Flow-induced Oscillations Research Articles

Flow-induced oscillations of low-aspect-ratio flexible plates with various tip geometries

The distinctive oscillations and induced wake of wall-mounted thin plates with flat, elliptic, and aristate tips were experimentally studied under nearly uniform flows for various Cauchy numbers, Ca. Particle tracking velocimetry and planar particle image velocimetry were used to characterize the dynamics of the plates and surrounding flow. The results show that the oscillations of the plates were dominated by their natural frequency across Ca. The structures underwent distinctive reconfigurations which led to asymmetric distributions of the tip fluctuations with respect to the equilibrium position. Interestingly, the oscillation intensity of the plates reached a local maximum at a critical Cauchy number Cacri, which increased with narrower tip. This is explained with a basic model that accounts for the nonlinear structure bending and the flow fluctuations in the vicinity of the structures. The very near wake was characterized by two recirculation regions at sufficiently low Ca, one at the base of the plates and another right above it extending up to the tip. However, the recirculation region near the tip eventually vanished under large structure bending at sufficiently high Ca, which led to increased mean shear. Also, the turbulence levels were dominated by the streamwise component of the velocity fluctuations around the tips, where the aristate tip induced the lowest levels.

Sawtooth wave-like pressure changes in a syrup eruption experiment: implications for periodic and nonperiodic volcanic oscillations

This study is based on the observation of sawtooth wave-like pressure changes (STW) observed during repetitive gas emissions in a syrup eruption experiment. Similar waveforms are observed at many active volcanoes as geodetic signals. By studying the physics of such experiments, we often find new ideas and insights that are applicable to natural volcanic phenomena. We consequently try identifying the features common to both our experimental system and natural volcanic systems. We infer that the oscillatory mechanism in our experiment is similar to flow-induced oscillation controlled by a coupling between elastic capacitance and variable flow resistance. We developed an elementary pipe–chamber system to quantitatively test this hypothesis. We observed three distinct oscillatory patterns: periodic STW, non-STW, and nonperiodic STW. A mathematical model is constructed to support the hypothesis and to enable comparison with existing models of volcanic systems. Models of flow-induced volcanic oscillations are mathematically similar to the model derived from our experimental system. Our results indicate that flow pattern transitions are essential for oscillatory behavior during gas emission. An important finding is that cycle periodicity and fluctuation are controlled by interactions between upward and downward flow within the pipe, especially during the termination of gas emission and the transition to the next cycle. Similar interactions occur in natural volcanoes: during the termination of an explosive eruption, fall-back or drain-back of ejected materials may modulate the explosivity and periodicity of the next eruption cycle. Our experimental system may provide a useful tool for understanding the oscillatory behavior of natural volcanic systems. This study may also give a proof that the original eruption experiment provides a useful tool for explaining the dynamics of oscillatory and eruptive behaviors of natural volcanic systems in educational (e.g., Open Day) demonstrations.

Flow-induced oscillation of a rigid rectangular plate hinged at its leading edge

In the present study, the flow-induced oscillation of an elongated rectangular cylinder (a rigid plate with flat leading and trailing edges), hinged at its leading edge, is studied experimentally. Effects of various parameters such as the Reynolds number, chord, thickness, and moment of inertia on oscillation characteristics of the plate were investigated. Flow visualization and hotwire measurements were carried out to study the nature of the flow field around the plate. The angular deflection and angular velocity of the oscillating plate were obtained using a high-speed camera. There exists a critical chord to thickness ratio, below which the rectangular plate hinged at the leading edge starts oscillating beyond a critical wind speed. The flow-induced oscillation of the plate begins with a small amplitude oscillation which increases with time, and the plate finally attains a limit cycle oscillation. Impinging shear layer vortices, originating at the leading edge, play a key role in the initiation of flow-induced oscillation of the plate. The frequency of the limit cycle oscillation increases almost linearly with the wind speed and decreases with an increase in the chord to thickness ratio of the plate. A dimensional analysis is carried out to identify the important dimensionless parameters, and the same has been used for extracting the scaling for oscillation frequency. Good agreement between the present experimental results and the derived analytical expressions is obtained.

Study on model design and dynamic similitude relations of vibro-acoustic experiment for elastic cavity

Coupling between aero-acoustic noise and structural vibration under high-speed open cavity flow-induced oscillation may bring about severe random vibration of the structure, and even cause structure to fatigue destruction, which threatens the flight safety. Carrying out the research on vibro-acoustic experiments of scaled down model is an effective means to clarify the effects of high-intensity noise of cavity on structural vibration. Therefore, in allusion to the vibro-acoustic experiments of cavity in wind tunnel, taking typical elastic cavity as the research object, dimensional analysis and finite element method were adopted to establish the similitude relations of structural inherent characteristics and dynamics for distorted model, and verifying the proposed similitude relations by means of experiments and numerical simulation. Research shows that, according to the analysis of scale-down model, the established similitude relations can accurately simulate the structural dynamic characteristics of actual model, which provides theoretic guidance for structural design and vibro-acoustic experiments of scaled down elastic cavity model.

Large-Eddy Simulation of Internal Flow through Human Vocal Folds

The phonatory process occurs when air is expelled from the lungs through the glottis and the pressure drop causes flow-induced oscillations of the vocal folds. The flow fields created in phonation are highly unsteady and the coherent vortex structures are also generated. For accuracy it is essential to compute on humanlike computational domain and appropriate mathematical model. The work deals with numerical simulation of air flow within the space between plicae vocales and plicae vestibulares. In addition to the dynamic width of the rima glottidis, where the sound is generated, there are lateral ventriculus laryngis and sacculus laryngis included in the computational domain as well. The paper presents the results from OpenFOAM which are obtained with a large-eddy simulation using second-order finite volume discretization of incompressible Navier-Stokes equations. Large-eddy simulations with different subgrid scale models are executed on structured mesh. In these cases are used only the subgrid scale models which model turbulence via turbulent viscosity and Boussinesq approximation in subglottal and supraglottal area in larynx.

STIM1 and TRPV4 regulate fluid flow-induced calcium oscillation at early and late stages of osteoclast differentiation

Bone resorption is mainly mediated by osteoclasts (OCs), whose formation and function are regulated by intracellular Ca2+ oscillation. Our previous studies demonstrated that fluid shear stress (FSS) lead to Ca2+ oscillation through mechanosensitive cation-selective channels. However, the specific channels responsible for this FSS-induced Ca2+ oscillation remain unknown. In the present study, we examined the expression of several Ca2+ channels in OCs, including STIM1, ORAI1, TRPV1, TRPV4, TRPV5, and TRPV6, by western blotting and reverse transcription-polymerase chain reaction. The results showed that STIM1 was highly expressed in early stage OCs, while TRPV4 was highly expressed in late stage OCs. We observed intracellular Ca2+ responses in OCs that were mechanically stimulated by FSS. When we blocked STIM1-dependent store-operated Ca2+ entry or inhibited TRPV4 using siRNA or drug inhibition, FSS-induced Ca2+ oscillations were almost undetectable in early and late stage OCs, respectively. These results indicate that STIM1 and TRPV4 act as mechanical transduction channels for OCs during the early and late differentiation stages, respectively, suggesting that these calcium channel could serve as markers of osteoclastogenesis or bone resorption.

Effect of rounded corners on two degree of freedom naturally oscillating square cylinder

PurposeThe paper aims to study the influence of rounded corners on the flow-induced oscillation of a square cylinder that is free to oscillate in two degrees of freedom.Design/methodology/approachThe finite volume code in conjunction with the moving mesh scheme was implemented via a user-defined function to carry out the computations in two dimensions. The Reynolds number (Re) chosen for the present study is fixed at 100, and the frequency ratio, Fr = fs/fn (where fs is the vortex shedding frequency and fn is the natural frequency of cylinder) is used as a varying parameter. The computational model was validated for flow past a stationary cylinder with R/D = 0 and 0.5, and the results showed good agreement with the literature.FindingsThe aerodynamic characteristics, amplitude response, trajectories of cylinder motion and vortex shedding modes are obtained by conducting a series of simulations under different frequency ratios of the cylinder. It was found that the minimum transverse amplitude, drag force and lift force obtained for a naturally oscillating square cylinder are quite different when compared with a stationary and forced oscillating cylinder, where the maximum drag and lift forces were observed for a square cylinder and a minimum around R/D = 0.2 was observed.Originality/valueThe present work identified the significant effect of the varying frequency ratio and R/D on the VIV modes of the cylinder. It was observed that the cylinder wake exhibits the (2S) vortex shedding mode for R/D = 0 to 0.2 at all Fr, whereas the C (2S) mode appeared for R/D > 0.2 at Fr = 1.1.

Vortex-induced vibration and galloping of prisms with triangular cross-sections

Flow-induced oscillations of a flexibly mounted triangular prism allowed to oscillate in the cross-flow direction are studied experimentally, covering the entire range of possible angles of attack. For angles of attack smaller than $\unicode[STIX]{x1D6FC}=25^{\circ }$ (where $0^{\circ }$ corresponds to the case where one of the vertices is facing the incoming flow), no oscillation is observed in the entire reduced velocity range tested. At larger angles of attack of $\unicode[STIX]{x1D6FC}=30^{\circ }$ and $\unicode[STIX]{x1D6FC}=35^{\circ }$, there exists a limited range of reduced velocities where the prism experiences vortex-induced vibration (VIV). In this range, the frequency of oscillations locks into the natural frequency twice: once approaching from the Strouhal frequencies and once from half the Strouhal frequencies. Once the lock-in is lost, there is a range with almost-zero-amplitude oscillations, followed by another range of non-zero-amplitude response. The oscillations in this range are triggered when the Strouhal frequency reaches a value three times the natural frequency of the system. Large-amplitude low-frequency galloping-type oscillations are observed in this range. At angles of attack larger than $\unicode[STIX]{x1D6FC}=35^{\circ }$, once the oscillations start, their amplitude increases continuously with increasing reduced velocity. At these angles of attack, the initial VIV-type response gives way to a galloping-type response at higher reduced velocities. High-frequency vortex shedding is observed in the wake of the prism for the ranges with a galloping-type response, suggesting that the structure’s oscillations are at a lower frequency compared with the shedding frequency and its amplitude is larger than the typical VIV-type amplitudes, when galloping-type response is observed.

Interaction between a simplified soft palate and compressible viscous flow

Fluid–structure interaction in a simplified 2D model of the upper airways is simulated to study flow-induced oscillation of the soft palate in the pharynx. The goal of our research has been a better understanding of the mechanisms of the Obstructive Sleep Apnea Syndrome and snoring by taking into account compressible viscous flow. The inspiratory airflow is described by the 2D compressible Navier–Stokes equations, and the soft palate is modeled as a flexible plate by the linearized Euler–Bernoulli thin beam theory. Fluid–structure interaction is handled by the arbitrary Lagrangian–Eulerian formulation. The fluid flow is computed by utilizing 4th order accurate summation by parts difference operators and the 4th order accurate classical Runge–Kutta method which lead to very accurate simulation results. The motion of the cantilevered plate is solved numerically by employing the Newmark time integration method. The numerical schemes for the structure are verified by comparing the computed frequencies of plate oscillation with the associated second mode eigenfrequency in vacuum. Vortex dynamics is assessed for the coupled fluid–structure system when both airways are open and when one airway is closed. The effect of mass ratio, rigidity and damping coefficient of the plate on the oscillatory behavior is investigated. An acoustic analysis is carried out to characterize the acoustic wave propagation induced by the plate oscillation. It is observed that the acoustic wave corresponding to the quarter wave mode along the length of the duct is the dominant frequency. However, the frequency of the plate oscillation is recognizable in the acoustic pressure when reducing the amplitude of the quarter wave mode.

Numerical investigation of flow-induced rotary oscillation of circular cylinder with rigid splitter plate

Numerical results of fluid flow over a rotationally oscillating circular cylinder with splitter plate are presented here. Different from the previous examinations with freely rotatable assembly, the fluid and structure interactions are treated as a coupled dynamic system by fully considering the structural inertia, stiffness, and damping. The hydrodynamic characteristics are examined in terms of reduced velocity Ur at a relatively low Reynolds number Re = 100 for different plate lengths of L/D = 0.5, 1.0, and 1.5, where Ur = U/(Dfn), Re = UD/υ and fn = (κ/J)0.5/2π with U the free stream velocity, D the diameter of the circular cylinder, υ the fluid kinematic viscosity, fn the natural frequency, J the inertial moment, κ the torsional stiffness, and L the plate length. Contrast to the freely rotating cylinder/plate body, that is, in the limit of κ → 0 or Ur →∞, remarkable rotary oscillation is observed at relatively low reduced velocities. For the typical case with L/D = 1.0, the maximum amplitude may reach five times that at the highest reduced velocity of Ur = 15.0 considered in this work. At the critical reduced velocity Ur = 4.2, notable hydrodynamic jumps are identified for the rotation amplitude, response frequency, mean drag coefficient, lift amplitude, and vortex shedding frequency. Moreover, the phase angle between the fluid moment and rotary oscillation abruptly changes from 0 to π at Ur = 6.5. Due to the combined effect of fluid moment, rotation response, and phase difference, the natural frequency of the rotating body varies in flow, leading to a wide regime of lock-in/synchronization (Ur ≥4.2, for L/D = 1.0). The phenomenon of rotation bifurcation, i.e., the equilibrium position of the rotary oscillation deflects to a position which is not parallel to the free stream, is found to only occur at higher reduced velocities. The longer splitter plate has the lower critical reduced velocity. The occurrence of bifurcation is attributed to the anti-symmetry breaking of the wake flow evolution. The resultant asymmetric mean pressure distribution on the splitter plate gives rise to the net lift force and the deviated moment on the assembly, leading to the offset mean position of splitter plate. The global vortex shedding is identified to be the classic 2S mode for both cases with and without the bifurcation, although the second vortex formation and the shedding pattern in the near wake for the bifurcate case are different from the non-bifurcate case with lower reduced velocities.

Numerical study of flow-induced oscillations of two rigid plates elastically hinged at the two ends of a stationary plate in a cross-flow

The flow-induced oscillation (FIO) of bluff bodies is commonly encountered in the fluid structure interaction (FSI) problems. In this study, we use an unstructured moving grid strategy and simulate the FIO of two rigid plates, which are elastically hinged at the two ends of a fixed flat plate in a cross-flow. We use a hybrid finite-element-volume (FEV) method in an arbitrary Lagrangian–Eulerian (ALE) framework to study FIO of the two hinged plates. The current simulations are carried out for wide ranges of flow Reynolds number (50–175), spring stiffness coefficient, and the two hinged plates' moment of inertia magnitudes. The influences of these parameters are investigated on the magnitudes of maximum deflection angle, the amplitude of oscillation, the total lift and drag coefficients, and so on. The study is also carried out in the transition period to describe the in-phase and out-of-phase angular oscillations occurring for the two elastically hinged plates with respect to each other. After the transition period, the two hinged plates eventually arrive to a similar periodic oscillation; however, with some phase lags. We find that the achieved phase lag is equal to the phase lag between the two pairs of flow vortices, which are alternatively shed into the flow from the upper and lower hinged plates. Similar to past FIO problems, the current model also exhibits two important lock-in and phase-switch FSI phenomena; however, in angular directions. There is a phase jump of approximately 170° between the aerodynamic lift coefficient and angular oscillations of hinged plates, which nearly occurs in the middle of lock-in region. Indeed, our literature review shows that this is the first time to report the phase-switch phenomenon in angular oscillations of three-element bluff bodies in a FSI problem.

Hydroelastic Buffeting Assessment Over a Vertically Hinged Flat Plate

This study explores hydroelastic buffeting of a flat plate experimentally. The flat plate is hinged vertically to an elastic axis, and its only degree of freedom, therefore, is rotation in pure yaw about this axis. Buffeting is a type of flow-induced oscillation which is caused by unsteadiness in the incoming flow. The laboratory technique presented here was applied for buffeting stimulation of the flat plate in water current. A bluff body was placed in the upstream of the flat plate to impose the unsteadiness. During the experiments, the distance between the bluff body and the flat plate was changed several times to assess the effect of distance on buffeting behavior. According to results, for distance ratios greater than 1, the buffeting responses are independent of distance ratio, although they are dependent on Reynolds number. Also the feasibility of hydrokinetic energy harvesting through buffeting is discussed.

Vortex formation and shedding from a cyber-physical pitching plate

We report on the dynamics of the formation and growth of the leading-edge vortex and the corresponding unsteady aerodynamic torque induced by large-scale flow-induced oscillations of an elastically mounted flat plate. All experiments are performed using a high-bandwidth cyber-physical system, which enables the user to access a wide range of structural dynamics using a feedback control system. A series of two-dimensional particle image velocimetry measurements are carried out to characterize the behaviour of the separated flow structures and its relation to the plate kinematics and unsteady aerodynamic torque generation. By modulating the structural properties of the cyber-physical system, we systematically analyse the formation, strength and separation of the leading-edge vortex, and the dependence on kinematic parameters. We demonstrate that the leading-edge vortex growth and strength scale with the characteristic feeding shear-layer velocity and that a potential flow model using the measured vortex circulation and position can, when coupled with the steady moment of the flat plate, accurately predict the net aerodynamic torque on the plate. Connections to previous results on optimal vortex formation time are also discussed.

Marine current energy extraction through buffeting

The research is an experimental study on the feasibility of marine current energy extraction through the phenomenon of buffeting. This phenomenon is a type of flow-induced oscillation of an elastic structure that is caused by the unsteadiness of the incoming flow. In order to prove the concept, a preliminary unit turbine model is designed working based on the buffeting phenomenon. The turbine model is an elastic structure, consisting of a rectangular flat plate located in a current flume. The elasticity is provided by attachment of four linear springs with equal rates (k) at the edges of the flat plate, to give an equivalent torsion spring rate. The unsteadiness is due to the wake of a bluff body located for this purpose in the upstream of the flat plate. The von Kármán street in the wake formed behind the bluff body induces oscillating moments in the flat plate which makes it to yaw with small angles of attack. The results demonstrate that one such system has the ability to extract hydrokinetic energy with efficiencies of up to 60%. The model tests results then are used to estimate the performance of the array of turbines, through the similarity analysis. Finally, a rough estimate of the manufacturing, installation and operation cost of the buffeting turbine subsea farm is presented.

Precursors to self-sustained oscillations in aeroacoustic systems

In this study, we attempt to provide a repertoire of measures to forewarn the onset of impending flow-induced mechanical oscillations via online health monitoring. To illustrate the principles, the flow of air through a pipe terminated by a circular orifice plate is investigated at various flow velocities using a suitably placed pressure transducer. It is observed that the regimes corresponding to the production of a tone is presaged by operating conditions that display temporarily intermittent bursts of periodic pressure oscillations that emerge from a background of lower-amplitude aperiodic fluctuations. The various model-free measures prescribed in this paper serve as efficient precursors by characterizing these intermittent states, which can potentially arise in aeroacoustic systems when the flow is highly unsteady or turbulent.

Experimental study on transverse flow-induced oscillations of a square-section cylinder at low mass ratio and low damping

This article presents experimental results concerning the transverse flow-induced oscillations of a square-section cylinder for several values of the mass ratio, ranging from 2.2 to 14.4, and low mechanical damping. Experiments were carried out in a re-circulated free-surface water channel. A rigid square-section cylinder was fixed to an elastic system made with dual (parallel) metallic blades connected by a rigid block. By this way the square-section cylinder is free to oscillate in the transverse (normal to flow) direction. The elastic system presents linear behavior in terms of stiffness and damping. The functional dependence between the steady-state amplitude and frequency of oscillations with reduced velocity is characterized. A re-normalization plot appears when steady-state amplitude of oscillations is plotted against the “true” reduced velocity, which is defined as the reduced velocity divided by the dimensionless frequency of oscillations. Experiments also allow to discuss other aspects, like the reduced velocity at which oscillations are expected to start depending on the value of the mass ratio, how close to sinusoidal (say, fixed amplitude and frequency) the oscillations are, or when quasi-steady conditions are expected to be met.

Flutter of spring-mounted flexible plates in uniform flow

A fluid–structure interaction (FSI) system is studied wherein a cantilevered flexible plate aligned with a uniform flow has its upstream end attached to a spring mounting. This allows the entire system to oscillate in a direction perpendicular to that of the flow as a result of the mounting׳s dynamic interaction with the flow-induced oscillations, or flutter, of the flexible plate. We also study a hinged-free rotational-spring attachment as a comparison for the heaving system. This variation on classical plate flutter is motivated by its potential as an energy-harvesting system in which the reciprocating motion of the support system would be tapped for energy production. We formulate and deploy a hybrid of theoretical and computational modelling for the two systems and comprehensively map out their linear-stability characteristics at low mass ratio. Relative to a fixed cantilever, the introduction of the dynamic support in both systems yields lower flutter-onset flow speeds; this is desirable for energy-harvesting applications. We further study the effect of adding an inlet surface upstream of the mount as a means of changing the destabilising mechanism from single-mode flutter to modal-coalescence flutter which is a more powerful instability more suited to energy harvesting. This strategy is seen to be effective in the heaving system. However, divergence occurs in the rotational system for low spring natural frequencies and this would lead to its failure for energy production. Finally, we determine the power-output characteristics for both systems by introducing dashpot damping at the mount. The introduction of damping increases the critical speeds and its variation permits optimal values to be found that maximise the power output for each system. The addition of an inlet surface is then shown to increase significantly the power output of the heaving system whereas this design strategy is not equally beneficial for the rotational system.

Investigating coupled flow-structure-acoustic interactions of human vocal fold flow-induced vibration

Flow-induced vibration of the human vocal folds is a central component of sound production for voiced speech. During vocal fold oscillation, tightly coupled flow, structure, and acoustic dynamics form a system that is rich in multi-physics phenomena, such as large deformation and large strain of exceedingly flexible and multi-layered tissues, repeated collision between vocal folds, coupling between structural modal frequencies and acoustic resonances, and the presence of non-trivial flow features such as the Coanda effect, flow separation, and axis switching. One of the aims of voice production research is to better understand these physical phenomena. In this presentation, tools and techniques for studying vocal fold flow-structure interactions will be discussed. Synthetic vocal fold replicas that exhibit flow-induced oscillations comparable to those of the human vocal folds will be introduced. These replicas are fabricated using three-dimensional prototyping, molding, and casting techniques, in which the multi-tissue layer structure of the human vocal folds is simulated using multiple layers of silicone of differing material properties. Experimental techniques used to characterize replica dynamic responses will be presented. Computational models that include fully coupled fluid, solid, and acoustic domains to simulate vocal fold vibration will be introduced. Several applications of these models and approaches will be discussed.

Fatigue fracture of composite insulator sheds utilized in strong wind areas

This paper presents the fracture failure mechanism of composite insulators that have been deployed for only one year on 750 kV transmission lines in strong wind areas. Investigations indicate that the flexible shed structure was vulnerable to flow-induced oscillation, which further brought cracking to the shed root. The interactive process of fatigue and fracture was reviewed by inspecting the failed samples to gain insight into the cracking mechanism. The fatigue theory of silicone rubber was introduced to illustrate the fatigue resistance of the material. Simulations of wind load and stress distribution were performed to demonstrate the load condition and stress concentration at the root area on sheds. Wind tunnel tests show the oscillation of two types of 750 kV insulators under strong winds of up to 60 m/s. Four samples of silicone rubber from major insulator manufacturers were tested to investigate the fatigue resistance property, and withstand limit of high- and lowperformance silicone rubbers. Results indicate that, composite insulators without specific design and wind tunnel tests were risky to apply on transmission lines in strong wind areas, which is common type of climate in northwestern China.

Large amplitude flow-induced oscillations and energy harvesting using a cyber-physical pitching plate

The dynamics of an elastically mounted flat plate in a uniform stream undergoing flow-induced pitching oscillations is studied with the aid of a cyber-physical system which is capable of simulating arbitrary structural dynamics. The system measures the angular position and velocity, and combines them with appropriate gains to provide a torque to a computer-controlled servomotor that emulates arbitrary torsional stiffness and damping. A series of experiments were carried out over a wide range of parameter space and the results demonstrate that inertial scaling of the stiffness and damping effectively captures the system behavior. As the torsional stiffness decreases, the system exhibits several bifurcations in its dynamical behavior. Firstly, a subcritical transition through a saddle-node bifurcation results in small-amplitude asymmetric limit-cycle oscillations and later, a subcritical transition through a Hopf bifurcation gives rise to large-amplitude symmetric limit-cycle oscillations. At very low stiffness the plate does not oscillate. Energy harvesting from the flow is only ~1% efficient, but is found to be optimized at a non-dimensional stiffness close to unity while increasing the Reynolds number is found to extend the range of damping over which large scale oscillations and energy harvesting can be sustained. Although velocity measurements are not made, the details of torque-position phase plane are used to infer the details of the formation time and stability of the leading-edge vortex associated with the rapidly pitching plate.