Aeroelastic Coupling Research Articles

Passive Control for a 2D Wind Turbine Blade with a Locally Flexible Structure at Low Reynolds Number

A passive control method is proposed for the performance enhancement of a 2D wind turbine blade using the aeroelastic coupling of locally flexible structure (LFS). An aeroelastic model of LFS airfoil is presented using the theory of shallow arch with large deformation. After the verification of numerical algorithm, the interaction between flow and LFS airfoil on the performance is studied with various elastic stiffness. The results show that the unsteady interaction between fluid and structure has a great potential to improve the aerodynamic performance of the airfoil. The lift-drag ratio of the airfoil is enhanced significantly at proper structural parameters, compared with the rigid airfoil. The idea for locally flexible structure can be considered as a smart structure for enhancing the aerodynamic performance of the wind turbine blade.

A modification to the flutter derivative model

There is an ambiguity in the original equations for the identification of flutter derivatives. The analysis in this paper shows that, for the two degrees of freedom (2DOF) model, these derivatives in these equations are, in fact, three-dimensional functions instead of one dimensional functions of reduced frequency. Alternative governing equations are proposed in this paper to solve this problem. These new equations “decouple” the aeroelastic coupling effect by approximated modeling strategy. The number of independent aeroelastic parameters is reduced from eight to five. Nonlinear nature of the aeroelastic coupling effect is unveiled. The nonlinear identification method is presented to retrieve the new set of aeroelastic parameters. Wind tunnel experiments were conducted to show the effectiveness of the newly proposed equations.

Aeroelastic Analysis of Horizontal Axis Wind Turbine Based on Multi-body Model

The alternate loads like aerodynamic loads, inertial force and elastic force can lead to the coupled vibration of flexible wind rotor, tower and other components, and this coupled vibration can dramatically affects the aerodynamic characteristics and service life of wind turbine. The establishment of aeroelastic coupling mathematical model of horizontal axis wind turbine and its numerical integration method are studied. The super-element(SE) which can describe elastic deformation is used to distretize the flexible component into a series of rigid bodies connected by rotational joints with force element springs and dampers. The dynamic equations of a constrained multi-body system(MBS) of wind turbine is automatically set up and numerically solved through a designed simulation program based on the theory of dynamics of multi-body system and the modelling method of hybrid multi-body system(HMBS). The analysis of dynamic characteristics of the system is realized through Fourier spectrum analysis method. The natural frequencies and modal shapes of a 5 MW offshore wind turbine of United States National Renewable Energy Laboratory(NREL) are analyzed and the effectiveness of the program and the correctness of the modeling method are verified. Aerodynamic loads acting on each rigid body of blade under deformation state are calculated based the blade element momentum(BEM) theory, and the coupling between the structural and aerodynamic analysis is carried out in real time during the process of numerical integration. The results show that super-element can accurately describe the coupling among the aerodynamic loads, inertial loads and elastic loads of wind turbine. The program developed in this study is a useful analysis platform for the aeroelastic coupling, stability analysis and the design of control system of wind turbine.

Hybrid Electroactive Wings Morphing for Aeronautic Applications

As part of the RTRA funded EMMAV (Electroactive Morphing for Micro-Airvehicles) research program electroactive materials were studied as well as their applications as actuators for morphing wings. The aim of this research program is to study both the actuation with large displacements at low frequencies as well as low displacement, high frequency actuation. The large displacement actuation, which targets primarily the flight control, is achievable using Shape Memory Alloys (SMA) while high frequency; low displacement actuation can be achieved using piezoelectric actuators [. This high frequency actuation is especially interesting for improving the aeroelastic coupling effect inducing both noise and drag. This paper describes the construction of a prototype incorporating piezoelectric and SMA based actuation mechanisms. Furthermore, a cooling mechanism for SMAs is described aiming at improving the cycle time of the actuator. The developed prototype is to be evaluated during wind-tunnel experiments showing the influence of the actuation on the fluid.

Coupled Flight-Dynamic and Low-Order Aeroelastic Model for a Slender Launch Vehicle

This paper presents a method to develop a low-order aeroelastic model that qualitatively captures some of the phenomena experienced by launch vehicles, suitable for use in preliminary controller design. Equations of motion for the two-dimensional dynamics are derived by treating the vehicle as a beam with a gimbaled nozzle attached at the aft end. The flexible-body dynamics are kinematically described using a modal representation. An aerodynamic model focuses on flow separations at diameter transitions in the transonic regime that can lead to lengthwise variations in the applied aerodynamic force. Additionally, convective effects are modeled that lead to time lag in the aerodynamic forces. The equations of motion are tenth order when neglecting convective effects and twelfth order when including convective effects. The model demonstrates some of the possible coupling that occurs between rigid-body, flexible-body, and aerodynamic states. For representative parameter values, the aeroelastic coupling can destabilize the flexible-body motion. The resulting linearized model is not fully controllable, however, is stabilizable.

Aeroelastic response of rocket nozzles to asymmetric thrust loading

The aeroelastic response of rocket nozzles subjected to combined axial thrust and side loads is investigated using a particular computational technique. The technique uses two-way “loose” coupling between an accurate flow solver and an accurate structural-dynamics solver to accurately capture the behavior of the internal flow, the behavior of the nozzle wall, and the interactions between the fluid-dynamic and structural-dynamic phenomena. It is shown that the technique captures the fluid-dynamic phenomena that are known to contribute to nozzle side loads, including the asymmetry in the propagation of the initial blast wave, the asymmetry in the separation lines, the pressure pulsations at the separation lines, the transition of separated flow patterns, and various flow instabilities. It is also shown that the computational technique couples the fluid-dynamic and the structural-dynamic solutions with an accuracy and a resolution that are sufficient for accurate prediction of the aeroelastic response modes in the system. The fluid-dynamics solver is validated for shock-induced flow separations in a sub-scale J-2S nozzle, while the structural-dynamic solver is validated for a typical dynamic response in a rocket nozzle. The two-way loose coupling methodology is validated for the flutter of a flat plate in supersonic flow, and the validation results are discussed and assessed with respect to the fitness of the computational technique for the prediction of the aeroelastic response of typical actual nozzle configurations. Finally, the side loads are computed for a J-2S nozzle with rigid walls and with flexible walls, and the results for the two types of walls are analyzed and compared. It is found that allowing aeroelastic coupling significantly affects the response of the nozzle and the predicted side loads. It is speculated that the computational technique investigated in this work combines all the elements required to accurately predict and simulate the aeroelastic response of a rocket nozzle to non-symmetric thrust, and that such a technique can be used to study and investigate side load phenomena in rocket nozzles in detail and also as a design tool for rocket nozzles. The technique is also speculated to be effective and appropriate for other fluid–structure interaction problems within an appropriate range of aeroelastic regimes.

Linearization method of nonlinear aeroelastic stability for complete aircraft with high-aspect-ratio wings

A linearization method and an engineering approach for the geometric nonlinear aeroelastic stability analysis of the very flexible aircraft with high-aspect-ratio wings are established based on the little dynamic perturbation assumption. The engineering practicability of the method is validated by a complex example. For a high-altitude long-endurance unmanned aircraft, the nonlinear static deformations under straight flight and the gust loads are calculated. At the corresponding nonlinear equilibrium state, the complete aircraft is linearized dynamically and the vibration modes are calculated considering the large deformation effects. Then the unsteady aerodynamics are calculated by the double lattice method. Finally, the aeroelastic stability of the complete aircraft is analyzed. The results are compared with the traditional linear calculation. The work shows that the geometric nonlinearity induced by the large structural deformation leads to the motion coupling of the wing chordwise bending and the torsion, which changes the mode frequencies and mode shapes. This factors change the aeroelastic coupling relationship of the flexible modes leading to the decrease of the flutter speed. The traditional linear method would give not only an imprecise flutter speed but also a possible dramatic mistake on the stability. Hence, for a high-altitude long-endurance unmanned aircraft with high-aspect-ratio wings, or a similar very flexible aircraft, the geometric nonlinear aeroelastic analysis should be a necessary job in engineering practice.

Bridge Buffeting Analysis Based on POD and Aeroelastic Coupling Method

The bridge buffeting response is a type of response varying with the time, space and frequency, in this paper, the bridge buffeting response analysis method based on Proper Orthogonal Decomposition (POD) and aeroelastic coupling is proposed, which can consider the contribution of effective turbulence on the bridge buffeting response. To test the proposed technique, a cable-stayed bridge is used to compare current analysis with the results using the traditional buffeting simulation method.

Nonlinear Modeling and Aeroelastic Analysis of an Adaptive Camber Wing

stations using actuators for which actuation force magnitudes are determined iteratively using linearized influence coefficients. Flow solutions are obtained using a high-order panel method, and aeroelastic coupling is performed using the in-house code SAMOA. For morphed-wing configurations, induced drag is reduced by creating washout. By conducting static aeroelastic analysis actuation, force magnitudes are determined and favorable aerodynamic effects are identified. Results indicate that use of a nonlinear structural model is essential in capturing the stiffening behavior observed during application of spanwise variable camber.

Analytical modeling of sound transmission across finite aeroelastic panels in convected fluids

An analytical approach is formulated to account for the effects of mean flow on sound transmission across a simply supported rectangular aeroelastic panel. The application of the convected wave equation and the displacement continuity condition at the fluid-panel interfaces ensures the exact handling of the complex aeroelastic coupling between panel vibration and fluid disturbances. To explore the mean flow effects on sound transmission, three different cases (i.e., mean flow on incident side only, on radiating side only, and on both sides) are separately considered in terms of refraction angular relations and sound transmission loss (STL) plots. Obtained results show that the influence of the incident side mean flow upon sound penetration is significantly different from that of the transmitted side mean flow. The contour plot of refraction angle versus incident angle for the case when the mean flow is on the transmitted side is just a reverse of that when the mean flow is on the incident side. The aerodynamic damping effects on the transmission of sound are well captured by plotting the STL as a function of frequency for varying Mach numbers. However, as the Mach number is increased, the coincidence dip frequency increases when the flow is on the incident side but remains unchanged when in the flow is on the radiating side. In the most general case when the fluids on both sides of the panel are convecting, the refraction angular relations are significantly different from those when the fluid on one side of the panel is moving and that on the other side is at rest.

Fluid-structure interaction in rocket engines

Numerical approaches to predict side-loads on over-expanded launcher engines, resulting from the aeroelasticity, are proposed in this study: a stability model and a fluidstructure model. The main idea is to offer a better understanding of the repercussions likely to appear from the aeroelastic coupling in terms of side loads resulting from the motion of the compression shock and that may be responsible of damage effects on the current engines. It is notably shown the existence of a given natural torsional frequency of the nozzle for which the measured side loads are maximum, phenomenon associated with a transversal wave in the flow between both walls. These studies aim to improve the only current aeroelastic stability model in over expanded nozzle.

Tumbling Dynamics of Passive Flexible Wings

The influence of flexibility on the flight of autorotating winged seedpods is examined through an experimental investigation of tumbling rectangular paper strips freely falling in air. Our results suggest the existence of a critical length above which the wing bends. We develop a theoretical model that demonstrates that this buckling is prompted by inertial forces associated with the tumbling motion, and yields a buckling criterion consistent with that observed. We further develop a reduced model for the flight dynamics of flexible tumbling wings that illustrates the effect of aeroelastic coupling on flight characteristics and rationalizes experimentally observed variations in the wing's falling speed and range.

Fluid-structure interaction in rocket engines

Numerical approaches to predict side-loads on over-expanded launcher engines, resulting from the aeroelasticity, are proposed in this study: a stability model and a fluidstructure model. The main idea is to offer a better understanding of the repercussions likely to appear from the aeroelastic coupling in terms of side loads resulting from the motion of the compression shock and that may be responsible of damage effects on the current engines. It is notably shown the existence of a given natural torsional frequency of the nozzle for which the measured side loads are maximum, phenomenon associated with a transversal wave in the flow between both walls. These studies aim to improve the only current aeroelastic stability model in over expanded nozzle.

Flight Control for Flexible, High-Aspect-Ratio Flying Wings

High-aspect-ratio flying wing configurations designed for high altitude, long endurance missions are characterized by high flexibility, which may lead to significant static aeroelastic deformation and coupling between aeroelasticity and flight dynamics. Using the concept of the mean axis, a 6-degree-of-freedom reduced-order model of the flight dynamics is constructed that minimizes the coupling between rigid-body modes and structural dynamics while accounting for the nonlinear static aeroelastic deformation of the flying wing. A multistep nonlinear dynamic inversion controller based on this reduced-order model is coupled with a nonlinear guidance law to design a flight controller for path following. The configuration of the flying wing used in the reduced-order model is updated at each time instant to account for static aeroelastic deformation. Simulation results presented in this paper show that the controller is able to successfully follow both straight line and curved ground paths while maintaining/moving to the desired altitude. The controller is also shown to be able to handle an abrupt change in the external loading, which is simulated by a step change in the payload mass.

Optimal design of an aeroelastic wing structure with seamless control surfaces

This article presents an investigation into the concept and optimal design of a lightweight seamless aeroelastic wing (SAW) structure for small air vehicles. Attention has been first focused on the design of a hingeless flexible trailing edge (TE) control surface. Two innovative design features have been created in the SAW TE section: an open sliding TE and a curved beam and disc actuation mechanism. This type of actuated TE section allows for the SAW having a camber change in a desirable shape and minimum control power demand. This design concept has been simulated numerically and demonstrated by a test model. For a small air vehicle of large sweep back wing, it is noted that significant structural weight saving can be achieved. However, further weight saving is mainly restricted by the aeroelastic stability and minimum number of carbon/epoxy plies in a symmetric layup rather than the structural strength. Therefore, subsequent effort was made to optimize the primary wing box structure. The results show that an initial structural weight can be reduced significantly under the strength criterion. The resulting reduction of the wing box stiffness and aeroelastic stability and control effectiveness can be improved by applying the aeroelastic tailoring. Because of the large swept angle and resulting lightweight and highly flexible SAW, geometrical non-linearity and large bending—torsion aeroelastic coupling have been considered in the analysis.

Aeroelastic coupling of geometrically nonlinear structures and linear unsteady aerodynamics: Two formulations

Two different time domain formulations of integrating commonly used frequency-domain unsteady aerodynamic models based on a modal approach with full order finite element models for structures with geometric nonlinearities are presented. Both approaches are tailored to flight vehicle configurations where geometric stiffness effects are important but where deformations are moderate, flow is attached, and linear unsteady aerodynamic modeling is adequate, such as low aspect ratio wings or joined-wing and strut-braced wings at small to moderate angles of attack. Results obtained using the two approaches are compared using both planar and non-planar wing configurations. Sub-critical and post-flutter speeds are considered. It is demonstrated that the two methods lead to the same steady solution for the sub-critical case after the transients subside. It is also shown that the two methods predict the amplitude and frequency of limit cycle oscillation (when present) with the same accuracy.

Analytical solutions for a single blade in vertical axis turbine motion in two-dimensions

An analytical model for a time dependent two dimensional flow around a moving profile is developed. The model is suitable for fast aerodynamic and aeroelastic coupling calculations. It determines the inviscid pressure distribution in the vicinity of one blade and the force on the blade in arbitrary two dimensional motion. The method is more flexible than previous analysis: it can represent any profile, pitching motion and blade attachment position. The method is based on conformal mapping techniques and Laurent's series decomposition and is faster and more accurate than standard panel methods. A main idea is to directly treat the singularities of the flow in a mapped plane where any geometrical plane is simplified to a circle. The vorticity is assumed to be shed in the form of a continuous vortex sheet near the trailing edge.

Chaotic Response of an Airfoil due to Aeroelastic Coupling and Dynamic Stall

Flight-test data of helicopters indicate that vibratory levels in the fuselage exhibit a wide spectrum of frequencies, including a few below the rotor revolutions per minute. It is well known that helicopter blades operate in a complex aerodynamic environment, involving time-varying heave, pitch, and pulsating oncoming flow. During operation, some sections of the rotor blade undergo dynamic stall once in a revolution. This paper attempts to understand the reason for the existence of several frequencies in the response of the fuselage and the possible cause for this observed phenomenon by analyzing the effects of dynamic stall and aeroelastic couplings on the response of 2-D airfoil. The ONERA dynamic stall model developed by Petot is modified by incorporating a higher-order rational approximation of Theodorsen's lift deficiency function. This improved model is shown to provide a better correlation with experimental stall data. The response characteristics of a 2-D airfoil undergoing pitching and plunging motion in a pulsating oncoming flow, simulating the response of a cross section of a helicopter rotor blade in forward flight are analyzed. This study shows significant difference in the response characteristics of the airfoil for unsteady (dynamic stall model) and quasi-steady aerodynamic models. It is observed that the nonlinear aerodynamics (dynamic stall effects) in association with aeroelastic couplings above a certain level lead to a bounded chaotic motion of the airfoil.

Variation of modal parameters of a cable-stayed bridge identified from ambient vibration measurements and FE modelling

During construction, the first cantilever of the Second Severn Crossing cable-stayed bridge was monitored for dynamic effects. Modal parameters of many modes were identified from ambient vibration measurements, and mode shapes and natural frequencies were matched with modes from a Finite Element (FE) model. The accuracy of the measured natural frequencies and damping ratios was estimated, and variations in these parameters were identified from long-term records. The contributions of aerodynamic and structural damping were separated, and the magnitude of the components is discussed. Variations in natural frequencies with wind velocity and, for the finished bridge, traffic loading were identified, but no changes due to temperature were detected, although the temperature range experienced was small. The variations are compared with results from the FE analysis and the significance of the measurement accuracy and natural variations are discussed in relation to Model Updating and Structural Health Monitoring. Finally, evidence of aeroelastic coupling of vertical and torsional modes, during construction, is presented. This arose as the natural frequencies of the two modes were much closer than anticipated, highlighting the need for detailed FE modelling to determine this important frequency ratio accurately.

Numerical validation of a stability model for a flexible over-expanded rocket nozzle

A numerical approach for the aeroelastical stability of an over-expanded rocket engine is proposed in this paper. The main idea is to offer a better understanding of the repercussions likely to appear from the aeroelastic coupling in terms of side loads that may be responsible for damage effects on the engine. After a brief description of the stability model issued from previous works (Pekkari's team) and details upon a numerical fluid–structure coupling code, comparative calculations are conducted. The stability model is then called into question and a finer analysis is proposed to explain its major tendency to over-predict the aeroelastic frequency shift in comparison with numerical coupling results. Copyright © 2005 John Wiley & Sons, Ltd.