Aeroelastic Vibrations Research Articles

Panel flutter suppression with nonlinear energy sinks: Numerical modeling and analysis

High-speed flight can cause aircraft skin to exhibit an aeroelastic instability typically called panel flutter. Due to the risk of fatigue failure imposed by this undesired phenomenon, several techniques have been proposed over the years to passively or actively control such aeroelastic vibrations. One relatively new method that has been proven effective for controlling various aeroelastic phenomena is the use of nonlinear energy sinks (NES). Here, for the first time, this technique is employed for suppressing panel flutter and reducing the intensity of limit cycle oscillations (LCOs). The present work consists of a comprehensive study on the numerical modeling of a NES, its coupling to an aeroelastic finite element plate model, and several solutions attained through the resulting system. In order to simulate LCOs, a geometrically nonlinear plate model is employed. The supersonic aerodynamic loads are modeled by piston theory, and the final aeroelastic equations of motion are solved directly in time by an implicit integrator. The energy dissipated by the NES and the energy injected in the panel by the flow are obtained numerically as a function of time. This provides important insights on the mechanism through which a NES can either suppress flutter or mitigate LCOs by partially balancing the aerodynamic work. The performance of the NES is tested in three regimes: At the pre-flutter regime, the NES leads the panel to a faster return to equilibrium after a perturbation; at higher speeds, the NES is still able to suppress flutter, whereas an uncontrolled panel would exhibit LCOs; at even higher speeds, the NES is no longer able to completely suppress the motion but can pump enough energy to reduce the amplitude of the LCOs. In any of these scenarios, the practical outcome would be a longer lifespan for the skin structure. Furthermore, a parametric study is conducted to assess how different NES parameters such as damping coefficient and nonlinear stiffness affect the aeroelastic response. The results reveal that a NES can be used as a lightweight device for passively controlling panel flutter, and that such technique is suitable for optimization-driven design.

Adaptive Feedforward Control for Gust-Induced Aeroelastic Vibrations

This paper demonstrates the implementation of an adaptive feedforward controller to reduce structural vibrations on a wing typical section. The aeroelastic model includes a structural nonlinearity, which is modelled in a polynomial form. Aeroelastic vibrations are induced by several gusts and atmospheric turbulence, including the discrete “one-minus-cosine” and a notably good approximation in the time-domain to the von Kármán spectrum. The control strategy based on the adaptive feedforward controller has several advantages compared to the standard feedback controller. The controller gains, which are updated in real-time during the gust encounter, are found solving a minimization problem using the finite impulse responses as basis functions. To make progress with the application in aeroelasticity, a single-input single-output controller is designed measuring the wing torsional deformation. For both deterministic and random atmospheric shapes, the controller was found successful in alleviating the aeroelastic vibrations. The impact of the control action on the unmeasured structural modes was found minimal.

Identification of Volterra kernels for improved predictions of nonlinear aeroelastic vibration responses and flutter

Aeroelastic structural systems are intrinsically nonlinear and accurate predictions of dynamic responses of nonlinear aeroelastic systems have become of paramount importance since these directly affect the accuracy and reliability of subsequent stability analyses. Such nonlinear systems can be generally represented with Volterra series whose kernels have been found to be effective in their dynamic characterizations. This paper examines how first- and second-order Volterra kernels of nonlinear aeroelastic systems can be accurately identified and then incorporated into the theoretical models of aeroelastic analyses such as predictions of dynamic response and onset of aeroelastic flutter. A novel identification method based on correlation analysis to extract frequency components has been developed which can be applied to general nonlinear aeroelastic systems to obtain accurately the required Volterra transfer functions. The method is very accurate and extremely robust against measurement noise contaminations in both input and output signals, due to the correlation scheme which effectively filters uncorrelated signal components. Detailed aeroelastic behavior of a representative pitch-plunge airfoil dynamic model with nonlinear pitch stiffness has been examined. Its Volterra transfer functions are then identified which are found to be close to their exact analytical counterparts, though interaction between kernels becomes apparent as input level increases. Once inverse Fourier transformed, these identified Volterra kernels are then included in the modeling of the dynamics of aeroelastic systems for vibration response and flutter. Extensive numerical simulation results have demonstrated that the proposed method is very accurate and resilient to measurement errors when applied to the identification of second-order Volterra kernels, and the improvement in predictions of vibration response and flutter become significant when the contributions of these second-order Volterra kernels are included in the overall aeroelastic system dynamics. The identification and subsequent inclusion of second-order Volterra kernels into system dynamics model offer improved design capabilities of nonlinear aeroelastic structural systems.

Active suppression of freeplay aeroelastic vibrations of ailerons by robust control methods with incomplete measurements

PurposeThe purpose of this paper is to analyze the active suppression of the nonlinear aeroelastic vibrations of ailerons caused by freeplay by robust H∞ and linear quadratic Gauss (LQG) methods of control in case of incomplete measurements of the state of the system.Design/methodology/approachThe flexible wing with nonlinear aileron with freeplay is treated as a plant-controller system with H∞ and LQG controllers used to suppress the aeroelastic vibrations. The simulation approach was used for analyzing the impact of completeness of measurements on the efficiency and robustness of the controllers.FindingsThe analysis shows that the H∞ method can be effectively used for suppression of nonlinear aeroelastic vibrations of aileron, although its efficiency depends essentially on completeness and types of measurements. The LQG method is less effective, but it is also able to prevent aileron vibrations by reducing their amplitudes to acceptable, safe level.Research limitations/implicationsOnly numerical analysis was carried out for the problem described; thus, the proposed solution is of theoretical value at this stage of analysis, and its application to the real suppression of aeroelastic vibrations requires further research.Practical implicationsThe work presents a potentially useful solution to the problem of interest and results are a theoretical basis for further research.Social implicationsThis work may lead to a hot debate on the advantages and drawbacks of the active suppression of vibrations in the aeroelasticians community.Originality/valueThe work raises the important questions of practical stabilizability of the nonlinear aeroelastic systems, their dependence on completeness and types of measurements and robustness of the controllers.

A harmonic piecewise linearisation-wavelet transforms method for identification of non-linear vibration “black box” systems: Application in wind-induced vibration of a high-rise building

In this work, a harmonic piecewise linearisation-wavelet transform method is proposed for identification of some kind of non-linear/linear vibration “black box” systems. The proposed method firstly looks at the non-linear vibration system as piecewise linear, and then identifies the time-varying damping ratio and frequency by the segmented amplitude log analysis and wavelet transform method, secondly it obtains the relationship between damping and frequency to time or amplitude by least squares method, and finally establishes its non-linear oscillation governing equation. The proposed harmonic piecewise linearisation-wavelet transform method is verified by two numerical examples and then is used to identify the wind-induced cross-wind non-linear/linear aerodynamic parameters of Shanghai’s World Financial Centre by aeroelastic model vibration measurement wind tunnel test. The results show that the proposed harmonic piecewise linearisation-wavelet transform method is feasible for identification of some kind of non-linear/linear “black box” vibration systems, including in a flexible high-rise building subjected to strong wind: in this case, the non-linear/linear aeroelasticity effect cannot be ignored, which would generate additional weak non-linear damping and negative aerodynamic stiffness.

Optimal Placement of Piezoelectric Element for Energy Harvesting from Supersonic Panel Flutter Oscillation of Plates

The present paper deals with the piezoelectric energy harvesting from aeroelastic vibration with simply supported plates. The supersonic panel flutter energy harvester is composed of a piezoelectric element attached to the structures and a conventional harvesting circuit. A diode bridge of four diodes is connected to the piezoelectric element and it provides a mechanism of current rectification. Aeroelastic analysis of simply supported plates with a piezoelectric element is based on a finite element method and a quasi-steady aerodynamics. The effect of location of the piezoelectric element on converged voltage in the harvesting system is examined through the numerical examples.

On the effect of moving blade grid on the flow field characteristics

The motivation of this paper is the continual development of the blades for the last stage of the steam turbine. The biggest problem is the slenderness of such blades and the extreme sensitivity to aeroelastic vibrations (flutter) caused by the instabilities present in the flow. This experimental research is dealing with the aeroelastic binding of the moving blades located in the blade grid with the flow field and vice versa. A parallelogram is used to ensure one order of freedom of the blade. The grid has five blades in total, three of them are driven by force control using three shakers. The deviation as well as force response is measured by strain gauges and dynamometers. The flow field statistical as well as dynamical characteristics are measured by optical method Particle Image Velocimetry. The grid is connected to the blow-down wind tunnel with velocity range up to 40 m/s. The aeroelastic binding is investigated in dependency on used actuation frequency and maximal amplitude (the intensity of force actuation) and on different Reynolds numbers. The flow field and the wake behind each individual blade are studied and the maximal interaction is examined for individual inter-blade phase angle of the grid.

Measurement of Aeroelastic Vibration of Multi-connected Structures in a Narrow Passage

Measurement of Aeroelastic Vibration of Multi-connected Structures in a Narrow Passage

Sliding Mode Control for a Nonlinear Aeroelastic System through Backstepping

This paper considers an aeroelastic two-dimensional wing-flap system with uncertainty and external disturbance. Nonlinear stiffness in pitch is expressed as the second-order polynomial form. In the uncontrolled aeroelastic model, limit cycle oscillation (LCO) appears at postflutter speed. By utilizing leading-edge and trailing-edge control surfaces as the control input, a sliding mode control scheme using varying boundary layers based on backstepping design is developed to suppress aeroelastic vibration. The proposed control strategy combines the advantages of both sliding mode control with varying boundary layers and backstepping technique. In contrast to the conventional sliding mode control design, the proposed method not only avoids chattering but also obtains stronger robustness. The results of numerical simulation clearly demonstrate the effectiveness of the proposed control strategy for aeroelastic vibration suppression under saturated controller input.

Robust output feedback control for aeroelastic vibration suppression of a 2-DOF airfoil under quasi-steady flow

In this paper, a robust output feedback control design is developed for suppression of aeroelastic vibration of a 2-DOF nonlinear wing section system. The aeroelastic system operates in a quasi-steady aerodynamic incompressible flowfield and is actuated using a combination of a leading-edge (LE) and a trailing-edge (TE) flap. By only utilizing measurements of pitching and plunging deflections, an innovative Lyapunov-based procedure is used to design sliding mode control inputs for the LE and TE control surface deflections. The closed-loop system is shown to have semi-global asymptotic stability even in the presence of model uncertainty and unknown external gust loading. Extensive simulation results under a variety of scenarios show the effectiveness of the control strategy.

Aerodynamic forces and galloping instability for a skewed elliptical cylinder in a flow at the critical Reynolds number

The mechanism of large-amplitude aeroelastic vibrations of cylindrical bodies in the critical Reynolds number range are still unclear. This study concerns the aerodynamic forces acting on elliptical cylinders and the induced galloping instability resulting from skew flows (i.e., the direction of the flow is angled 0°–45° with respect to the central axis of the cylinder) for Reynolds numbers in the range of 37–235 k. The effects of the critical Reynolds number and the skew angle on the aerodynamic forces and the galloping instability are investigated with pressure wind tunnel tests. In all of the cases investigated in the present study, a sharp decrease in the lift coefficient with increasing angle of attack and a reduction in the drag coefficient at the critical Reynolds number could be responsible for the galloping instability. Variations in the torque coefficient leads to a torsional aerodynamic instability at the critical Reynolds number. Furthermore, the skew flow cause a critical flow state at lower Reynolds numbers. One possible reason for this behavior is that the longer effective cross section allows the flow to reattach.

Effects of aerodynamic modification mechanisms on interference from neighboring buildings

This paper investigates the effect of the single and multiple aerodynamic modification mechanisms on the dynamic behavior of the principal building that is interfered by a very closely located building. For the study, aeroelastic vibration tests and high-frequency force balance tests are conducted to compare responses and wind forces in a well-simulated turbulent boundary layer flow. The principal building is manufactured with three different building configurations to represent the single and multiple aerodynamic modification treatments; the neighboring building which produces interference effects is made into a square prism model. Results show that the multiple modification treatment is efficient in reducing wind forces in all interference location series. However, it is also found that in certain critical conditions, such treatment is sensitive to reduced velocity, and may amplify the interference effect and result in larger displacements.

Types of aeroelastic vibrations of turbomachinery bladed discs

A real process was analyzed for presence of self-oscillations. Absence of self-oscillations was revealed as its attributes such as progressive wave and beating were not discovered. A model of oscillatory process was proposed on the basis of the kind of oscillograms and the linear superposition theory for several simultaneously excited oscillation modes. In accordance with the theory two vibrations with frequencies equal to 919 and 469Hz were summed up. As a result of addition of these frequencies and comparison of the derived oscillograms with the real process it was found that the proposed model agrees with the original well enough. Moving further into the process, a third frequency component equal to 1002Hz originates in addition to those of 465 and 918 Hz. Addition of three frequencies carried out similarly to the first example also yields a satisfactory result. Oscillograms of real and model processes have a similar appearance. The frequency components calculated on the basis of the oscillogram of the model process practically coincided with the real process frequencies. Classification of aeroelastic oscillations was analyzed in order to determine the oscillatory type of the process under investigation. Three main phenomena were distinguished: forced oscillations, flutter and non-synchronous oscillations. Almost all foreign engine companies faced the latter phenomenon, sometimes mistaken for flutter. The analyzed example was labeled as non-synchronous oscillations according to features of non-synchronous oscillations, namely, aliquant rotation frequency and resonant behavior of oscillations.

Analysis of aeroelastic vibration of rectangular cylinder in a uniform flow by a Large Eddy Simulation formulated in a non-inertial moving coordinate system

A large-eddy simulation method of calculating the flow past an elastically-supported body, has been constructed based on the finite difference method formulated in the coordinates fixed on the moving body. The boundary conditions on the body can be applied in the same way as on a fixed body. Reconstruction of computational mesh or interpolation of the computational points is not needed as the body moves. The coding of the method is simple and the computation is efficient, but the equations of motion must be written in the non-inertial coordinates and the boundary conditions on the outer boundaries of the flow domain that also move must be adjusted as the orientation of the body changes. The method has been tested in the computation of the flow past a rectangular cylinder supported by a spring and dashpot and the motion of the cylinder for which experimental data are available. Comparison with experimental data in the case of a rectangular cylinder of depth to breadth ratio of D/B=2 indicates that the characteristics of the translational oscillation corresponding to the heaving of long beams and the rotational oscillation corresponding to the torsional deformation are reproduced well.

Dynamic instability of shallow shells in three-dimensional incompressible inviscid potential flow

The system of the hypersingular integral equations with respect to the aerodynamic derivatives of the shell pressure drop is obtained to analyze the interaction of the shallow shell with three-dimensional incompressible potential air flow. This system of the integral equations is very applicable to analyze aeroelastic vibrations of thin-walled structures. The numerical approach based on the discrete vortices method is suggested to solve the system of the hypersingular integral equations. Using the assumed-mode method, the finite degrees of freedom dynamical system is derived to analyze the shallow shell dynamic instability. The dynamic instability of the shallow shell equilibrium in the subsonic air flow is analyzed numerically. This type of instability results in flutter. The influence of the structure parameters on the dynamic instability is analyzed. The parameters of the dynamic instability are compared with the data, which are calculated by the software ANSYS.

Aeroelastic and aerodynamic interference effects on a high-rise building

The 492m high Shanghai World Financial Centre is an interesting high-rise building in that the cross-section varies with the height. It is surrounded by a number of other buildings, including some of about half the height of it, a slightly lower nearby Jin Mao building, and a higher Shanghai Centre (not considered here). A series of wind tunnel tests for aeroelastic model vibration measurement and rigid model pressure measurement were carried out to investigate the aeroelasticity and aerodynamic interference effects on this building. The results show that, the densely clustered high-rise buildings (excluding the Jin Mao building) located upstream may cause the mean positive pressures of the lower obscured surface of the World Financial Centre to decrease, and the RMS pressures to increase or decrease for different configuration, which would exert some influences on the average and RMS responses. When buildings are located downstream, the wind-induced aerodynamic interference is negligible. When the Jin Mao building lies upstream, the mean positive pressures on the obscured surface of the World Financial Centre would decrease and even may become negative (the maximum absolute values may increase obviously), and the RMS pressures may also increase significantly, which cause the average displacement responses along-wind to reduce, the RMS horizontal displacement responses to increase, and the rotational displacement responses to increase for asymmetrical pressure. Moreover, it will weaken the energy and periodicity of vortex shedding of the World Financial Centre so as to reduce the possibility or amplitude of vortex resonance. More specifically, when the Jin Mao building lies downstream, the interference effects of the wind loads and wind-induced vibration of rigid World Financial Centre are less obvious, however, when the aeroelasticity is considered at the same time, a significant vortex resonance arose in the direction along the first order modal of the World Financial Centre at a lower wind speed, for which the natural frequency is close to the vortex shedding frequency. The findings derived from the present study are believed to be useful to gain further insight into the aeroelasticity and aerodynamic interference effects of high-rise buildings.

Higher-order moments of eigenvalue and eigenvector distributions for the nonlinear stochastic dynamics of cable networks

Cross-ties are employed on cable-stayed bridges to reduce wind-induced stay vibration. The occurrence of a number of failures has motivated investigations on the nonlinear dynamics of these systems. The non-linear behavior is possible at incipient failure of the cross-tie. This paper combines the features of the Equivalent Linearization Method, recently derived to study non-linear behavior in the cross-ties at moderate stay vibration amplitudes, with implementation of the Stochastic Approximation (SA) to account for the presence of parameter variability during aeroelastic vibration. The problem becomes an equivalent random eigenvalue problem with eigenvalues (equivalent frequencies) depending on a random vibration amplitude parameter. Novel implementations of the SA are considered to evaluate higher-order statistical moments of eigenvalue distributions; the consequent randomness in the eigenvectors is also examined.

Aeroelastic Vibration of a Sheet under the Influence of Periodic Gust

Aeroelastic Vibration of a Sheet under the Influence of Periodic Gust

NONLINEAR MODEL OF STABILITY STUDY OF SYSTEM "SURFACE CONTROL – ACTUATOR" OF MANEUVERABLE UNMANNED AERIAL VEHICLE

One of the important problems of the designing of maneuverable unmanned aerial vehicles (UAV) is to ensure aeroelastic stability with automatic control system (ACS). One of the possible types of aeroelastic instability of UAV with ACS is loss of stability in the system "surface control – actuator". A nonlinear model for the study of the stability of the system "surface control – actuator" is designed for solving problems of joint design of airframe and ACS with the requirements of aeroelasticity. The electric actuator is currently the most widely used on highly maneuverable UAV. The wide bandwidth and the availability of frequency characteristic lifts are typical for the modern electric actuator. This exacerbates the problem of providing aeroelastic stability of the UAV with ACS, including the problem of ensuring the stability of the system "surface control – actuator". In proposed model the surface control, performing bending-torsion oscillations in aerodynamic flow, in fact, is the loading for the actuator. Experimental frequency characteristics of the isolated actuator, obtained for different levels of the control signal, are used for the mathematical description of the actuator, then, as dynamic hinge moment, which is determined by aeroelastic vibrations of the surface control in the air flow, is calculated. Investigation of the stability of the system "surface control – actuator" is carried out by frequency method using frequency characteristics of the open-loop system. The undeniable advantage of the proposed model is the simplicity of obtaining the transfer functions of the isolated actuator. The experiment by its definition is a standard method of determining frequency characteristics of the actuator in contrast to time-consuming experiments for determining the dynamic stiffness of the actuator (with the surface control) or the transfer function of the actuator using electromechanical simulation of aeroelastic loading of the surface control, that also used in research stability problems of the system "surface control – actuator".

Downstream interference effect of high-rise buildings under turbulent boundary layer flow

This paper addresses the interference effects of high-rise building caused by either an upstream or downstream adjacent building. During the study, aeroelastic vibration tests and high-frequency force balance tests are conducted for two identical square prism models of buildings with an aspect ratio of 8 in a turbulent boundary layer flow. Later, critical locations for interfering buildings are identified by examining interfered force spectral characteristics and various patterns of response trajectories. Finally, an idealized 2D numerical simulation of CFD is tried for the simulations of some critical cases. Although the absolute values from CFD results cannot be perfectly compared with the experimental results, the simulated vorticity movement generated from the existence of both buildings provides an intuitive way for the enhancement of the downstream interference mechanism.