Aeroelastic Structures Research Articles

Event-Based Measurement of Aeroelastic Structure in High-Speed Flow

In high-speed aerodynamics research, point sensors are ideal for embedding in test models but lack spatial resolution, whereas high-speed cameras offer spatiotemporally resolved measurement but involve significant footprint, cost, and data size. To address these tradeoffs, this study explores the application of nascent event-based cameras for high-speed tests. Event-based cameras support continuous, data-sparse kilohertz-equivalent imaging at 720p resolution, on form factors as small as 36 mm and 40 grams in mass, combining the benefits of point sensors and high-speed cameras. However, these attributes come from asynchronous pixels that necessitate unique operating and postprocessing approaches. Here, the authors adapted event-based cameras for two-/three-dimensional photogrammetric tracking of aeroelastic structures, demonstrating an event-based workflow and two tracking algorithms (mean-shift filtering and circle fit). Bench-top validations achieved three-dimensional precision of 0.35 mm/s on 20 mm/s motion across a 259 mm field of view, while two-dimensional measurements of an aeroelastic titanium panel in Mach 0.76 transonic flow successfully identified millimeter-scale vibrations at 43.7, 120, and 270 Hz, validated against a laser displacement and high-speed camera. The transonic test’s raw data were 145.8 MB on the event-based camera, compared to 88.5 GB on the high-speed camera. The presented results demonstrated the viability of event-based techniques in high-speed aerodynamic testing, while highlighting challenges such as polarity switching and pixel latency.

Prediction and suppression of chaos following flutter in wind turbines

Chaotic motion in a fluttering wind turbine blade is investigated by the development of an efficient analytical predictive model that is then used to suppress the phenomenon. Flutter is a dynamic instability of an elastic structure in a fluid, such as an airfoil section of a wind turbine blade. It is presently modelled using generalised two degree of freedom coupled modes of a blade airfoil section (pitch and plunge) combined with local unsteady aerodynamics, based on flutter derivatives and a continuous bilinear lift curve under damping. The mode coupling causes instability and limit cycle flutter due to a Hopf bifurcation. Following the critical flutter speed, the response can transition to chaos through successive other bifurcations like period doubling. New closed-form conservative analytical conditions for chaos following blade flutter are identified and discussed for the wind turbine section taking into account the blade geometry and optimal design of the wind turbine. These predictions are numerically verified for a range of conditions including stall slope and damping. The results confirm that chaos following blade flutter can occur due to nonlinearities in the aerodynamics, i.e. due to a bilinear lift law. This phenomenon is then suppressed to unrealistically high wind speeds and/or eliminated by quantified variation of system parameters using the predictive model. The results show that small changes in tip speed ratio (−15%), and stall slope factor (−17%) can eliminate or suppress chaos following flutter, while, in general, larger magnitude changes in dynamic parameters (i.e. mass, inertia > 81%, stiffness > 97%, damping > 100%) are required to achieve the same, by detuning the coupled plunge and pitch natural frequencies or damping out overlapping parametric resonances. These results also highlight that the analytical predictions can remarkably be generalized to any parameter set and provide almost instantaneous calculations representing many thousands of numerical simulations from many bifurcation diagrams (computational acceleration factor of 107 times). General insight is also provided into the occurrence and suppression of airfoil chaos following flutter in aeroelastic structures like wind turbines.

Modelling of physical systems with a Hopf bifurcation using mechanistic models and machine learning

We propose a new hybrid modelling approach that combines a mechanistic model with a machine-learnt model to predict the limit cycle oscillations of physical systems with a Hopf bifurcation. The mechanistic model is an ordinary differential equation normal-form model capturing the bifurcation structure of the system. A data-driven mapping from this model to the experimental observations is then identified based on experimental data using machine learning techniques. The proposed method is first demonstrated numerically on a Van der Pol oscillator and a three-degree-of-freedom aeroelastic model. It is then applied to model the behaviour of a physical aeroelastic structure exhibiting limit cycle oscillations during wind tunnel tests. The method is shown to be general, data-efficient and to offer good accuracy without any prior knowledge about the system other than its bifurcation structure.

Multi-Objective Reliability-Based Partial Topology Optimization of a Composite Aircraft Wing

Reliability-based partial topology optimization (RBPTO) is a new approach for aircraft structural design; however, it brings computational complexity and makes aeroelasticity analysis quite challenging. Therefore, the present study proposed the multi-objective reliability-based partial topology optimization of a composite aircraft wing using a fuzzy-based metaheuristic (MRBPTOFBMH) approach. The objective is to obtain an optimal layout including partial topology and sizing of the aircraft wing structure. Here, an optimal aeroelastic structure is designed by taking into account the uncertain nature of material properties and the permitted transverse displacement. To increase computational efficiency in the design process, a non-probabilistic approach called a possibilistic safety index-based design optimization (PSIBDO) with fuzzy uncertainties is proposed to quantify the uncertainties due to aeroelastic and structural constraints. Various optimum partial topological shapes and sizing of aircraft wing structures with various PSI values in the range of [0.001, 1.0] have been obtained in a single optimization run. These outcomes, including deterministic and reliable optimal aircraft wing structures, demonstrate the high effectiveness of the proposed MRBPTOFBMH technique to alleviate the complexity of unconventional aircraft wing structure design. The findings also reveal the ease in cooperation of the suggested technique with a high-performance multi-objective evolutionary algorithm (MOEA) and its application in real-world multi-objective design optimization (MODO) problems with the least computational requirements against the traditional method’s multiple runs. Furthermore, the proposed methodology can generate potential aircraft wing structures in a range of m = [89.38–127.84] kg, and flutter speed = [285.61–632.78] m/s, that adhere to all the constraints requirements.

Data-Driven Unsteady Aeroelastic Modeling for Control

Aeroelastic structures, from insect wings to wind turbine blades, experience transient unsteady aerodynamic loads that are coupled to their motion. Effective real-time control of flexible structures relies on accurate and efficient predictions of both the unsteady aeroelastic forces and airfoil deformation. For rigid wings, classical unsteady aerodynamic models have recently been reformulated in state space for control and extended to include viscous effects. Here, we further extend this modeling framework to include the deformation of a flexible wing in addition to the quasi-steady, added mass, and unsteady viscous forces. We develop low-order linear models based on data from direct numerical simulations of flow past a flexible wing at a low Reynolds number. We demonstrate the effectiveness of these models to track aggressive maneuvers with model predictive control while constraining maximum wing deformation. This system identification approach provides an interpretable, accurate, and low-dimensional representation of an aeroelastic system that can aid in system and controller design for applications where transients play an important role.

Experimental investigation on load mitigation of a 5 MW wind turbine in digital fluid power pitch system test rig based on predictive load mitigation controller

ABSTRACT The pitch control system plays a vital role in generating rated power and mitigates mechanical loads during strong wind gusts. Generally, the hydraulic pitch system deployed in a large-scale wind turbine uses a proportional/servo valve to adjust the pitch angle. However, these valves suffer from low operating efficiency which leads to poor pitching performance. Thus, a high-speed digital hydraulic valve-based Digital Fluid Power Pitch System (DFPPS) test rig was proposed in this paper. Moreover, the conventional controller was not able to produce adequate pitching performance during high wind flux. Therefore, a machine learning-based pitch controller was implemented in the developed test rig. The developed DFPPS test rig comprises simulation loop (Wind Generator System (WGS) model, pitch gear model, Predictive Load Mitigation Controller (PLMC), and pitch load model), and DFPPS-hardware interfaced through an I/O interface. Here, PLMC was developed to improve the load mitigation performance without compromising the generator performance at high wind speeds. The proposed PLMC is a cascade arrangement of Recurrent Elman Neural Network (RENN) tuned Fractional-Order Proportional Integral Derivative (FOPID). Further, for collecting the optimal datasets to train the RENNs in PLMC, a Customized Particle Swarm Optimization (CPSO) algorithm was utilized. Two experiments are performed considering the developed WGS model and benchmark Fatigue Aero-elastic Structure Turbulence (FAST) simulator deployed in the DFPPS test rig. The experimental results of PLMC (experiment) were compared to simulation results PLMC (simulation). Further, the experimental output of the PLMC was compared to established controllers in the literature (Radial Basis Function tuned Fractional-Order Proportional Integral Derivative (RBF-FOPID) and baseline Proportional Integral (PI)). In the experiment using FAST, relative to RBF-FOPID, the PLMC (experiment) decreased the Standard Deviation (STD) of blade and tower moment by 81.5% and 81.4%, respectively. Consequently, compared to baseline PI, the PLMC (experiment) reduced the STD of blade and tower moment by 85.6%, and 84.4%, respectively. The proposed controller effectively mitigated the load and also the generator performance was close to the rated value than its counterparts.

Graphene Technology for Design Efficiency of the Solar Hybrid Electrical Cryoplane and Airships

The problems of introducing graphene technologies into the design studies of complex aviation solutions of minimal weight are relevant for the development of high-strength and lightweight composite structures with surface solar nano film energy storage for hybrid electric cryogenic aircraft (hydrogen cryoplanes LH2) and airships (disk-shaped Thermoplane MAI). The optimal design is directly related to the higher specific characteristics of liquid hydrogen fuel systems together with cryocooling systems, taking into account the use of new graphene-based materials and thin flexible solar cells, which is considered for SOLARSTRATOS and MAI projects or for any projects of hybrid electric aircraft/airships and their engines. A design analysis has been carried out to improve the design capabilities when introducing graphene technologies with their unique strength, electrical superconductivity, gas tightness and low mass in the component modification of a hybrid electric propulsion (HEP) and aero elastic energy-recoverable aircraft structures. The choice of rational design solutions using combined graphene composites, quartz dampers-vibration accumulators of structures and film solar energy cells allows you to reduce the weight of larger fuel tanks with liquefied hydrogen at high and low internal pressures and at the same time include electric motors in the cryocooling system – generators, power cables and batteries with additional solar energy charging, which increases the efficiency of on-board electrical systems and reduces the initial energy level and allows to increase energy efficiency and reduce weight costs during design studies.

One-way fluid-material interaction study on a plunging UAV wing

This research is the one-way interaction study of fluid loading on a plunging NACA-2412 modeled UAV wing. Fluid and Structures are two separate domains combined to study both, and their effects on the Aeroelastic structure. This study consists of experimental and computational analysis of the wing structure and the results were compared for effectiveness. The modeled wing was mounted on a cantilever structural setup by restricted to a single degree of freedom. The experimentation was comprised of the static aeroelastic responses of the wings under the aerodynamic loading produced in the wind tunnel for various Reynolds numbers and different angles of attack (α). After the deflection calculations, the model analysis was done using the ANSYS Workbench solid structural analysis and the results were compared with experimental values. The results of the Fluid-Structure Interaction (FSI) study of the wing were concluded for all angles of attack for different Reynolds numbers and aerodynamic loading. The rate of change of deflection due to the fluid loading at every α = 100 angles of attack was found to be 30% at the lowest Reynold’s numbers and a hike up to 60% for higher values.

Automation and New Capabilities in the University of Florida NHERI Boundary Layer Wind Tunnel

The Natural Hazards Engineering Research Infrastructure (NHERI) experimental facility at the University of Florida provides a diverse suite of experimental resources to support wind hazard research. The 40-m long Boundary Layer Wind Tunnel (BLWT) simulates boundary layer flows to characterize wind loading on rigid structural models and assess the response of aeroelastic structures. The use of experimental automation tools provides researchers unparalleled flexibility in their test configurations while supporting high throughput testing and data collection. The Terraformer, an array of 1116 roughness elements, can be rapidly reconfigured to generate terrain conditions in less than 90 seconds, the test section turntable can move automatically through a range of wind approach angles, and the instrumentation gantry can traverse preset paths to collect wind field measurements anywhere in the tunnel cross-section. These test automation tools, along with mechatronic structural models and real-time data transfer and processing, provide new opportunities in experimental wind tunnel testing. Recent cyber-physical wind tunnel testing projects highlight the benefits of these experimental automation tools. This paper will also discuss the most recent addition to the BLWT, the Flow Field Modulator (FFM). It consists of a 2D array of 319 individually controlled ducted fans driven by electronic speed controllers. The FFM expands the BLWT capabilities by supporting the simulation of non-monotonic profiles and nonstationary events such as gust fronts and downbursts, where mean velocity and turbulence distributions change over short spatial scales. The ability to simulate these flow conditions in a wind tunnel enables the investigation of a wide range of damaging conditions and the solutions for mitigating their impact on structures.

An Experimental Analysis and Parametric Simulation of Vibration-Based Piezo-Aeroelastic Energy Harvesting Using an Aerodynamic Wing Profile

In this study, an aeroelastic structure is formed by connecting an aerodynamic profile to the end of a piezoelectric material-glued beam element. The aerodynamic profile is continuously vibrated at the first natural frequency of the structure using an air nozzle. Due to the resonance effect occurred by the vibration of the system, harvested energy from the aeroelastic structure is maximized. In this work, an experimental study on the energy harvesting from the piezo-aeroelastic structure is carried out. In addition, a theoretical model is developed in MATLAB/Simulink environment and obtained analytical results are validated with experimental results. The effect of air velocity on energy harvesting is investigated considering different air velocities. Besides, energy conversion efficiency is calculated and the effects of damping ratio, frequency ratio and coupling factor of the piezoelectric material on the energy conversion efficiency are examined.

Hybrid pressure integration and buffeting analysis for multi-row wind loading in an array of single-axis trackers

Unrestrained single-axis solar trackers are uniquely flexible structures that can withstand more deflections than typical aeroelastic structures such as long-span bridges or aircraft wings. These distinctive features and the aerodynamics of multi-row arrays lead to the requirement of novel techniques for design wind loads. The present study presents a hybrid technique using two separate wind tunnel tests: (i) a pressure model at a relatively small scale, and (ii) a sectional model at a larger scale. The pressure model test is used to measure the buffeting forces acting on each row of the array and the sectional model study is used to extract the variation in aerodynamic stiffness and damping as a function of wind speed. The results of these separate studies are combined to numerically simulate the buffeting response of the tracker, which predicts significant inertial and self-excited forces. Comparisons between the proposed hybrid method and wind loads estimated using a dynamic amplification approach are performed. It is shown that at high wind speeds, the self-excited forces become significant and the peak design moments exceed those predicted using pressure data alone. The proposed methodology is expected to complement aeroelastic modeling as a convenient and efficient design tool.

Aeroelastic global structural optimization using an efficient CFD-based reduced order model

Abstract In aeroelastic structure optimization, the structural model needs to be modified repeatedly to meet all targets. Although computational fluid dynamics (CFD) based reduced order models (ROMs) have been successfully applied to transonic aeroelastic analysis, the existing CFD-based ROMs are at a fixed flight condition for a frozen aeroelastic model configuration. The aerodynamic and structural model have to be reconstructed to ensure accuracy, when a structural modification was made. These reconstructions take a considerable time and the computational costs become prohibitive. To overcome the realistic challenge, we have developed an efficient CFD-based ROM, which is robust to aeroelastic system. This paper presents a new optimization process using the efficient method for aeroelastic global structural optimization. The optimization process employs Genetic Algorithms (GAs) as optimization tool. In order to assess the performance of presented optimization process, the AGARD 445.6 wing model is taken as numerical example. The results show that the most feasible and optimal solutions are effectively obtained by the presented optimization process.

Axially functionally graded beams and panels in supersonic airflow and their excellent capability for passive flutter suppression

Mode localization in the airflow direction can be observed in aeroelastic structures. On the contrary, can the aeroelastic stability of the structure be enhanced if the geometric sizes and material properties vary along the airflow direction? The present study mainly solves this problem. The mode localization phenomenon in panel flutter in supersonic airflow is displayed. Three arbitrary thickness functions according to the flutter mode are taken into account. Their panel flutter behaviors show that although the consistency between the variation of thickness and flutter mode can increase the flutter bound slightly, the increments are limited. Consequently, a novel strategy for passive control of the panel flutter is proposed by the optimal axially functionally graded (AFG) design of the panel. By investigating the sensitivity of each element in the aerodynamic stiffness matrix to the aeroelastic stability of the structure, the optimal thickness and Young's modulus functions are given out. Simulation results show that the optimal AFG design in this study can suppress the flutter essentially. It can increase the flutter bound of the structure by changing the flutter modes rather than only makes a slight extension on the original basis. Moreover, the designed thickness and Young's modulus are reasonable and applicable.

Elastic shape morphing of ultralight structures by programmable assembly

Ultralight materials present an opportunity to dramatically increase the efficiency of load-bearing aerostructures. To date, however, these ultralight materials have generally been confined to the laboratory bench-top, due to dimensional constraints of the manufacturing processes. We show a programmable material system applied as a large-scale, ultralight, and conformable aeroelastic structure. The use of a modular, lattice-based, ultralight material results in stiffness typical of an elastomer (2.6 MPa) at a mass density typical of an aerogel . This, combined with a building block based manufacturing and configuration strategy, enables the rapid realization of new adaptive structures and mechanisms. The heterogeneous design with programmable anisotropy allows for enhanced elastic and global shape deformation in response to external loading, making it useful for tuned fluid-structure interaction. We demonstrate an example application experiment using two building block types for the primary structure of a 4.27 m wingspan aircraft, where we spatially program elastic shape morphing to increase aerodynamic efficiency and improve roll control authority, demonstrated with full-scale wind tunnel testing.

A Method of Panel Flutter Suppression and Elimination for Aeroelastic Structures in Supersonic Airflow

In traditional active flutter control, piezoelectric materials are used to increase the stiffness of the aeroelastic structure by providing an active stiffness, and usually the active stiffness matrix is symmetric. That is to say that the active stiffness not only cannot offset the influence of the aerodynamic stiffness which is an asymmetric matrix, but also will affect the natural frequency of the structural system. In other words, by traditional active flutter control method, the flutter bound can just be moved backward but cannot be eliminated. In this investigation, a new active flutter control method which can suppress the flutter effectively and without affecting the natural frequency of the structural system is proposed by exerting active control forces on some discrete points of the structure. In the structural modeling, the Kirchhoff plate theory and supersonic piston theory are applied. From the numerical results, it can be noted that the present control method is effective on the flutter suppression, and the control effects will be better if more active control forces are exerted. After being controlled by the present control method, the natural frequency of the structure remains unchanged.

Investigations on precursor measures for aeroelastic flutter

Wind tunnel experiments carried out on a pitch-plunge aeroelastic system in the presence of fluctuating flows reveal that flutter instability is presaged by a regime of intermittency. It is observed that as the flow speed gradually increases towards the flutter speed, there appears intermittent bursts of periodic oscillations which become more frequent as the wind speed increases and eventually the dynamics transition into fully developed limit cycle oscillations, marking the onset of flutter. The signature from these intermittent oscillations are exploited to develop measures that forewarn a transition to flutter and can serve as precursors. This study investigates a suite of measures that are obtained directly from the time history of measurements and are hence model independent. The dependence of these precursors on the size of the measured data set and the time required for their computation is investigated. These measures can be useful in structural health monitoring of aeroelastic structures.

Reliability-based design optimization with frequency constraints using a new safest point approach

ABSTRACTIn the reliability-based design optimization (RBDO) model, the mean values of uncertain variables are usually applied as design variables, and the cost is optimized subject to prescribed probabilistic constraints as defined by a nonlinear mathematical programming problem. Therefore, an RBDO solution that reduces the structural weight in non-critical regions provides not only an improved design, but also a higher level of confidence in the design. Solving such nested optimization problems is extremely expensive for large-scale multidisciplinary systems that are likewise computationally intensive. This article focuses on the study of a particular problem representing the failure mode of structural vibration analysis. A new method is proposed, called safest point, that can efficiently give the reliability-based optimum solution under frequency constraints, and then several probability distributions are developed, which are mathematically nonlinear functions, for the proposed method. Finally, the efficiency of the extended approach is demonstrated for probability distributions such as log-normal and uniform distributions, and its applicability to the design of structures undergoing fluid–structure interaction phenomena, especially the design process of aeroelastic structures, is also demonstrated.

LPV model reduction methods for aeroelastic structures

The aeroservoelastic modeling of flexible structures leads in general to large dimensional linear parameter varying (LPV) models. These models are intractable by the most analysis and control synthesis algorithms. Therefore, efficient model order reduction methods are needed. This paper investigates two approaches. Both are based on decoupling the large-scale model into smaller dynamical components, that are easier to reduce by balanced reduction techniques. While the first approach exploits the a-priori known, specific structure of the model, the second method algorithmically generates the low-level subsystems by performing a modal transformation and clustering the modes of similar dynamical behavior. The properties of the methods are analyzed via the reduction of the large-scale aeroelastic model of the BAH jet transport wing.

A generalized gust factor technique for evaluating the wind–induced response of aeroelastic structures sensitive to vortex-induced vibrations

This paper develops a unified procedure for dealing with gust-excited vibrations and aeroelastic phenomena on slender structures and structural elements in the framework of the Generalized Gust Factor technique. The structure is arbitrarily inclined and constrained, and excited on its fundamental mode. Galloping phenomenon is taken into account considering linearized effects only; vortex-induced oscillations are simulated through a nonlinear equivalent damping based on the classic Vickery and Basu approach. The effectiveness of the procedure is discussed and verified over a selection of circular-shaped structures, object of extensive experimental measures. The model proposed is fully suitable to reproduce the effective structural aeroelastic behavior, also in the synchronization region at lock-in. Large uncertainties, however, arise from the choice of the model parameters, on which the literature is still poor. Particular attention is devoted to the limiting magnitude (which governs the non-linear aerodynamic damping) and to the peak factor (which supplies the maximum response), both these quantities having a crucial role in the assessment of vortex-induced vibrations.

Multi-fractality in aeroelastic response as a precursor to flutter

Wind tunnel tests on a NACA 0012 airfoil have been carried out to study the transition in aeroelastic response from an initial state characterised by low-amplitude aperiodic fluctuations to aeroelastic flutter when the system exhibits limit cycle oscillations. An analysis of the aeroelastic measurements reveals multi-fractal characteristics in the pre-flutter regime. This has not been studied in the literature. As the flow velocity approaches the flutter velocity from below, a gradual loss in multi-fractality is observed. Measures based on the generalised Hurst exponents are developed and are shown to have the potential to warn against impending aeroelastic flutter. The results of this study could be useful for health monitoring of aeroelastic structures.