Aeroelastic Response Research Articles

Characterization of aeroelastic response and aerodynamic stiffness effect of an airfoil in the presence of dynamic stall

Characterization of aeroelastic response and aerodynamic stiffness effect of an airfoil in the presence of dynamic stall

Parked aeroelastic field rotor response for a 20% scaled demonstrator of a 13‐MW downwind turbine

AbstractAeroelastic parked testing of a unique downwind two‐bladed subscale rotor was completed to characterize the response of an extreme‐scale 13‐MW turbine in high‐wind parked conditions. A 20% geometric scaling was used resulting in scaled 20‐m‐long blades, whose structural and stiffness properties were designed using aeroelastic scaling to replicate the nondimensional structural aeroelastic deflections and dynamics that would occur for a lightweight, downwind 13‐MW rotor. The subscale rotor was mounted and field tested on the two‐bladed Controls Advanced Research Turbine (CART2) at the National Renewable Energy Laboratory's Flatiron Campus (NREL FC). The parked testing of these highly flexible blades included both pitch‐to‐run and pitch‐to‐feather configurations with the blades in the horizontal braked orientation. The collected experimental data includes the unsteady flapwise root bending moments and tip deflections as a function of inflow wind conditions. The bending moments are based on strain gauges located in the root section, whereas the tip deflections are captured by a video camera on the hub of the turbine pointed toward the tip of the blade. The experimental results are compared against computational predictions generated by FAST, a wind turbine simulation software, for the subscale and full‐scale models with consistent unsteady wind fields. FAST reasonably predicted the bending moments and deflections of the experimental data in terms of both the mean and standard deviations. These results demonstrate the efficacy of the first such aeroelastically scaled turbine test and demonstrate that a highly flexible lightweight downwind coned rotor can be designed to withstand extreme loads in parked conditions.

Numerical investigations of coupled aeroelastic performance of wind turbines by elastic actuator line model

With the economic considerations of wind power generation, blades of wind turbines are gradually becoming longer and more flexible. Therefore, it is necessary to research the influence of coupling aerodynamic and elastic structure of blades of wind turbines. Moreover, in a wind farm, the turbulent wind is generated by the irregular movement of the atmosphere, which makes the elastic deformations of blades and the complex wake interference effect between wind turbines. Therefore, the coupled aeroelastic performance of the blade is applied to analyze the interaction of the wake flow field of two tandem wind turbines based on the elastic actuator line model and the Euler-Bernoulli beam theory. Firstly, the correctness of the coupled aeroelastic model is illustrated by comparing the aeroelastic response with OpenFAST software. Secondly, the influence of aeroelastic performances is researched on the structural deformations of blades. It is found that the aerodynamic power and thrust decrease after considering the deformations of blades. Furthermore, the interference effect on the thrust is more evident than aerodynamic power. Thirdly, the far wake region is affected apparently when the deformations of blades are taken into consideration, which causes the tip vortex interference phenomenon to delay. Finally, compared with the uniform inflow condition, the displacement and speed of the deformations of the blades are both increased under the turbulent inflow condition. Meanwhile, the disturbance of the wake vortex in the far wake region by the turbulent wind is stronger. Besides, by comparing the differences between the four vortex identification methods, it is significant that the ΩR method can better capture the vortex system behind the rotor plane without manual intervention threshold selection. The vortex structure also represents the vorticity of the wake vortex evolution.

Large eddy simulations of a utility‐scale horizontal axis wind turbine including unsteady aerodynamics and fluid‐structure interaction modelling

AbstractGrowing horizontal axis wind turbines are increasingly exposed to significant sources of unsteadiness, such as tower shadowing, yawed or waked conditions and environmental effects. Due to increased dimensions, the use of steady tabulated airfoil coefficients to determine the airloads along long blades can be questioned in those numerical fluid models that do not have the sufficient resolution to solve explicitly and dynamically the flow close to the blade. Various models exist to describe unsteady aerodynamics (UA). However, they have been mainly implemented in engineering models, which lack the complete capability of describing the unsteady and multiscale nature of wind energy. To improve the description of the blades' aerodynamic response, a 2D unsteady aerodynamics model is used in this work to estimate the airloads of the actuator line model in our fluid–structure interaction (FSI) solver, based on 3D large eddy simulation. At each section along the actuator lines, a semi‐empirical Beddoes‐Leishman model includes the effects of noncirculatory terms, unsteady trailing edge separation, and dynamic stall in the dynamic evaluation of the airfoils' aerodynamic coefficients. The aeroelastic response of a utility‐scale wind turbine under uniform, laminar and turbulent, sheared inflows is examined with one‐ and two‐way FSI coupling between the blades' structural dynamics and local airloads, with and without the enhanced aerodynamics' description. The results show that the external half of the blade is dominated by aeroelastic effects, whereas the internal one is dominated by significant UA phenomena, which was possible to represent only thanks to the additional model implemented.

Predictive and stochastic reduced-order modeling of wind turbine wake dynamics

Abstract. This article presents a reduced-order model of the highly turbulent wind turbine wake dynamics. The model is derived using a large eddy simulation (LES) database, which cover a range of different wind speeds. The model consists of several sub-models: (1) dimensionality reduction using proper orthogonal decomposition (POD) on the global database, (2) projection in modal coordinates to get time series of the dynamics, (3) interpolation over the parameter space that enables the prediction of unseen cases, and (4) stochastic time series generation to generalize the modal dynamics based on spectral analysis. The model is validated against an unseen LES case in terms of the modal time series properties as well as turbine performance and aero-elastic responses. The reduced-order model provides LES accuracy and comparable distributions of all channels. Furthermore, the model provides substantial insights about the underlying flow physics, how these change with respect to the thrust coefficient CT, and whether the model is constructed for single wake or deep array conditions. The predictive and stochastic capabilities of the reduced-order model can effectively be viewed as a generalization of a LES for statistically stationary flows, and the model framework can be applied to other flow cases than wake dynamics behind wind turbines.

Aeroelastic modeling and analysis of wings in proximity

This study investigates the effects of aerodynamic interaction on the aeroelastic characteristics of wings in proximity which is considered to be aerodynamically more efficient than the single wing case. The structural model of the wing is based on the first-order shear deformation theory and analyzed by the finite element method. The flow surrounding the wings is governed by potential theory and the aerodynamic loads are computed using the unsteady vortex lattice method. The aeroelastic simulation framework is established by integrating the structural and aerodynamic models within the flexible multibody dynamic program to solve the aeroelastic equations of motion in the time domain. Cantilever wings in proximity, arranged in biplane configuration, with different geometric parameters such as gap, sweep angle, dihedral angle, and stagger are considered in the analysis. The numerical results show that the smaller gap between the wings magnifies the aerodynamic interaction effect and decreases the onset of flutter speed. Sweeping the wing backward further reduces the flutter boundaries of the biplane model mainly due to changes in the wing's structural modal whereas the effect of dihedral angles is understated. The effect of stagger is more evident for a smaller gap and mostly leads to an increase in the flutter boundary. The aeroelastic responses are independent of initial disturbance and show a natural tendency to oscillate with a phase difference. This investigation warrants the importance of considering biplane wings as a single system for a gap lower than wing chord length as well as the significance of other geometric parameters in the aeroelastic calculation.

Active flutter suppression of damaged variable stiffness laminated composite rectangular plate with piezoelectric patches

The present study investigated the effect of damage on the free vibration and the aeroelastic response of variable stiffness (VS) composite laminated plates. The ineluctable defects or internal flaws are incorporated into the finite-element coding through anisotropic damage formulation and coupled with MSC.Nastran using Direct Matrix ion Program to generate the aerodynamic forces at discrete values of reduced frequencies for performing aeroelastic analysis. The sensitive location of damage for the free vibration and flutter characteristics is identified. Thereafter, the best placement of piezoelectric patches is determined to enhance the aeroelastic boundary of the damaged VS plates with active feedback control.

Atmospheric rotating rig testing of a swept blade tip and comparison with multi-fidelity aeroelastic simulations

Abstract. One promising design solution for increasing the energy production of modern horizontal axis wind turbines is the installation of curved tip extensions. However, since the aeroelastic response of such geometrical add-ons has not been characterized yet, there are currently uncertainties in the application of traditional aerodynamic numerical models. The objective of the present work is twofold. On the one hand, it represents the first effort in the experimental characterization of curved tip extensions in atmospheric flow. On the other hand, it includes a comprehensive validation exercise, accounting for different numerical models for aerodynamic load prediction. The experiments consist of controlled field tests at the outdoor rotating rig at the Risø campus of the Technical University of Denmark (DTU), and consider a swept tip shape. This geometry is the result of an optimized design, focusing on locally maximizing power performance within load constraints compared to an optimal straight tip. The tip model is instrumented with spanwise bands of pressure sensors and is tested in atmospheric inflow conditions. A range of fidelities of aerodynamic models are then used to aeroelastically simulate the test cases and to compare with the measurement data. These aerodynamic codes include a blade element momentum (BEM) method, a vortex-based method coupling a near-wake model with a far-wake model (NW), a lifting-line hybrid wake model (LL), and fully resolved Navier–Stokes computational fluid dynamics (CFD) simulations. Results show that the measured mean normal loading can be captured well with the vortex-based codes and the CFD solver. The observed trends in mean loading are in good agreement with previous wind tunnel tests of a scaled and stiff model of the tip extension. The CFD solution shows a highly three-dimensional flow at the very outboard part of the curved tip that leads to large changes of the angle of the resultant force with respect to the chord. Turbulent simulations using the BEM code and the vortex codes resulted in a good match with the measured standard deviation of the normal force, with some deviations of the BEM results due to the missing root vortex effect.

Local mesh deformation using a dual-restricted radial basis functions method

In order to study the motion of a deformable configuration in the air flow, mesh deformation technology is essentially needed. The radial basis function (RBF) interpolation algorithm is a popular mesh deformation algorithm. For local mesh deformation, such as aeroelastic response simulation of aircraft components and deflection of flight control surface, we do not need to move all grid nodes. On the contrary, only part of the mesh near the deformation boundary needs to be moved. If we can limit the deformation range caused by the RBF method to a small region, the computational cost could be saved. In addition to the consideration of efficiency, another reason is that in the position optimization of aircraft components, we need to keep most of the meshes unchanged to reduce the error caused by mesh inconsistency. In the previous research [24], a restricted RBF interpolation algorithm was constructed and coupled with a multi-level interpolation technique to reduce the volume mesh nodes for global mesh deformation problem. The restricted RBF method is improved in the current research and a so-called dual-restricted RBF algorithm is proposed. This new method constructs a restricting function using the distance both to the moving and static boundaries so that the stationary wall could be preserved exactly. Theoretical analysis and numerical examples also have confirmed that the new algorithm is beneficial to improve the efficiency of mesh deformation process compared with the restricted RBF algorithm because mesh nodes near the stationary boundary no longer need to be moved.

Preset angle, aspect ratio, and stall representations effects on the aeroelastic responses of small-scale wind turbine blades

This work provides an inclusive nonlinear aeroelastic study on the effects of horizontal axis Small-Scale Wind Turbines (SSWTs)’s preset angle of attack as well as aspect ratio on its aerodynamic stability. For this purpose, an enhanced nonlinear reduced-order model is established and a state–space formulation for the aerodynamics of a comprehensive wing-based aeroelastic system is constructed. This representation captures the influence of the preset angle of attack where the blade no longer displays an oscillatory motion around its zero-equilibrium point in the post-flutter region, instead, a new equilibrium point is introduced to the system through the incorporation of the preset angle. The effect of the preset angle on the blade stability is investigated by jointly performing linear analyses that provide insights on this parameter’s influence on the onset of flutter, accompanied with nonlinear analyses that evaluate the latter design parameter’s effect on the blade bending and torsion amplitudes in the post-flutter regime. Additionally, the Duhamel’s principle specific to low angles of attack is extended to represent unsteady flows with non-conventional lift curves of low aspect ratio blades while considering different representations of the aerodynamic stall. Results show the significant effect of the preset angle on both the onset of flutter as well as the post flutter amplitudes where the higher the preset angle, the higher the flutter speed. Furthermore, this research effort also demonstrates the importance of accurate lift modeling since ill-evaluated lift characteristics leads to a potential over- or under-estimation of the blade’s torsional and bending amplitudes post-flutter, and the conventional 2π slope assumption frequently used in various reduced-order-models within the literature seldom predicts erroneous onset of flutter values as well as inaccurate amplitudes in the post-flutter regime especially for the case of low aspect-ratio systems.

Influence of the installation of a trailing edge flap on the vortex induced vibrations of a wind turbine blade

This work explores the use of a trailing edge flap for the mitigation of the Vortex Induced Vibrations (VIV) of a wind turbine blade in standstill. Fluid structure interaction simulations are performed, coupling a computational fluid dynamics code with a multi-body structural solver. A publicly available 96 m long wind turbine blade is selected for the study, and labeled as IEA 10MW. The effects of a flap actuation in the range [−90°, 90°] are analyzed, for a given set of inflow conditions, and compared to the aeroelastic response of the original IEA 10MW geometry. While the consideration of the flap does lead to reductions on the amplitude of the edgewise vibrations, these are only noticeable when operating outside of the design limits of the aerodynamic device. A simple non-dimensional analysis seems to indicate that the potential of the flap installation with regards to VIV mitigation is very limited, as the instability is only shifted towards different inflow conditions and the expected effects on the maximum amplitude of vibrations are minor.

Motion characteristics and aero-elastic responses of floating offshore wind turbine under coupling action of waves and winds

As a complex multi degree of freedom coupling system, floating offshore wind turbine usually works in complex and changeable marine environment. The research of floating offshore wind turbine often needs to consider the coupling effect of wind, wave and current at the same time. Under the action of wind and wave load, floating offshore wind turbine will produce a large motion response. Generally, the surge motion and pitch motion are the most obvious aspects, and their motion amplitude is also the largest, and large-scale motion of the floating platform will directly affect the relative velocity of the wind loaded flow. Therefore, the surge motion and pitch motion usually have the most obvious impact on the aerodynamic load of wind turbine. In this paper, the motion characteristics and aeroelastic responses of the DTU-10MW semi-floating offshore wind turbine under coupling action of wind and wave are simulated. The motion characteristics and aeroelastic responses of floating offshore wind turbine are calculated under different wind and wave combination conditions, and the influence law of wind and wave action are summarized. The results show that wind load affects the mean value of motion of the floating offshore wind turbine, and wave load affects the amplitude of motion; Finally, the dynamic response of the wind turbine with the included angle of the wind and wave load is discussed and analyzed, and the influence of the incident angle of the wind wave on the motion response of the floating offshore wind turbine is analyzed.

Response of a plate with piezoelectric elements to turbulent pressure fluctuation in supersonic flow

The aeroelastic response of a plate with supersonic freestream flow on one side and a shallow cavity on the other to turbulent pressure fluctuations is investigated computationally and experimentally. An empirical model is developed for the pressure fluctuations in a turbulent boundary layer that accounts for spatial and spectral variations in the pressure field. Supersonic wind tunnel tests were conducted in a Mach 2.5 flow with and without an impinging shock at the plate surface. In both cases the boundary layer was turbulent. The impinging shock creates shock-wave boundary-layer interaction, which alters the characteristics of the pressure fluctuations. Pressure-sensitive paint was used to measure the unsteady pressure on the surface of a rigid plate and characterize the pressure field (local mean, rms, and the spatial coherence length) and piezoelectric patches were used as sensors to measure the response of an elastic plate. The extracted pressure parameters were used to simulate the fluid–structure response and correlate with experiments. The computed pressure perturbation due to plate motion is found to be small relative to the natural pressure fluctuation for the fluid/structural configuration studied. Computed and measured power spectra of the piezoelectric element voltage show good agreement over a wide range of structural natural frequencies. Aeroelastic response sensitivity to pressure fluctuation coherence length was also investigated computationally. It is found that with small fluid elements, which represent small-scale uncorrelated noise, the structural response is relatively small because the excitation is filtered by the plate dynamics. Experimental results suggest that the effective excitation spatial scale is on the order of the boundary layer thickness.

On the Effect of Geometric Nonlinearity on the Dynamics of Flared Folding Wingtips

Recent studies have considered the use of flared folding wingtips (FFWTs) to enable higher aspect ratios—reducing overall induced drag—while reducing gust loading and meeting airport operational requirements. The majority of these analyses have employed linear assumptions despite the presence of large wingtip deformations. In this paper the effect of geometric nonlinearities introduced by an FFWT on the static and dynamic aeroelastic response of a wing is assessed. A geometrically exact expression was formulated to describe the change in the local angle of attack in the chordwise direction across all fold angles. This expression highlighted that the aerodynamic stiffness of an FFWT and, therefore, quantities such as the linear flutter speed are a function of the fold angle and, hence, the attitude of the wing. This effect was verified using both a wind tunnel model of a flexible semispan wing and a numerical model utilizing MSC Nastran, which linearized the model about the equilibrium position of the wingtip. These experiments showed that the geometric nonlinearities introduced due to the large deformations of FFWTs can significantly affect the dynamics of the system, with flutter speeds varying by over 28%, simply by changing the angle of attack of the model.

Three-Dimensional Uncoupled Fluid Structure Analysis of Small Unmanned Aerial Vehicle Wing

Analyzing the interaction between a fluid and an exposed structure is vital in the design of numerous applications, including the optimization of aerodynamic bodies such as the flexible wings of small unmanned aerial vehicles. Various techniques currently exist to study such fluid/structure interactions, including experimental approaches and numerical techniques that couple fluid and structural analysis tools. In this study, a novel uncoupled static aeroelastic analysis method is demonstrated for determination of the aeroelastic response of three-dimensional wings for the first time. The results indicate that this uncoupled static aeroelastic analysis method is capable of accurately predicting the pressure and displacement fields of a flexible wing, exhibiting at most a 2% error relative to traditional coupled aeroelastic analysis results and a 2.3% error relative to experimental results. Additionally, because the fluid and structural solvers are uncoupled, it is herein demonstrated that the uncoupled static aeroelastic analysis method is capable of reusing a single computationally expensive fluid data set to evaluate multiple internal wing structural configurations across a range of angles of attack while maintaining similar levels of accuracy, thereby reducing the total computational expense by as much as 70%. Furthermore, it is also demonstrated that the uncoupled static aeroelastic analysis method is suitable for preliminary design optimization of the internal structure of wings, which may experience either simple (bending only) or complex (bending and twisting) deformation.

Study on Nonlinear Aerodynamic Characteristics of a Semi-Closed Box Bridge Deck Based on Coupled Amplitude Variation

This paper explores the nonlinear characteristics of the self-excited aerodynamic forces of a semi-closed box deck section to perfect the theory of aeroelastic response analysis. A numerical wind tunnel model was established based on the computational fluid dynamics (CFD) method. The heaving-pitching coupled motion is realized by loading user-defined function (UDF) and dynamic grid technology. The self-excited aerodynamic forces varying with amplitude are identified and analyzed, and the reliability of the aerodynamic results obtained by numerical simulation is verified in the wind tunnel test. In the heaving-pitching coupled motion, the results show that the nonlinear characteristics of aerodynamic forces, especially the aerodynamic moment, are mainly affected by the pitching motion. The phenomenon of high-order harmonic energy transfer is observed with the increase in pitching amplitude, and the main component of high-order harmonic can be determined by the pitching amplitude. The contribution of heaving motion to aerodynamic forces nonlinear components is small, but its influence on nonlinear characteristics is complex. Small amplitude heaving motion plays a positive damping role in heaving-pitching coupled motion, and its scope and effect of positive damping action are affected by pitching motion. The extreme value heaving amplitude of positive damping action is observed in the aerodynamic lift.

Aeroelastic Design and Comprehensive Analysis of Composite Rotor Blades Through Cluster-Based Kriging

Excessive vibration during high-speed flights is a critical issue in modern rotorcraft. The aeroelastic design of the composite rotor blades can relieve the hub vibratory loads of the rotor that are a dominant source of these vibrations. However, it has many technical problems, such as a high-dimensional design space, narrow and discontinuous feasible regions, and highly nonlinear design indices. Although the surrogate-based design is known to improve the design results, the conventional surrogate models have an insufficient capacity to represent the nonlinearity of aeroelastic responses. Because the inaccuracy of the surrogate model is combined with the discontinuity of feasible regions during the optimization process, it is likely to converge to local optima. Cluster kriging could accurately represent the complicated physical relations using machine learning techniques, such as soft clustering and classification algorithms. The accurate surrogate model results in a global optimization with 56% vibration reduction, whereas the global approximation using a single kriging yields local optima with 39% vibration reduction. Furthermore, a comprehensive design analysis, conducted to investigate the underlying physics of the vibration reduction, confirms the global optimization. The proposed design method can fully use the multidisciplinary nature of aeroelastic responses, which is not considered in conventional design methods.

Geometrically Nonlinear Effects on the Aeroelastic Response of a Transport Aircraft Configuration

This paper investigates geometrically nonlinear aeroelastic effects on an industry-level aircraft configuration. Results are obtained via a new approach that uses geometrical information to compute nonlinear effects and seamlessly integrates with conventional, linear aeroelastic load packages. The starting point is a generic finite-element model and frequency-domain aerodynamic influence coefficient matrices, and the resulting system includes control inputs, longitudinal trim, or gust disturbances in time domain, which can be employed separately or in combination to obtain a nonlinear dynamic model for loads and stability analysis. The aeroelastic response of a long range aircraft is studied, highlighting the difference between linear and nonlinear approaches in the calculations of the aircraft flying equilibrium, dynamic perturbations, and variations to the flutter boundary. Results justify the need for geometrically nonlinear analysis in the production environment of airplanes with ultrahigh-aspect-ratio wings.

Experimental investigation on the synchronization characteristics of a pitch-plunge aeroelastic system exhibiting stall flutter.

This study focuses on characterizing the bifurcation scenario and the underlying synchrony behavior in a nonlinear aeroelastic system under deterministic as well as stochastic inflow conditions. Wind tunnel experiments are carried out for a canonical pitch-plunge aeroelastic system subjected to dynamic stall conditions. The system is observed to undergo a subcritical Hopf bifurcation, giving way to large-amplitude limit cycle oscillations (LCOs) in the stall flutter regime under the deterministic flow conditions. At this condition, we observe intermittent phase synchronization between pitch and plunge modes near the fold point, whereas synchronization via phase trapping is observed near the Hopf point. Repeating the experiments under stochastic inflow conditions, we observe two different aeroelastic responses: low amplitude noise-induced random oscillations (NIROs) and high-amplitude random LCOs (RLCOs) during stall flutter. The present study shows asynchrony between pitch and plunge modes in the NIRO regime. At the onset of RLCOs, asynchrony persists even though the relative phase distribution changes. With further increase in the flow velocity, we observe intermittent phase synchronization in the flutter regime. To the best of the authors' knowledge, this is the first study reporting the experimental evidence of phase synchronization between pitch and plunge modes of an aeroelastic system, which is of great interest to the nonlinear dynamics community. Furthermore, given the ubiquitous presence of stall behavior and stochasticity in a variety of engineering systems, such as wind turbine blades, helicopter blades, and unmanned aerial vehicles, the present findings will be directly beneficial for the efficient design of futuristic aeroelastic systems.

Sloshing reduced-order model based on neural networks for aeroelastic analyses

A thorough understanding of the effects of sloshing on aircraft dynamic loads is of great relevance for the future design of flexible aircraft to be able to reduce their structural mass and environmental impact. Indeed, the high vertical accelerations caused by the vibrations of the structure can lead to the fragmentation of the fuel free surface. Fluid impacts on the tank ceiling are potentially a new source of damping for the structure that has hardly been considered before when computing the dynamic loads of the wings. This work aims at applying recently developed reduced-order models of sloshing to the case of a research wing to investigate their effects on the wing aeroelastic response under pre-critical and post-critical conditions. The vertical sloshing dynamics is considered using neural networks trained with experimental data from a scaled tank and then integrated into the aeroelastic system following a suitable scaling procedure. The results concern the aeroelastic response of the wing to gust input under pre-critical (flutter) conditions as well as post-critical conditions highlighting the onset of limit cycle oscillations caused by sloshing, the only nonlinear phenomenon modelled in the present simulation framework. Moreover, the load alleviation performances will be assessed for a typical landing input.