Gust Load Research Articles

Flexible aircraft gust encounter simulation using subspace projection model reduction

Reduced-order modelling of multidisciplinary computational fluid dynamics can enable routine industrial gust load analysis. Key to the herein presented model reduction approach is that it modularly builds upon the widely accepted modal structural reduced-order model, where high-fidelity aerodynamics are simply projected onto the structural degrees-of-freedom, while opening a novel route to account for dominant modal aerodynamics in multidisciplinary edge-of-the-envelope flight physics. The influence of the flexible structure is captured by aeroelastic eigenmodes which originate in the structural equations. Such global aeroelastic modes for a large aircraft case with nearly 50 million degrees-of-freedom are computed for the first time using exact aerodynamics from computational fluid dynamics via the Schur complement method. The influence of atmospheric gusts is captured by modes from proper orthogonal decomposition applied to sinusoidal gust responses at discrete frequencies. Afterwards, the two sets of modes are combined and the linearised operator of the Reynolds-averaged Navier–Stokes equations coupled with a modal structural model is projected onto the resulting subspace. The constructed low-dimensional model can be solved rapidly for practical gust analysis giving accurate agreement with the full-order reference results throughout. Feasibility of aeroelastic model reduction, using the industry-grade computational fluid dynamics package DLR–TAU, for a relevant use case in transonic flight gust encounter is demonstrated.

Gust load alleviation in a flexible smart idealized wing

Among the aeroelastic phenomena most commonly affecting flexible and very flexible aircraft, those caused by gusts deserve special attention due to their potential either in degrading flying qualities and ride comfort or in increasing structural loads. It is then of interest to structural loads and flight controls engineers that solutions be developed to attenuate the effects of gusts on aircraft. Particularly, the use of piezoelectric transducers arises as one of the potential solutions in the design of gust load alleviation and structural mode suppression systems. In this paper, the gust load alleviation on a flexible smart idealized wing using only piezoelectric transducers is analyzed and experimentally tested. The numerical model includes a finite-element model of the wing, employing two-node, seven-degree-of-freedom smart beam elements, assuming small deformations and neglecting transverse shear. A quasi-steady, strip-theory-based aerodynamic model is used. Two control laws are evaluated: one based on output feedback, and the other based on feedback of observed states of a truncated system. Using a gust generator, wind-tunnel tests were performed at different flow speeds and gust frequencies to validate the computational model and to verify the performance of piezoelectric transducers. The results show a considerable attenuation of the wing root bending moment, especially using two piezoelectric actuators. Important performance improvements were overall verified with feedback of observed states when compared with static output feedback, specially to decrease the participation of the elastic modes in the gust response.

Gust load alleviation for a long-range aircraft with and without anticipation

This paper presents an overview of the DLR activities on active load alleviation in the CleanSky Smart Fixed Wing Aircraft project. The investigations followed two main research directions: the multi-objective, multi-model, structured controller design for the feedback load alleviation part and the use of Doppler LIDAR technologies for gust/turbulence anticipation. On this latter topic, the prior work made in the AWIATOR European FP6 project constituted a reference in terms of demonstrations and the objective was not to repeat these previous investigations with a real sensor in flight test but to develop new ideas for the exploitation of the Doppler LIDAR measurements for gust alleviation purposes. Very fruitful exchanges between industry partners and research organizations took place during this project and all the work presented in this paper has been made using a generic long-range benchmark provided by Airbus on the basis of the XRF-1 model.

Experimental and numerical investigations of a 2D aeroelastic airfoil encountering a gust in transonic conditions

In order to make substantial progress in reducing the environmental impact of aircraft, a key technology is the reduction of aircraft weight. This challenge requires the development and the assessment of new technologies and methodologies of load prediction and control. To achieve the investigation of the specific case of gust load, ONERA defined a dedicated research program based on both wind tunnel test campaigns and high-fidelity simulations. To reach the experimental objectives, a set-up was designed, manufactured, and implemented within the ONERA S3Ch transonic wind tunnel facility. The first component, called gust generator, consists of two oscillating airfoils installed upstream of the wind tunnel test section and allows to produce air flow deflections. The second component, the test model, is a two degrees-of-freedom aeroelastic model of a supercritical airfoil. A test campaign has been performed leading to the generation of databases for high-fidelity tools validation. These databases have been used in order to assess the capabilities of the elsA code (ONERA-Airbus-Safran property) using its aeroelastic module and a gust model based on the field velocity method. A validation process has been defined to move from experimental results obtained in the wind tunnel with wall boundaries to industrial modeling computed with farfield boundaries. The full process was applied to a transonic case with sine gust excitation signals.

A Generalized State-Space Aeroservoelastic Model Based on Tangential Interpolation

In this work, a new approach for the generation of a generalized state-space aeroservoelastic model based on tangential interpolation is presented. The resulting system of differential algebraic equations (DAE) is reduced to a set of ordinary differential equations (ODE) by residualization of the non-proper part of the transfer function matrix. The generalized state-space is of minimal order and allows for the application of the force summation method (FSM) for the aircraft loads recovery. Compared to the classical rational function approximation (RFA) approach, the presented method provides a minimal order realization with exact interpolation of the unsteady aerodynamic forces in tangential directions, avoiding any selection of poles (lag states). The new approach is applied first for the generation of an aerodynamic model for the bidimensional unsteady incompressible flow in the time domain. Next, an application on the generation of an aeroservoelastic model for loads evaluation of the flutter reduced order assessment (FERMAT) model under atmospheric disturbances is done, showing an excellent agreement with the reference model in the frequency domain. The proposed aeroservoelastic model of minimal order is suited for loads analysis and multivariable control design, and an application to a gust loads alleviation (GLA) strategy is shown.

Integrity assessment of an oyster farm mooring system through in-situ measurements and extreme environment modeling

Design practices for aquaculture systems in northeastern US estuarine areas have evolved over time based on experience of operators and on project economics in an increasingly competitive market. There has been growing awareness among aquaculture practitioners of the need to evaluate the integrity of their designs under increasing severity and/or frequency of extreme environments, such as winter storms and hurricanes. A parametric study is conducted in this paper to assess the adequacy of mooring ropes for oyster cages under severe environmental conditions, including high tides, storm surges and 100-year extreme wave heights. The test site for this study, comprising of 1080 cages arranged in 24 rows, is located in the upstream estuarine area of the Damariscotta River along the coast of Maine, where significant aquaculture farming takes place. Field measurements were conducted on the main mooring lines at three different locations, by using several load cells and custom-designed data acquisition systems. Environmental data at the site were collected from oceanographic buoys, as well as from local tidal and wind stations. Statistical results of the field data show that the measured mooring tensions are strongly correlated with tide-induced water level variations. While current is not the dominating contributor to mooring tension at high tide, it is a dominant force for rope tension at low to mid tidal level. Also notable is the effect of reduced current on the mooring load as the flow passes through the net cages at the front and back of the farm. The mooring tensions at the front (seaward direction) are observed to be 30–50% larger than the back (landward) loads. In addition, wind gust load also constitutes a significant contribution to mooring tension. At low and high tides, it can add up to 200% and 30%, respectively, to the total mooring load. Formulations commonly used in offshore engineering were applied to evaluate the environmental forcing and the line tensions. By employing recommended inertia and drag coefficients, good agreement with measured data were obtained. These formulations are subsequently used to evaluate the safety factor of the under various scenarios, including larger farm sizes, shorter rope lengths, and effects of extreme storm surge conditions and 100-year return wave height.

Preliminary aeroelastic design of composite wings subjected to critical gust loads

Including a gust analysis in an optimization framework is computationally expensive as the critical load cases are not known a priori and hence a large number of points within the flight envelope have to be analyzed. Model order reduction techniques can provide significant improvement in computational efficiency of an aeroelastic analysis. In this paper, after thorough analysis of 4 commonly used model order reduction methods, balanced proper orthogonal decomposition is selected to reduce the aerodynamic system which is based on potential flow theory. The reduced aerodynamic system is coupled to a structural solver to obtain a reduced-order aeroelastic model. It is demonstrated that the dominant modes of the aerodynamic model can be assumed to be constant for varying equivalent airspeed and Mach number, enabling the use of a single reduced model for the entire flight envelope. A dynamic aeroelastic optimization method is then formulated using the reduced-order aeroelastic model. Results show that both dynamic and static loads play a role in optimization of the wing structure. Furthermore, the worst case gust loads change during the optimization process and it is important to identify the critical loads at every iteration in the optimization.

Higher-Order Estimations of Load Factor from Derived Gust Velocity

Pratt’s formula, in which the aircraft is modeled as a single-degree-of-freedom system, is used extensively to extract derived gust velocities from measured normal acceleration. There is evidence that airframe mass distribution along the longitudinal axis can affect the gust load alleviation factor. In this paper, the inclusion of the pitching degree of freedom in the model is explored, accounting for the aircraft mass distribution along the longitudinal axis. Two different two-degree-of-freedom models are presented and numerical solutions are offered. A typical heavy transport aircraft is used for illustration. It is shown that the inclusion of the second degree of freedom results in as much as 10% reduction in the magnitude of the gust load alleviation factor.

Modeling of airfoil aeroelastic response to gust and rain coupled loads

The aeroelastic response of an airfoil featuring plunging-pitching coupled motion in an incompressible flow field to gust and rain coupled loads is primarily investigated in this paper. The equations of airfoil coupled motion are derived by the “typical section” approach and the aeroelastic model is achieved by coupling these to 2D unsteady aerodynamics models for incompressible potential flows. The modelings of the unsteady aerodynamic and gust loads are established by the use of the Wagner’s and the Küssner’s functions, respectively. The incorporation of the rain effect is implemented via substituting the values of the slope of lift curve in the rain conditions in the aeroelastic response equations. Numerical modelings of the aeroelastic response to gusts of different velocity distributions and comparisons with the solution obtained for the rigid and elastic models featuring plunging or pitching motion alone and plunging-pitching coupled motion are provided. The pertinent results agree well with the results from other literature. The influences of gust, rain and airfoil control parameters are specially presented and discussed in detail. These results seem new and should have important reference value to the optimization of aircraft structure for the industrial aviation community.

Buffeting response evaluation of long-span bridges with emphasis on the three-dimensional effects of gusty winds

A generalized gust loading model applicable to the buffeting response predictions of both slender, line-like long-span bridges and bridges with small aspect-ratios (the ratio of the span to the width of a deck) is established based on the three-dimensional (3D) theory. The spectra of the buffeting forces are determined by the equivalent 3D two-wavenumber aerodynamic admittance function (AAF) to deal with the 3D effects of gusty winds. The mode shape influence function is introduced to take into account the three-dimensionality of the modal force. Since the mode shape influence function involves the aspect-ratio term, the three-dimensionality of modal force therefore shows a dependence on the aspect-ratio of the bridge deck. The gust response analysis can easily be conducted in the frequency domain through the use of the generalized gust loading model. Numerical examples of bridges with different aspect-ratios in service states and construction states are performed to evaluate the discrepancies in the buffeting displacements calculated by both the generalized gust loading model and the traditional gust loading model. The results show that the traditional gust loading model can generally be applied to the buffeting analysis of long-span bridges with large aspect-ratios in both the service and construction states. However, for bridges with small aspect-ratios, such as in the construction stages, the traditional gust model may overestimate the buffeting response of a bridge deck to a larger degree (neglecting any end effect), indicating the invalidation of the strip assumption in these cases.

Aeroelastic Instability of Cantilever Wing with Added Engine Mass and Thrust Loads Under the Gust Envelope

Dynamic analysis and instability prediction of immersed bodies under variable loading conditions is an important issue in fluid dynamics. This work presents the dynamic analysis of a wing section by considering the engine mass and the resulting thrust effect using a distributed parameter model. The equations of motion of cantilever wing are obtained using the extended Hamilton’s principle by accommodating the engine mass and the follower force terms. Initially, the stability regions of the clean wing without engine are identified using modal reduction scheme. The critical frequencies and flutter speed obtained from the model are validated with available results. In order to study the influence of other parameters such as position of engine mass, number of store masses and the generated thrust, a series of parametric studies are conducted. To make the study more realistic, the cantilever wing with engine mass is further subjected to a vertical discrete gust loading. It is found that the model is reliable and interactive in further analysis.

Sliding Mode Observer Controller Design for a Two Dimensional Aeroelastic System with Gust Load

AbstractThe aim of this paper is to develop state estimation and sliding mode control schemes for the vibration suppression of an underactuated wing aeroelastic system in the presence of a gust load disturbance. Ignoring structural elastic deformation and using the concentrated elastic system (spring) to simulate the overall elastic deformation, this aeroelastic model consists of a straight wing and spring system, describing flap and pitch freedoms. The corresponding dynamic motion equation is established using the Lagrange method, and the gust is modeled as a typical “1‐cosine” gust. The aerodynamic lift and moment on the wing are computed by strip theory. The open loop system exhibits the limit cycle oscillations (LCOs) at a certain freestream velocity. The objective is to design a control system for suppressing the LCOs. For the purpose of control, a single trailing‐edge control surface is used. It is assumed that only the pitch angle is measured and the remaining state variables needed for full state feedback are estimated by the designed observer. Then an integral sliding surface is put forward on the estimation space; a new continuous reaching law is proposed to reduce the chattering phenomena. The finite‐time reachability of the predesigned sliding surface is proved and guaranteed by the designed sliding mode control law. The sufficient condition for the asymptotic stability of the closed‐loop system composed of the sliding mode dynamics and the error dynamical system is derived in terms of linear matrix inequality (LMI). The effectiveness of the proposed strategy is finally demonstrated by simulation results.

Characteristics of wind-induced displacement of super-large cooling tower based-on continuous medium model wind tunnel test

A cooling tower is a wind-sensitive structure, and wind loading is used as the control load for its design. In this study, the displacement characteristics of a large concrete cooling tower with respect to a turbulent wind is investigated by using a continuous medium model wind tunnel test. Firstly, based on the similarity relations of the continuous medium model for the cooling tower, which were derived and verified in the present study, a continuous medium model with a geometrical scale of 1:400 is designed and processed for a 200 m high cooling tower at a nuclear power plant in China. Subsequently, wind tunnel tests are carried out to measure the wind-induced displacement using a high-precision laser displacement sensor. Based on the results, the displacement and characteristics of the power spectra of some typical measurement points are analyzed. The analysis result indicates that the continuous medium model can accurately simulate the similarity relations of mass, stiffness, and damping. It is also found that the maximum displacement occurs in the windward side of the throat, and the size is about 20% of the throat thickness. In addition, background response plays a major role in the total displacement response, and the dynamic amplification effect is not obvious. The mean dynamic amplification coefficient is 1.02, which is slightly less than the value of 1.05 specified in VGB-R 610Ue, 2010. At the same time, the test result shows that gust loading factor decreases with increase in height, which is consistent with VGB-R 610Ue, 2010, and the mean value is slightly less than the specified value.

Gust response envelope approach to the equivalent static wind load for large-span grandstand roofs

Current equivalent static wind load (ESWL) format is expressed by either gust response factor (GRF) approach or the combination of mean, background and resonant components. Due to the complexity of fluctuating wind load characteristics and dynamic structural features of large-span roofs, the current ESWL formats are either too simplified or too complicated to be codified in wind load codes. In consideration of engineering convenience, this paper presents a gust response envelope (GRE) approach to express the ESWL model by the combination of mean and RMS wind pressures to envelope the wind-induced responses. The coefficient multiplied by the RMS wind pressure is defined as the GRE factor, which can be expressed by the product of the background and resonant factors. The background factor, based on gust loading envelope (GLE, Chen and Kareem, 2004), reflects the effect of asynchronous fluctuating wind loads on structures. The resonant factor denotes the dynamic effect of structures. The ESWL format is applied to the codification of wind loads on large-span cantilevered grandstand roofs. Parametric wind-induced response analyses were carried out, and the empirical formula of the GRE factor were provided for engineering reference.

Computational analysis on actuator failures of flexible aircraft

A coupled model of aeroservoelasticity and hydraulic actuator used for failure simulation is presented. The mathematical model composites rigid-body modes, elastic modes, control surface modes, unsteady aerodynamic forces and failure models (jam, loss of control (LOC), oscillatory failure, and hydraulic fluid leakage). A clear framework of coupling method of airplane aeroelastic equation and control surface dynamic equation is provided to study the impacts of surface failures on rigid-elastic motion of airplane. The coupled model is shown to be effective in evaluation of gust response in both discrete gust and continuous turbulence conditions compared with results obtained from the 3-order simplified actuator. Examples of gust load alleviation (GLA) system with LOC of ailerons are given. Results show that total loss of function of GLA system is caused by the LOC. With continuous turbulence excitation, the failure loads is several times larger than that without GLA system.

Testing of a Hinged Wingtip Device for Gust Loads Alleviation

Recent aircraft designs have considered higher-aspect-ratio wings to reduce induced drag for improved fuel efficiency; however, to remain compliant with airport gate requirements, folding wingtips have been introduced as a solution to the increased wingspan. Recent numerical studies suggest that a folding wingtip solution may be incorporated with spring devices to provide an additional gust loads alleviation ability in flight as well. In this work, a series of low-speed steady and dynamic wind tunnel tests was conducted using a prototype of such a concept. It was found that a folding wingtip with a nonzero relative angle of the folding hinge axis to the streamwise direction could provide gust loads alleviation. The level of load alleviation varied with hinge spring stiffness and lifting condition, with the best performance achieving a 56% reduction in peak loading.

The Adaptive Camber Concept—A passive approach for gust load alleviation on wind turbines

AbstractA new passive sectional approach to alleviate gust loads on wind turbines—the Adaptive Camber Concept— is introduced. The concept entails fluid‐structure interaction, where flow conditions at the leading edge affect the airfoil shape and vice versa. A two‐dimensional airfoil equipped with the Adaptive Camber Concept is experimentally investigated under steady and unsteady conditions in a wind tunnel. Under steady conditions, the adaptive camber airfoil de‐cambers gradually with increasing angle of attack, yielding a lift curve with declined slope. Unsteady angle of attack fluctuations of various reduced frequencies are generated by means of an active grid. Under unsteady conditions, the adaptive camber airfoil is found to alleviate up to 60% of the fluctuating loads, while generating higher mean lift compared to a rigid reference airfoil.

Computational-Fluid-Dynamics-Based Aeroservoelastic Analysis for Gust Load Alleviation

Gust load alleviation using computational fluid dynamics as source of the aerodynamic predictions is carried out in the time domain. To this goal, an aeroservoelastic reduced-order model is generated. The model capitalizes on two key aspects: a dimensional reduction through proper orthogonal decomposition, further enhanced via balanced truncation, and an analytically derived mechanism to reproduce the gust effects in the reduced-order model. The compact model in state-space form thus obtained was then used for control design synthesis. Assuming that information on the structural motion only is accessible for feedback, a linear quadratic regular was designed first on the reduced model and then validated on the large computational model. Results are presented for two configurations: an aerofoil and the modified AGARD 445.6 wing, both with a trailing-edge control surface. Studies are presented for the gust response to discrete gusts and continuous turbulence. In particular, for the latter, the standard deviation of the loads and the structural motion was reduced as much as 77%.

Spatial Distribution of Gusty Loads on a Rectangular Prism in Boundary Layer Flows

The pressure fluctuations acting on a stationary rectangular prism with cross configuration (2:1/1:2) are investigated by wind tunnel testing. The synchronous surface pressures on the rigid model were measured in simulated atmospheric boundary layer flow and the unsteady forces were calculated by numerical integral of surface pressures. The effects of the wind fields and the model’s side ratio on the aerodynamic coefficients, spectral characteristics and spatial correlation of aerodynamic loading were investigated. The main purpose of this study is to further analyze the spatial structure of the fluctuating wind loads acting on a rectangular prism as well as to use the cross correlation coefficient and coherence function to explore their distribution trends. The results show that the along-wind gust loading is consistent with the longitudinal turbulence and the across-wind aerodynamic force is mainly induced by the vortex-shedding, recirculation and reattachment of separation shear layers on the lateral sides. In practical applications, the Strouhal number is approximately constant along the height. Extended empirical coherence models of the fluctuating along-wind and across-wind loads are proposed with consideration to the effects of wind fields, the structure characteristics and dimensions of the prism and length scale.

Gust and Ground Loads Integration for Aircraft Landing Loads Prediction

A methodology is described to couple unsteady aerodynamic loads with flexible bodies in multibody simulations. A representative civil jet aircraft aeroelastic model is used to evaluate the structural loads due to the combination of gust and ground loads. The implementation of unsteady aerodynamic loads is validated by comparing the results for the wing root bending moment and tip displacement with Nastran transient aeroelastic analyses for a series of gust lengths of a linear free–free aircraft model. The main objective of the paper is to prove that, with the implementation of a time domain formulation for the unsteady aerodynamic loads, a multibody simulation can be used to predict the aircraft aeroelastic loads that are comparable with traditional finite element-based methods. However, the use of a multibody simulation software offers the simulations to other maneuvers such as landings and the use of nonlinear and active structures.