Aeroelastic Instability Research Articles

Ensuring the aerodynamic stability of the long-span bridges through studies in the wind tunnel

The main ideas of the aerodynamic studies of large bridges are presented in present paper. Main types of aero-elastic instability for bridges with spans over 100 meters are considered. A two-step modeling approach is presented. At the first stage, the aerodynamic characteristics of the span fragment are considered, at the second.stage the characteristics of the whole bridge. Methods for investigation of bridge oscillations in a special-purpose experimental facility – the Landscape Wind Tunnel – are described. Examples of tests with elastic similar models of bridges are given, and measurements to mitigate dangerous oscillations early in the bridge design process are described.

On mathematical modelling of aeroelastic problems with finite element method

This paper is interested in solution of two-dimensional aeroelastic problems. Two mathematical models are compared for a benchmark problem. First, the classical approach of linearized aerodynamical forces is described to determine the aeroelastic instability and the aeroelastic response in terms of frequency and damping coefficient. This approach is compared to the coupled fluid-structure model solved with the aid of finite element method used for approximation of the incompressible Navier-Stokes equations. The finite element approximations are coupled to the non-linear motion equations of a flexibly supported airfoil. Both methods are first compared for the case of small displacement, where the linearized approach can be well adopted. The influence of nonlinearities for the case of post-critical regime is discussed.

Aircraft Active Flutter Suppression: State of the Art and Technology Maturation Needs

Active flutter suppression, which is a part of the group of flight vehicle technologies known as active controls, is an important contributor to the effective solution of aeroelastic instability problems when they pop up late in the development of a new aircraft or, if used from the start of the design process, it is a key element in multidisciplinary design optimization that could lead to more efficient aircraft. This work presents a thorough overview of more than 50 years of research and development in the active flutter suppression area. Along the way, key historical developments and the current state of the art in all supporting disciplines are surveyed. Technology gaps and research and technology needs are identified. Special attention is given to the vehicle safety issue and to research and development in the active flutter suppression area that would complement ongoing research and development in all areas of aeroelasticity, aeroservoelasticity, and active control. A thorough bibliography contains references that cover all building blocks of active flutter suppression technology. The work would, hopefully, contribute to the preservation of the treasures of experience and knowledge in this area so that they would not be forgotten and lost and would remain available to professionals working in this field.

Enhancement of flutter stability in wind turbines with a new type of passive damper of torsional rotation of blades

Classical flutter of wind turbine blades is defined as an aeroelastic instability where a torsional blade mode couples to a flapwise bending mode to result in a mutual rapid growth of the amplitude of the flapwise and torsional motions beyond certain rotational speed of the rotor. Till date, flutter instability of wind turbine rotor has not been considered to be a serious problem. However, with the advent of larger turbines fitted with relatively softer blades, classical flutter may become a more important design consideration. As an economically viable alternative to increasing the torsional rigidity (which is more expensive), the use of a type of viscous damper mounted inside the wind turbine blades is proposed to promote the flutter critical rotational speed of the rotor. The flutter critical rotational speed is determined by using a time-domain aeroelastic simulation technique based on the quasi-steady aerodynamic forces and also considering turbulence intensity of incoming wind. The optimal damper location and the optimal damping constant values are investigated for the DTU 10 MW wind turbine. It is shown that at an optimal tuning of the damper, the flutter critical rotational speed is only marginally dependent on the turbulence intensity of the incoming wind field, on the mean wind velocity and on the structural damping of the flutter torsional eigenmode. The novel tuned damper device proposed in this paper may increase the flutter critical rotational speed with more than 100%.

Energy harvesting from different aeroelastic instabilities of a square cylinder

This paper presents an experimental and numerical investigation of the power extraction from the oscillations of a square beam due to aeroelastic instabilities. The energy harvesting is performed using a coil-magnet arrangement connected to a variable resistance load with the target objective to auto-power a remote sensor. Two aeroelastic phenomena are investigated: Vortex Induced Vibration (VIV) and cross-flow galloping. The first instability (VIV) is analyzed on a free-standing vertical structure. A second experimental set-up is developed on a horizontal square cylinder supported by springs, free to oscillate vertically as a rigid body. In this case, both galloping and VIV interact, leading to interesting characteristics in order to harvest energy from the wind. The behavior of each electro-mechanical aeroelastic system is investigated for different reduced wind speeds and load resistances in a wind tunnel. Observed efficiencies are rather low, but large enough to power a remote sensor with an adapted measuring strategy. Both harvesting systems are then studied numerically using a wake oscillator model (for VIV) coupled to a quasi-steady model (for galloping) and an electric model (for the harvester). This mathematical model is used to extend the parametric space and to highlight the effectiveness of the high stable branch of the VIV-galloping curve to harvest energy.

Reliability-based aeroelastic design of composite plate wings using a stability margin

Reliability-based design is concerned with ensuring that constraints are enforced with acceptable probability under inherent variability in properties. In aircraft design, such a constraint may be that aeroelastic instability does not occur at velocities encountered by the aircraft. This approach can be complicated, as the aeroelastic instability speed is a discontinuous function of material properties, on account of particular modes only becoming unstable for some parameter values. In reliability analysis, it is common to use surrogate models due to the computational expense associated with Monte Carlo Simulation, however, such methods can be inaccurate when emulating discontinuous functions such as the aeroelastic instability speed. In this paper, an alternative approach is proposed in which Gaussian process surrogate models are fitted directly to each of the modal eigenvalues at the design air-speed, and used to emulate a stability margin based upon the most critical eigenvalue. Using this approach, it is shown that the reliability may be estimated for the aeroelastic stability using smooth emulators, thereby overcoming the problems associated with discontinuities. The method is demonstrated for layup optimisation of composite plate wings with uncertain ply angles, in which the probability of aeroelastic instability occurring is minimised for a prescribed air-speed. In uncertainty quantification, a good agreement is found with Monte Carlo Simulation with an order of two magnitudes reduction in model runs. Through reliability-based design, reductions in the probability of failure of up to 99.8% are achieved by increasing the stability margin at the design speed.

Modeling the coupled electro-mechanical response of a torsional-flutter-based wind harvester with a focus on energy efficiency examination

Wind energy is a rapidly evolving field of research because of the need for clean energy resources. Large horizontal axis wind turbines (HAWT) are often employed to increase output power and energy production. On the other hand, “specialized” wind-based energy systems have been proposed to capture the wind energy resource in the low wind speed range and for intermediate-scale applications, e.g., one or few residential housing units. Wind harvesters, triggered by various aeroelastic instability regimes, have been studied in recent years [e.g., Matsumoto et al. (2006)]. Along this line of research, the writer has examined a torsional-flutter-based apparatus for extracting energy from the wind flow. This paper presents some recent advancements, a new fully-coupled electro-mechanical model and the numerical results of an ongoing investigation.

Comparative Study on Uni- and Bi-Directional Fluid Structure Coupling of Wind Turbine Blades

The current trends of wind turbine blade designs are geared towards a longer and slender blade with high flexibility, exhibiting complex aeroelastic loadings and instability issues, including flutter; in this regard, fluid-structure interaction (FSI) plays a significant role. The present article will conduct a comparative study between uni-directional and bi-directional fluid-structural coupling models for a horizontal axis wind turbine. A full-scale, geometric copy of the NREL 5MW blade with simplified material distribution is considered for simulation. Analytical formulations of the governing relations with appropriate approximation are highlighted, including turbulence model, i.e., Shear Stress Transport (SST) k-ω. These analytical relations are implemented using Multiphysics package ANSYS employing Fluent module (Computational Fluid Dynamics (CFD)-based solver) for the fluid domain and Transient Structural module (Finite Element Analysis-based solver) for the structural domain. ANSYS system coupling module also is configured to model the two fluid-structure coupling methods. The rated operational condition of the blade for a full cycle rotation is considered as a comparison domain. In the bi-directional coupling model, the structural deformation alters the angle of attack from the designed values, and by extension the flow pattern along the blade span; furthermore, the tip deflection keeps fluctuating whilst it tends to stabilize in the uni-directional coupling model.

Robust and Reliability-Based Aeroelastic Design of Composite Plate Wings

Probabilistic design methods may be used to account for inherent variability in the manufacturing quality of aerospace structures. Two different probabilistic approaches are compared alongside a deterministic method for the aeroelastic design of composite plate wings with uncertain ply orientations. The instability speed is found to be a discontinuous function of the lamination parameters, which are themselves functions of the ply orientations. A surrogate modeling approach is presented in which Gaussian processes are combined with support vector machines to emulate the discontinuous instability speed to efficiently propagate uncertainty through the model. The surrogate model is used to calculate the objectives of two optimization strategies: a reliability-based design, in which the probability of aeroelastic instability occurring within the design envelope is minimized, and a robust design, in which the mean and standard deviation of the instability speed are traded off. A genetic algorithm is used to optimize the layup, and reductions in the probability of failure of 94% are achieved in the reliability-based design using a design speed, relative to benchmark deterministic designs. A 74% reduction in the standard deviation is achieved in the robust design, at the expense of a 1% reduction in the mean.

Numerical simulations of aeroelastic instabilities to optimize the performance of flutter-based electromagnetic energy harvesters

The article presents a two-dimensional fully coupled fluid–structure interaction model for simulating aeroelastic instabilities to evaluate and optimize the performance of flutter-based electromagnetic energy harvesters. The flutter-type unstable vibration of T-shaped cantilever systems is investigated as a driving mechanism for small-scale energy conversion. A flow solver based on the vortex particle method and a structural solver based on a corotational finite element formulation are coupled in order to accurately account for the geometrically nonlinear effects of such very flexible elements. A reference harvester is simulated considering the damping effects arising from the electromagnetic transducer. The estimated flutter wind speed and the predicted energy outputs under different electrical resistances are found to agree reasonably well with reference wind tunnel experiments. The influence of physical parameters such as length, thickness, and cantilever tip height on the performance of the harvester is investigated up to an envelope volume of 42 cm3. The maximum power output is found to be 5.3 mW at 8 m/s. The optimized harvester shows a better performance under low wind speeds by producing 0.65 mW at 4 m/s where the reference harvester produced no power.

Effects of aeroelasticity on the performance of hypersonic inlet

Effects of the aeroelasticity on the performance of the hypersonic inlet have been investigated numerically in this study. The aeroelasticity has been simulated using the coupled computational fluid dynamics/computational structural dynamics method, which is solved by the in-house code. The unsteady Reynolds-averaged Navier–Stokes equations have been solved in the computational fluid dynamics simulation, and the modal method has been adopted in the computational structural dynamics simulation. Two cases have been utilized to validate the numerical method. Finally, the aeroelasticity has been simulated for inlet plate with different thicknesses. The effects of aeroelasticity on performance parameters and flow structure have been discussed in detail. The results show that the generalized displacements present the “beat” phenomenon in the time domain. The power spectral density of the generalized displacements implies that the aeroelastic instability is mainly caused by the coupling between the fourth- and fifth-order modes. The time-average flow rate coefficient and pressure rise ratio increase relative to the initial value, while the total pressure recovery coefficient decreases. The fluctuation amplitude of the flow rate coefficient is small, while that of the total pressure recovery coefficient and pressure rise ratio are relatively large. Besides, the phases of the three performance parameters are greatly different. Furthermore, the aeroelasticity has significant effect on the shock wave structure especially at the exit of the inlet.

Aeroelastic Optimization of High-Speed Tiltrotor Wings with Wing Extensions and Winglets

Tiltrotors can experience an aeroelastic instability called whirl flutter, which is characterized by highly coupled wing–rotor vibrations, which are catastrophic in nature and limit the maximum speed of the aircraft. In this paper, an optimization study aimed at improving the whirl flutter speed of a tiltrotor equipped with a wing extension and a winglet is presented. Parametric studies are conducted on an aeroelastic model of a rotor mounted on a cantilevered wing with an extension and a winglet. The parameters that are investigated are stiffness, structural taper, composite couplings, extension sweep, and winglet cant angle. It is found that, when the wing has a structural taper and is equipped with a wing extension and a winglet, the whirl flutter speed can be increased by 15%. The parametric study is followed by an optimization study to determine the optimal combinations of these parameters that maximize whirl flutter speed. Genetic algorithms are used for the optimization process. Upper and lower bounds are placed as constraints on the design variables. It is shown that the optimized design has a flutter speed of 22% (65 kt) more than the baseline.

Non-linear aeroelastic response of high aspect-ratio wings in the frequency domain

ABSTRACTA current trend in the aeronautic industry is to increase the wing aspect ratio to enhance aerodynamic efficiency by reducing the induced drag and thus reduce fuel consumption. Despite the associated benefits of a large aspect ratio, such as higher lift-to-drag ratios and range, commercial aircraft usually have a relatively low aspect ratio. This is partially explained by the fact that the wing becomes more flexible with increasing aspect ratio and thus more prone to large deflections, which can cause aeroelastic instability problems such as flutter. In this work, an aeroelastic study is conducted on a rectangular wing model of 20 m span and variable chord for a low subsonic speed condition to evaluate the differences between linear and non-linear static aeroelastic responses. Comparisons between linear and non-linear displacements, natural frequencies and flutter boundary are performed. An in-house non-linear aeroelastic framework was employed for this purpose. In this work, the influence of the aspect ratio and geometric non-linearity (highly deformed states) is assessed in terms of aeroelastic performance parameters: flutter speed and divergence speed. A nearly linear correlation of flutter speed difference (relative to linear analysis results) with vertical-tip displacement difference is observed. The flutter and divergence speeds vary substantially as the wing aspect ratio increases, and the divergence speeds always remain above the flutter speed. Furthermore, the flutter mechanism was observed to change as the wing chord is decreased.

Aeroelastic Stability of Hingeless Rotor Blades in Hover Using Fully Intrinsic Equations

In this paper, the aeroelastic stability of a hingeless rotor blade in hover is investigated. The blade is modeled using the geometrically exact, fully intrinsic beam equations, whereas the aerodynamic loads applied on the blade are simulated based on the quasi-steady Greenberg aerodynamic theory. Using the generalized differential quadrature method, the resultant coupled aeroelastic equations are discretized and solved, and the eigenvalues of the linearized system are determined. By inspecting the eigenvalues, the stability of the system is examined for variations in different parameters. The obtained results are validated through comparison against those reported in the literature. Furthermore, the effects of offset of the aerodynamic center from the reference axis, the precone angle, the lead–lag frequency, static stall, and the drag coefficient on the stability boundaries of the system are evaluated. It is found that, by using the geometrically exact, fully intrinsic beam equations along with the generalized differential quadrature method, the aeroelastic instabilities of the hingeless rotor blades in hover can be determined accurately.

Loads and Acoustics Prediction on Deployed Weapons Bay Doors

Unsteady separated flow from deployed weapons bay doors can interact with the highly unsteady flow in the open bay cavity, which is known to exhibit strong acoustic content and could lead to fluid-resonance and high-intensity acoustic noise. The culmination of these unique flow physics can potentially excite structural modes of the doors, aircraft surfaces, or externally carried munitions and fuel tanks and can ultimately lead to aeroelastic instabilities, such as buffet, flutter, limit-cycle oscillations, or fatigue-induced failures. A hybrid Reynolds-averaged Navier–Stokes large eddy simulation (RANS/LES) method with low-dissipation schemes is developed to improve flow and acoustics predictive capabilities for supersonic weapons bays. Computational simulations are conducted for a weapons cavity with different deployed bay doors configurations, including the effect of dynamically moving doors, to assess the tonal content and unsteady aerodynamic loads on the doors. Wind tunnel testing is also carried out to provide unsteady experimental data for use in validating the high-fidelity simulation capability. The simulation results in terms of unsteady pressure, velocity fluctuations, and pressure resonant frequencies are computed and presented. The results suggest that the deployed doors energize the shear layer and cause it to go deeper into the cavity and produce higher unsteady fluctuations on the weapons cavity floor and aft wall. The deployed doors also cause a shift in the dominant resonant modes.

Investigations of Flutter and Aerodynamic Damping of a Turbine Blade: Experimental Characterization

Flutter is a self-excited and self-sustained aero-elastic instability, caused by the positive feedback between structural vibration and aerodynamic forces. A two-passage linear turbine cascade was designed, built, and tested to better understand the phenomena and collect data to validate numerical models. The cascade featured a center airfoil that had its pitch axis as a degree-of-freedom to enable coupling between the air flow and mechanical response in a controlled manner. The airfoil was designed to be excited about its pitch axis using an electromagnetic actuation system over a range of frequencies and amplitudes. The excitation force was measured with load cells, and the airfoil motion was measured with accelerometers. Extraordinary effort was taken to minimize the mechanical damping so that the damping effects of the airflow over the airfoil, that were of primary interest, would be observable. Assembling the cascade required specialized alignment procedures due to the tight clearances and large motion. The aerodynamic damping effects were determined by observing changes in the mechanical frequency response of the system. Detailed aerodynamic and mechanical measurements were conducted within a wide range of Mach numbers (Ma) from Ma = 0.10 to 1.20. Experimental results indicated that the aerodynamic damping increased from Ma = 0.10 to 0.65, dropped suddenly, and was then constant from Ma = 0.80 to 1.20. A flutter condition was identified in the interval between Ma = 0.65 and Ma = 0.80. The aerodynamic damping was also found to be independent of displacement amplitude within the tested range, giving credence to linear numerical approaches.

VIBRATION ANALYSIS OF A FLAT PLATE UNDER AERODYNAMIC LOADING

Aeroelastic instabilities may occur in aircraft surfaces, leading to fatigue or structure failure. Flutter is an aeroelastic instability that results in a self-excited divergent oscillatory behavior of the structure. A classical two-degree-of-freedom flutter is a combination of bending and torsion vibration modes. A flexible mount system has been developed for flutter tests with rigid wings in wind tunnel. This flexible mount has to provide a well-defined two-degrees-of-freedom system on which rigid wings encounter flutter. Experi-mental Modal Analysis (EMA) and Finite Element Model Analysis (FEM) were performed to verify the natural frequencies and modes prior to any wind tunnel flutter test. The equations of motion of the system were developed using Lagrange's equation. The critical flutter speed was determined by three different methods: the p-method for steady flow, the classical flutter analysis and the k-method for unsteady flow, and comparing with experimental results. قد يحدث عدم الاستقرار في أجنحة الطائرات مما يؤدي إلى الکسر او انهيار الهيکل بالکامل. وتعتبر الرفرفة أکثر أنواع عدم الاستقرارالتى تنشأ من التداخل بين قوى الهواء، والمرونة، وقوى القصور الذاتي. ولدراسة هذه الظاهرة نحصل على درجتان من درجات الحرية وهما عزم الثنى وعزم الالتواء وذلک عن طريق وضع انظمة مرنة وأخرى جاسئة ، ويتم وضع هذا النظام فى نفق هواء وذلک للحصول على هذه الظاهرة .ويتم استنتاج معادلات الحرکة عن طريق معادلة لاجرانج. ويتم حساب سرعة الهواء التى تحدث عندها هذه الظاهرة باستخدام ثلاثة طرق مختلفة، واحدة فى تدفق منتظم للهواء، والاثنان الأخريان فى تدفق غير منتظم للهواء

Dynamic stability analysis of a pressurized FG-CNTRC cylindrical shell interacting with supersonic airflow

The objective of this research is to investigate the aeroelastic buckling and flutter instability of a pressurized functionally graded carbon nanotube reinforced composite (FG-CNTRC) cylindrical shell subjected to supersonic airflow. The dynamic model of the FG-CNTRC cylindrical shell is established in accordance with the first-order shear deformation theory, Donnell kinematic theory along with the von Karman geometrical nonlinearity. The quasi-steady Krumhaar's modified piston theory by considering the effect of the panel curvature is used to estimate the aerodynamic pressure induced by the supersonic airflow. The dynamic equations are discretized using trigonometric expansion through the circumferential direction and harmonic differential quadrature (HDQ) method through the meridional direction. Effects of boundary conditions, geometrical parameters, volume fraction and distribution of CNTs and the Mach number on the flutter instability, onset of the buckling and deformation shapes of the cylindrical shell are put into evidence through a set of parametric studies. The simulation indicates that the critical flutter dynamic pressure may be significantly enhanced through functionally graded distribution of CNTs in a polymer matrix. Furthermore, it is found that presence of the aerodynamic pressure may completely change deformation shapes of the FG-CNTRC cylindrical shell.

Aeroelastic passive control optimization of supersonic composite wing with external stores

This paper provides a study on passive aeroelastic control optimization, by means of aeroelastic tailoring, of a composite supersonic wing equipped with external stores. The objective of the optimization is to minimize wing weight by considering the aeroelastic flutter and divergence instability speeds as constraints at several flight altitudes. The optimization variables are the composite ply angle and skin thickness of the wing box, wing rib and its control surfaces. The aeroelastic instability speed is set as constraint such that it should be higher than the flutter speed of a metallic base line model of supersonic wing having previously published. A finite element analysis is applied to determine the stiffness and mass matric of the wing and its multi stores. The boundary element method in the form of doublet lattice method is used to model the unsteady aerodynamic load. The results indicate that, for the present wing configuration, the high modulus Graphite/Epoxy composite provides a desired higher flutter speed and lower wing weight compare to that of Kevlar/Epoxy composite as well as the base line metallic wing materials. The aeroelastic boundary thus can be enlarged to higher speed zone and in the same time reduce the structural weight which is important for a further optimization process.

Flutter suppression for highly flexible wings using passive and active piezoelectric effects

This paper addresses both active and passive flutter suppressions for highly flexible wings using piezoelectric transduction. An active aeroelastic formulation is used in the studies, featuring a geometrically nonlinear beam formulation coupled with 2-D unsteady aerodynamic equations. The piezoelectric effect is involved in the dynamic nonlinear beam equations, allowing for the aeroelastic studies on multifunctional wings for both piezoelectric energy harvesting and active actuation. In this study, the active piezoelectric actuation is applied as the primary approach for the flutter suppression, with the energy harvesting, as a secondary passive approach, concurrently working to provide an additional damping effect on the wing vibration. The multifunctional system may also convert wing vibration energy to electric energy as an additional function. Moreover, a Linear Quadratic Gaussian controller is developed for the active control of wing limit-cycle oscillations due to the flutter instability. In the numerical studies, both the active and passive flutter suppression approaches are enabled for a highly flexible wing. The impact of the piezoelectric actuator and energy harvester placement on the wing flutter characteristic is explored. This paper presents a comprehensive approach to effectively suppress the aeroelastic instability of highly flexible piezoelectric wings, while allowing to harvest the residual vibration energy. The active multifunctional wing technology that is explored in the paper has the potential to improve the aircraft performance from both aeroelastic stability and energy consumption aspects.