV), vortex-induced vibration (VIV), vortex-induced motion (VIM), low aspect ratio, floating squared section cylinder

The mechanism of wake-induced vibrations (WIV) of a pair of cylinders in a tandem arrangement is investigated by experiments. A typical WIV response is characterized by a build-up of amplitude persisting to high reduced velocities; this is different from a typical vortex-induced vibration (VIV) response, which occurs in a limited resonance range. We suggest that WIV of the downstream cylinder is excited by the unsteady vortex–structure interactions between the body and the upstream wake. Coherent vortices interfering with the downstream cylinder induce fluctuations in the fluid force that are not synchronized with the motion. A favourable phase lag between the displacement and the fluid force guarantees that a positive energy transfer from the flow to the structure sustains the oscillations. If the unsteady vortices are removed from the wake of the upstream body then WIV will not be excited. An experiment performed in a steady shear flow turned out to be central to the understanding of the origin of the fluid forces acting on the downstream cylinder.

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.

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.

Fluid conveying pipes are used in all hydraulic systems and enter in all industrial fields, such as water, petroleum products and gases of all kinds. In spite of this widespread and critical importance of fluid conveying pipes, they suffer from major problems and one of these problems is the problem of vibrations that cause the collapse of the systems completely and cause significant economic losses if not avoided. For this reason, the researchers have dealt with this issue in all years, but the problem is not over. In this paper will highlight the problem of controlling the vibrations resulting from fluid flow inside the pipes. The research included the study of the dynamic behaviour of different types of stabilization of the pipes in the presence with no hydraulic damping (active control) and monitoring the stability of each case of stabilization. This study was carried out by deriving differential equations for pipes and for different types of fixation. Special equations with mode response and natural frequency equations were obtained in addition to the stability study of the pipes in the same manner, by using bode diagram technique. This is done by deriving the differential equation of motion and dealing with it for the purpose of reaching the general equation so that it can be easily converted within the method of state space and analysis of equations and find feedback with the change of inputs and outputs. With the help of MATLAB program where the final equations are programmed and the results are found. The main factors of dynamic and stability behaviour were calculated in addition to the main parameters on the pipe such as fluid velocity, pressure and mass ratios.

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 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.

Generalized procedure for designing solar water heater is discussed in this paper. Correlations are suggested for evaluating induced flow rate (Reynolds number) and heat transfer coefficient (Nusselt number). These correlations are used in evaluating various system parameters of the solar collector such as fluid outlet velocity, fluid outlet temperature, absorber plate temperature, heat gained by the fluid and efficiency of the collector. Numerical procedure is suggested for evaluating various system parameters. Experiment was conducted for evaluating and comparing the results obtained theoretically and was observed well within ±10%.

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.

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.

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.

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.

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.

In many industrial equipment such as boilers and heat exchangers, the cylindrical tubes are exposed to the gas-liquid two phase flow. For any immersed body in flow field vortex shedding is created with a frequency that may be constant or variable, according to conditions such as flow rates, geometry of body, and etc. The failure will happen in the equipment, when this frequency is close to one of the natural frequencies of them. This can cause noise and flow induced vibration problem which is one of the main defects in the heat exchangers. Therefore considering these flows can play a significant role in long-term reliability and safety of industrial and laboratory equipment. In this study Eulerian-Eulerian approach is employed to simulate two-phase flow around the cylindrical tube. Since the Reynolds Stress Model (RSM) accounts for the effects of streamline curvature, swirl, rotation, and rapid changes in strain rate in a more rigorous manner than other turbulence models, it has greater potential to give accurate predictions for complex flows. So in this study the RSM is used to recognize behavior of vortex shedding in the flow. Drag coefficient, Strouhal number, vortex shedding behind cylinder, void fraction and pressure coefficient distribution were investigated in air-water two phase flows. In order to verify validity of CFD model, inlet void fraction was set to zero, Strouhal number and it's relation with low Reynolds number (100-3000) in single phase flow were compared with experimental and numerical result of available literatures. The results show a good agreement between them. Having reasonable judgment of fluctuating lift force frequency, fast Fourier transform (FFT) was applied to lift coefficient of cylinder. The FFT produces averaged spectral coefficients that are independent of time and are useful to identify dominant frequencies in a signal. Some remedies were introduced to reduce vortex frequency of cylindrical tube subjected in gas-liquid two phase flows and protect tube from hard vibrations. For this purpose, some geometrical modifications were applied and results showed that in all cases drag coefficient and Strouhal number reduce.

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.

In spite of this widespread and critical importance of fluid conveying pipes, they suffer from major problems. One of these problems is the problem of vibrations that cause the collapse of the systems completely and cause significant economic losses if not avoided. Where, fluid conveying pipes are used in all hydraulic systems and enter in all industrial fields, such as water, petroleum products and gases of all kinds. For this reason, the researchers have dealt with this issue in all years, but the problem is not over. This research will highlight the problem of controlling the vibrations resulting from fluid flow inside the pipes, in addition to the study of reducing the vibrations of these pipes. This study is carried out by deriving differential equations for pipes and for different types of fixation. The research has studied the response and the natural frequency; the dynamic behavior of different types of stabilization of the pipes in the presence with no hydraulic damping (active control); and monitoring the response and stability of each case of stabilization. Where, in this paper using root locus technique to calculate the control for pipe conveying fluid. Thus, the investigation included calculated the effect for different damper parameters and fluid on the pipe stability. There, this work is expansion for previous study investigated the control for pipe vibration with using other stability and control theories, and comparison for presenting results with previous results, therefore, form comparison results found that the loot locus technique is a good technique can be using to investigation the stability and control for pipe fluid.

When a pair of tandem cylinders is immersed in a flow the downstream cylinder can be excited into wake-induced vibrations (WIV) due to the interaction with vortices coming from the upstream cylinder. Assi, Bearman & Meneghini (J. Fluid Mech., vol. 661, 2010, pp. 365–401) concluded that the WIV excitation mechanism has its origin in the unsteady vortex–structure interaction encountered by the cylinder as it oscillates across the wake. In the present paper we investigate how the cylinder responds to that excitation, characterising the amplitude and frequency of response and its dependency on other parameters of the system. We introduce the concept of wake stiffness, a fluid dynamic effect that can be associated, to a first approximation, with a linear spring with stiffness proportional to Re and to the steady lift force occurring for staggered cylinders. By a series of experiments with a cylinder mounted on a base without springs we verify that such wake stiffness is not only strong enough to sustain oscillatory motion, but can also dominate over the structural stiffness of the system. We conclude that while unsteady vortex–structure interactions provide the energy input to sustain the vibrations, it is the wake stiffness phenomenon that defines the character of the WIV response.

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 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.

This 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...

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 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.

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.

Keywords: Vortex-induced vibration Cross-flow and in-line motion Curved cylinder Particle image velocimetry a b s t r a c t Experiments have been conducted to investigate the two-degree-of-freedom vortex-induced vibration (VIV) response of a rigid section of a curved circular cylinder with low mass-damping ratio. Two curved configurations, a concave and a convex, were tested regarding the direction of the flow, in addition to a straight cylinder that served as reference. Amplitude and frequency responses are presented versus reduced velocity for a Reynolds number range between 750 and 15 000. Results for the curved cylinders with concave and convex configurations revealed significantly lower vibration amplitudes when compared to the typical VIV response of a straight cylinder. However, the concave cylinder showed relatively higher amplitudes than the convex cylinder which were sustained beyond the typical synchronisation region. We believe this distinct behaviour between the convex and the concave configurations is related to the wake interference taking place in the lower half of the curvature due to perturbations generated in the horizontal section when it is positioned upstream. Particle-image velocimetry (PIV) measurements of the separated flow along the cylinder highlight the effect of curvature on vortex formation and excitation revealing a complex fluid–structure interaction mechanism.

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.

When a pair of tandem cylinders is immersed in a flow the downstream cylinder can be excited into wake-induced vibrations (WIV) due to the interaction with vortices coming from the upstream cylinder. Assi, Bearman & Meneghini (J. Fluid Mech., vol. 661, 2010, pp. 365-401) concluded that the WIV excitation mechanism has its origin in the unsteady vortex-structure interaction encountered by the cylinder as it oscillates across the wake. In the present paper we investigate how the cylinder responds to that excitation, characterising the amplitude and frequency of response and its dependency on other parameters of the system. We introduce the concept of wake stiffness, a fluid dynamic effect that can be associated, to a first approximation, with a linear spring with stiffness proportional to Re and to the steady lift force occurring for staggered cylinders. By a series of experiments with a cylinder mounted on a base without springs we verify that such wake stiffness is not only strong enough to sustain oscillatory motion, but can also dominate over the structural stiffness of the system. We conclude that while unsteady vortex-structure interactions provide the energy input to sustain the vibrations, it is the wake stiffness phenomenon that defines the character of the WIV response.

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

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.

An important element of in-plant noise modeling for high-speed gas flow, in addition to the noise generated by control valves, is the noise due to piping system components such as tees which becomes significant at high mass flow rates. An effective tool for the modeling of noise due to piping system components is non-dimensional pressure fluctuation scaling. The current paper outlines procedures for characterization of the internal noise due to an equal tee with unequal outlet flows using non-dimensional pressure fluctuation scaling. Experimental data is presented demonstrating general collapse of the measured data onto a single characteristic curve for several combinations of flow velocity and gas density under steady-state flow conditions. Flow ratios of 1:2, 1:3, 1:4 and 2:3 between each of the two outlet legs from the equal tee are considered, as well as the affect of gas density over a range of 345kPaG to 1380kPaG.Copyright © 2003 by ASME

ABSTRACT This paper studies the effects of pre-tension on flow-induced vibration of a flexible cylinder with two degrees of freedom in a towing tank. A 0.45 m section of Tygon tubing with an outer and inner diameter of 7.9 mm and 4.8 mm respectively, was employed as a flexible test model, the mass ratio and aspect ratio of which is 0.77 and 57, respectively. It was towed from rest up to 0.8 m/s before slowing down to rest again over a distance of 1.6 m in still water with three cases of pre-tension from 0 to 8 N. The Reynolds number based on the cylinder’s outer diameter spanned from approximately 780–6300 while the reduced velocity ranged from 2 to 16. Emphasis was on revealing the possible relationship between pre-tension and response amplitude as well as the frequency. The vibration amplitude and frequency obtained during the brief constant towing speed were quantified and expressed in terms of reduced velocity. Narrowed lock-in bandwidth and reduced response amplitude were observed. The findings from this set of experiments are compared to the existing knowledge in the literature.

ABSTRACT Main steam control valves in power plants are required to operate underwide ranges of valve openings and pressure ratios. In the present paper, experimental and numerical investigations are conducted using rigid and flexible valve head supports to clarify the mechanisms of valve head vibrations that are caused by unsteady flows around the valve. The results obtained using the rigid support without valve head vibration show that the unsteady flow around the valve head causes pressure fluctuations on the valve head surface with random and impulsive wave forms. When using the flexible support, the valve head vibrates near the natural frequency of the valve head support system, and vibrations are excited around the operating conditions where the pressure fluctuation becomes greater when using the rigidly supported valve head. When the valve head vibration increases, the pressure fluctuation becomes periodic with the same frequency as the valve head vibration. The numerical results show that the response of the separated jet lags behind the valve head motion. As a result, the lateral fluid force adds negative damping on the vibration on the valve head.