197 results found
Duessler S, Palacios R, 2023, Enhanced unsteady vortex lattice aerodynamics for nonlinear flexible aircraft dynamic simulation, AIAA Journal: devoted to aerospace research and development, ISSN: 0001-1452
Several modelling extensions and numerical improvements to the unsteady vortex-lattice method are discussed for the simulation of very flexible aircraft dynamics. In particular, fuselage aerodynamics are included by means of linear source panels, polar corrections are incorporated on the aerodynamics under potentially large deformations, and a new wake discretization scheme is derived to accelerate the computations. The theory behind each approach is detailed and successfully verified on representative test cases. The resulting modelling environment is then exemplified in a study of the flight dynamics of a flexible aircraft demonstrator model. Results indicate that for large wing deformations, a noticeable effect of fuselage aerodynamic interference appears on the wing aeroelastic behaviour. The polar corrections improve the drag estimations and allow capturing lift decrements due to stall onset at high induced angles of attack, which then affects the gust loads. The wake discretisation scheme enables a reduction ofthe computational time for dynamic simulations on a full aircraft model by about 20 % while keeping the arising error negligible small.
Simiriotis N, Palacios R, 2023, A numerical investigation on direct and data-driven flutter prediction methods, JOURNAL OF FLUIDS AND STRUCTURES, Vol: 117, ISSN: 0889-9746
Lahooti M, Bao Y, Scott D, et al., 2023, LES/DNS fluid-structure interaction simulation of non-linear slender structures in<i> Nektar plus plus </i> framework, COMPUTER PHYSICS COMMUNICATIONS, Vol: 282, ISSN: 0010-4655
Jane-Ippel C, Bempedelis N, Palacios R, et al., 2023, High-fidelity simulations of wake-to-wake interaction in an atmospheric boundary layer over a complex terrain, 8th Wake Conference, Publisher: IOP PUBLISHING LTD, ISSN: 1742-6588
Goizueta N, Wynn A, Palacios R, 2022, Adaptive sampling for interpolation of reduced-order aeroelastic systems, AIAA Journal: devoted to aerospace research and development, Vol: 60, Pages: 6183-6202, ISSN: 0001-1452
A new strategy for the interpolation of parametric reduced-order models of dynamic aeroelastic systems is introduced. Its aim is to accelerate the numerical exploration of geometrically nonlinear aeroelastic systems over large design spaces or multiple flight conditions. The parametric reduced-order models are obtained from high-dimensional models by Krylov-subspace projection. They are subsequently interpolated to acquire realizations inexpensively anywhere in the parameter space, where having the state space as opposed to interpolating an output metric permits the use of linear analysis tools. The interpolation scheme is heavily conditioned by the available training points; thus, a novel methodology based on adaptive sampling and a combinatorial use of the available true-systems knowledge is presented, whereby we reuse all known data of the true models as different combinations of training and testing data to build statistical surrogates of the interpolation error across the parameter space and refine the sampling in those regions that need it. This minimizes the number of costly to evaluate function calls and ensures that parameter space regions are sampled according to the underlying system dynamics. The initial implementation of this adaptive sampling strategy is demonstrated on a very flexible wing with a complex stability envelope.
Otsuka K, Wang Y, Palacios R, et al., 2022, Strain-Based Geometrically Nonlinear Beam Formulation for Rigid-Flexible Multibody Dynamic Analysis, AIAA JOURNAL, Vol: 60, Pages: 4954-4968, ISSN: 0001-1452
Cea A, Palacios R, 2022, Geometrically nonlinear effects on the aeroelastic response of a transport aircraft configuration, Journal of Aircraft: devoted to aeronautical science and technology, Vol: 60, Pages: 205-220, ISSN: 0021-8669
This paper investigates geometrically-nonlinear aeroelastic effects on an industry-level aircraft configuration. Results are obtained via a new approach that uses geometrical informationto compute nonlinear effects and seamlessly integrates with conventional, linear aeroelasticload packages. The starting point is a generic finite-element model and frequency-domainaerodynamic influence coefficient matrices, and the resulting system includes control inputs,longitudinal trim, or gust disturbances in time domain, which can be employed separately orin combination to obtain a nonlinear dynamic model for loads and stability analysis. Theaeroelastic response of a long range aircraft is studied, highlighting the difference betweenlinear and nonlinear approaches in the calculations of the aircraft flying equilibrium, dynamicperturbations, and variations to the flutter boundary. Results justify the need for geometrically nonlinear analysis in the production environment of airplanes with ultra high aspect ratiowings.
Gomes P, Palacios R, 2022, Aerostructural topology optimization using high fidelity modeling, Structural and Multidisciplinary Optimization: computer-aided optimal design of stressed solids and multidisciplinary systems, Vol: 65, ISSN: 1615-147X
We investigate the use of density-based topology optimization for the aeroelastic design of very flexible wings. This is achieved with a Reynolds-averaged Navier–Stokes finite volume solver, coupled to a geometrically nonlinear finite element structural solver, to simulate the large-displacement fluid-structure interaction. A gradient-based approach is used with derivatives obtained via a coupled adjoint solver based on algorithmic differentiation. In the example problem, the optimization uses strong coupling effects and the internal topology of the wing to allow mass reduction while maintaining the lift. We also propose a method to accelerate the convergence of the optimization to discrete topologies, which partially mitigates the computational expense of high-fidelity modeling approaches.
Wang Y, Zhao X, Palacios R, et al., 2022, Aeroelastic simulation of high-aspect ratio wings with intermittent leading-edge separation, AIAA Journal, Vol: 60, Pages: 1769-1782, ISSN: 0001-1452
We present a medium-fidelity aeroelastic framework for computing intermittently separating three-dimensional (3-D) flows around high-aspect ratio wings with a significantly reduced computational cost compared to the computational-fluid- dynamics-based method. To achieve that, we propose a modified three-dimensional vortex panel method with leading-edge separation controlled by leading-edge suction parameter theory, and its incorporation in a coupled aeroelastic solver for the dynamic response of these systems. Numerical verifications and simulations are presented on both attached and separated aeroelastic test cases, demonstrating the method on postflutter limit-cycle oscillation of a cantilever wing, and leading-edge separation on a deploying wing, a complex kinematic response. In both cases we were able to capture three-dimensional interactions on intermittently separating dynamic flowfields using a low computational cost.
Gomes P, Palacios R, 2022, Pitfalls of discrete adjoint fixed-points based on algorithmic differentiation, AIAA Journal, Vol: 60, Pages: 1251-1256, ISSN: 0001-1452
Muñoz-Simón A, Wynn A, Palacios R, 2022, Some modelling improvements for prediction of wind turbine rotor loads in turbulent wind, Wind Energy, Vol: 25, Pages: 333-353, ISSN: 1095-4244
This paper investigates the accuracy of three aerodynamic models to compute loads on wind turbine rotors under turbulent inflow: Blade Element Momentum (BEM); Unsteady Vortex LatticeMethod (UVLM) and Large Eddy Simulation with Actuator Line (LES-AL). Turbulent inflow conditions are numerically generated with a new approach that combines control of turbulence andrealistic velocity spectrum by using Mann boxes and LES simulations, respectively. Several deficiencies of the tested models are found and overcome through proposed improvements. First,the BEM assumption of independent radial sections does not hold in turbulent cases with longblades. Thus, a spatial filter to account for the interaction of radial sections in BEM is designedthrough the analysis of these interactions with UVLM. Second, the absence of viscous drag inUVLM is observed to lead to a very high rotor power coefficient, and it is shown that this canbe mitigated by including drag in UVLM with a BEM-like-approach through look-up tables. Third,the free wake model in UVLM, required to accurately capture rotor thrust, significantly increasescomputational cost. For this reason, a new wake discretisation scheme for the wake convectionequation in UVLM is proposed, in which a coarse discretisation is employed far from the solidsurfaces, which significantly reduces the computational time. Finally, these improvements andthe performance of the three fidelities are analysed in a reference 10 MW wind turbine rotordemonstrating, in general, good agreement.
Goizueta N, Wynn A, Palacios R, et al., 2022, Flutter predictions for very flexible wing wind tunnel test, Journal of Aircraft: devoted to aeronautical science and technology, Vol: 59, Pages: 1082-1097, ISSN: 0021-8669
The stability boundaries of a very flexible wing are sought to inform a wind-tunnel flutter test campaign. The objective is twofold: to identify via simulation the relevant physical processes to be explored while ensuring safe and non-destructive experiments, and to provide a benchmark case for which computational models and test data are freely available. Analyses have been independently carried out using two geometrically nonlinear structural models coupled with potential flow aerodynamics. The models are based on a prototype of the wing for which static load and aeroelastic tests are available, and the experimental results have been successfully reproduced numerically. The wing displays strong geometrically nonlinear effects with static deformations as high as 50% of its span. This results in substantial changes to its structural dynamics, which display several mode crossings that cause the flutter mechanisms to change as a function of deformation. Stability characteristics depend on both the free-stream velocity and the angle of attack. A fast drop of the flutter speed is observed as the wing deforms as the angle of attack is increased, while a large stable region is observed for wing displacements over 25%. The corresponding wind tunnel dynamic tests have validated these predictions.
Düssler S, Goizueta N, Muñoz-Simón A, et al., 2022, Modelling and numerical enhancements on a UVLM for nonlinear aeroelastic simulation, AIAA SCITECH 2022 Forum, Publisher: American Institute of Aeronautics and Astronautics, Pages: 1-20
This work presents various improvements made to the Unsteady Vortex Lattice Method (UVLM) integrated into the open-source, nonlinear aeroelastic simulation environment SHARPy. The UVLM is extended by non-lifting body effects, polar corrections, and a new wake discretization scheme. The theory behind these enhancements is discussed and successfully verified. Finally, some of these enhancements are employed on a flexible aircraft demonstrator model. The results indicate an influence of the fuselage on the aeroelastic behavior of the wing, which becomes increasingly important for larger wing deformations and fuselage diameters. The polar corrections provide valuable refinements to the aerodynamic forces and moments, and the new wake discretization scheme significantly speeds up the simulations.
Düssler S, Palacios R, 2022, A CONTROL DESIGN FRAMEWORK FOR GUST LOAD ALLEVIATION IN MORE FLEXIBLE AIRCRAFT
This paper presents a control design framework for gust load alleviation systems of flexible aircraft. It considers geometrical nonlinearities, and includes a linearization methode to obtain ROMs for control synthesis. A UDP interface then enables control testing on the full-order nonlinear system. The FLEXOP model is implemented and simulated. Open-loop simulations considering discrete and continuous gust models are used to investigate the gust response of the aircraft, and interesting free-flight rigid body responses to spanwise-varying gusts are in particular investigated. Furthermore, the accurate capture of the aircraft dynamics with the linear model is verified, while deviations in magnitude and phase increase with stronger gusts due to the larger deformations obtained and thus enhanced nonlinear effects. Finally, the controllability of the FLEXOP with a simple PD controller is demonstrated, and preliminary linear closed-loop simulations with a more complex LQG controller are performed.
Otsuka K, Wang Y, Palacios R, et al., 2022, Strain-Based Geometrically Nonlinear Beam Formulation for Multibody Dynamic Analysis
The geometrically nonlinear strain-based beam formulation has the potential to analyze flexible multibody systems efficiently due to the minimum number of variables and the constant stiffness matrix. The objective of this paper is to extend the strain-based beam formulation to a generic multibody dynamic analysis. To achieve this objective, we describe the constraint equation by using the vector variables of the absolute nodal coordinate formulation that has a velocity-transformation relationship with the strain-based formulation. Then, we divide the Jacobian of the constraint equation into two terms. One term is equivalent to the velocity transformation matrix that has been implemented in the existing strain-based analysis framework. Therefore, additional programming effort and calculation are not needed. The other term is a simple constant or linear Jacobian defined by the orthonormal vectors of the absolute nodal coordinate formulation. This simple Jacobian description enables not only efficient analysis but also various choice of a time-integration method. We demonstrated that the proposed framework can be used with the explicit Runge-Kutta method and the implicit generalized-α method. The proposed strain-based multibody dynamic analysis method exhibited good agreement with and a better convergence than a conventional flexible multibody dynamic analysis method.
Correction Notice Email address in footnote, first page, for first author Nikolaos Simiriotis should be corrected. Correct email address is: email@example.com.
Nagy P, Jones B, Minisci E, et al., 2022, Multi-fidelity Nonlinear Aeroelastic Analysis of a Strut-braced Ultra-high Aspect Ratio Wing Configuration
Nonlinear aeroelasticity of a strut-braced ultra-high aspect ratio wing aircraft is investigated using a multi-fidelity simulation environment. A baseline aeroelastic solver based on an unsteady vortex-lattice method and geometrically-exact composite beam model is coupled with a data-driven model order reduction tool based on high-fidelity aerodynamics. This provides sectional corrections, retrieved from a basis created using proper orthogonal decomposition, to the lower-order tools. Those corrections incorporate the three-dimensional and compressibility effects of the full strut-brace wing aircraft geometry and thus result in a computationally-efficient analysis framework suitable for multi-disciplinary design optimization. Verification of the reconstruction process is first presented, and the static and dynamic aeroelasticity of the strut-braced wing configuration are finally investigated.
Simiriotis N, Palacios R, 2022, A COMPARISON OF DIRECT AND SAMPLING-BASED FLUTTER SOLUTION METHODS USING CFD
The paper reports on the development of a direct flutter-onset prediction framework, based on CFD frequency-domain techniques, for a robust and efficient search of the flutter boundary across the flight envelope. The implementation was carried out in the open-source SU2 solver and the methodology is examined in this contribution. This paper also provides numerical results from the application of the developed approach to 2D and 3D test cases operating in the transonic regime. The accuracy of the chosen strategy is shown, and the robustness and computational efficiency of the implementation are discussed. Finally, the cost-comparison between GAF-based identification and direct flutter prediction is addressed.
Burghardt O, Gomes P, Kattmann T, et al., 2022, Discrete adjoint methodology for general multiphysics problems A modular and efficient algorithmic outline with implementation in an open-source simulation software, STRUCTURAL AND MULTIDISCIPLINARY OPTIMIZATION, Vol: 65, ISSN: 1615-147X
Cea A, Palacios R, 2021, Parametric reduced order models for aeroelastic design of very flexible aircraft, AIAA Scitech, Publisher: ARC, Pages: 1-28
A description for flutter and dynamic loads prediction is laid out based on state-space systems around a nonlinear equilibrium. In order to reduce the computational times associated with building and manipulating these systems, a newmethodology is proposed to build parametric reduced-order models of the unsteady aerodynamic systems from optimal Latin-Hypercube design of experiments, frequency-limited balancing techniques and interpolation on the transfer functions of the resulting reduced systems. Optimization of the kernels encapsulating the parametric dependence is also studied. The developed tools are employed for the aeroelastic design optimization of a very flexible strut-braced wing with flutter constraints. Results show a promising approach to be deployed in design problems where dynamics and stability are to be included in the analysis and geometrically nonlinear effects are to be accounted for.
Goizueta N, Wynn A, Palacios R, 2021, Fast flutter evaluation of very flexible wing using interpolation on an optimal training dataset, AIAA SCITECH 2022 Forum, Publisher: American Institute of Aeronautics and Astronautics, Pages: 1-21
Machine learning strategies can be efficiently used to accelerate the exploration of the design space or flight envelope of highly flexible aeroelastic systems. In this paper, we explore the use of interpolation between parametric state-space realizations to, with few true systems sampled in the parameter space, produce with adequate accuracy a state-space model anywhere in the parameter space. The location of the sampling points is shown to be decisive thus the selection of these points takes the focus in this work. Several approaches are explored, putting emphasis on adaptive schemes that locate the optimal points in the parameter space that are needed to capture the changing system dynamics. Since the evaluation of the true system is costly, optimization techniques based on statistical surrogate models are sought, which need to be trained but are effective in locating the best locations to use as sampling data. A novel method inspired by Bayesian optimization is used to make the most out of a limited number of known state-spaces by taking different combinations as training and testing data of the statistical surrogate, leading to not only an accurate interpolation framework but also to a reduction of 50% of the number of costly full system evaluations compared to a standard Bayesian optimization set-up. These methods are demonstrated on the Pazy wing, a very flexible wing with a complex stability envelope, whereby we produce a very accurate representation of the flutter envelope at a reduced computational cost.
Simiriotis N, Palacios R, 2021, A flutter prediction framework in the open-source SU2 suite, AIAA SCITECH 2022 Forum, Publisher: American Institute of Aeronautics and Astronautics, Pages: 1-15
The paper reports on the development of a direct flutter-onset prediction framework, based on CFD frequency-domain techniques, for a robust and efficient search of the flutter boundary across the flight envelope. The implementation was carried out in the open-source SU2 solver. The complete methodology is examined in this contribution. The existing harmonic balance formulation was extended to treat arbitrarily deforming surfaces. A dedicated native solver was developed to integrate the linear structural equations of motion. Reduced order structural equations are built based on input modal shapes from external FE solvers. To this end, suitable interpolation schemes have been included to transfer data across the fluid-structure interface. This paper provides numerical results from the application of the developed approach to well-established 2D and 3D test cases operating in the transonic regime. The accuracy of the chosen strategy is shown, and the robustness and computational efficiency of the implementation are discussed.
Wynn A, Artola M, Palacios R, 2021, Nonlinear optimal control for gust load alleviation with a physics-constrained data-driven internal model, AIAA SCITECH 2022 Forum, Publisher: American Institute of Aeronautics and Astronautics, Pages: 1-22
A data-driven strategy is developed to improve the internal models used for predictive control in nonlinear aeroelastic applications. A nonlinear modal formulation of the structure is retained, while an identified quadratic model for both the gravitational forces and the aerodynamics are obtained from a least-squares fit with LASSO regularisation from simulated flights. This is first seen to improve the accuracy of the resulting reduced-order model for open-loop predictions on both gust response and a payload drop problem. Its superior performance as internal model for nonlinear control and estimation is finally demonstrated numerically.
Artola M, Rodriguez C, Wynn A, et al., 2021, Optimisation of Region of Attraction Estimates for the Exponential Stabilisation of the Intrinsic Geometrically Exact Beam Model, 2021 60th IEEE Conference on Decision and Control (CDC), Publisher: IEEE
Otsuka K, del Carre A, Palacios R, 2021, Nonlinear Aeroelastic Analysis of High-Aspect-Ratio Wings with a Low-Order Propeller Model, Journal of Aircraft, Pages: 1-14
Artola M, Goizueta N, Wynn A, et al., 2021, Aeroelastic control and estimation with a minimal nonlinear modal description, AIAA Journal: devoted to aerospace research and development, Vol: 59, Pages: 2697-2713, ISSN: 0001-1452
Modal-based, nonlinear Moving Horizon Estimation (MHE) and Model Predictive Control(MPC) strategies for very flexible aeroelastic systems are presented. They are underpinned by an aeroelastic model built from a 1D intrinsic (based on strains and velocities) description of geometrically-nonlinear beams and an unsteady Vortex Lattice aerodynamic model. Construction of a nonlinear, modal-based, reduced order model of the aeroelastic system, employing a state-space realisation of the linearised aerodynamics around an arbitrary reference point, allows us to capture the main nonlinear geometrical couplings at a very low computational cost. Embedding this model in both MHE and MPC strategies, which solve the system continuous-time adjoints efficiently to compute sensitivities, lays the foundations for real-time estimation and control of highly flexible aeroelastic systems. Finally, the performance and versatility of the framework operating in the nonlinear regime is demonstrated on two very flexible wing models, with notably different dynamics, and on two different control setups: a gust-load alleviation problem on a very high aspect ratio wing with slower dynamics, which involves substantial deflections; and flutter suppression on a flexible wing with significantly faster dynamics, where an unconventional nonlinear stabilisation mechanism is unveiled.
Artola M, Wynn A, Palacios R, 2021, Modal-based model predictive control of multibody very flexible structures, 21st IFAC World Congress on Automatic Control - Meeting Societal Challenges, Publisher: Elsevier, Pages: 7472-7478, ISSN: 2405-8963
A model predictive control strategy for flexible multibody structures undergoing large deformations is presented. The dynamics of such structures are highly nonlinear, with local effects introduced by the joint constraints and distributed effects arising from the structure’s increased flexibility, from which arbitrary large deflections and rotations can be expected. A modal-based nonlinear reduced order model of an intrinsic description (based on velocities and strains) of geometrically-exact beams is used to underpin the internal model. This low-order model, constructed using the linearised eigenfunctions of the constrained structures, is a set of nonlinear ordinary differential equations in time (i.e. no algebraic equations are present) thus facilitating analysis and demonstrating successful control. Numerical examples are presented based on a very flexible hinged two-link manipulator.
Artola M, Wynn A, Palacios R, 2021, Modal-based nonlinear model predictive control for 3D very flexible structures, IEEE Transactions on Automatic Control, Vol: 67, ISSN: 0018-9286
In this paper a novel NMPC scheme is derived, which is tailored to the underlying structure of the intrinsic description of geometrically exact nonlinear beams (in which velocities and strains are primary variables). This is an important class of PDE models whose behaviour is fundamental to the performance of flexible structural systems (e.g., wind turbines, High-Altitude Long-Endurance aircraft). Furthermore, this class contains the much-studied Euler-Bernoulli and Timoshenko beam models, but has significant additional complexity (to capture 3D effects and arbitrarily large displacements) and requires explicit computation of rotations in the PDE dynamics to account for orientation-dependent forces such as gravity. A challenge presented by this formulation is that uncontrollable modes necessarily appear in any finite dimensional approximation to the PDE dynamics. We show, however, that an NMPC scheme can be constructed in which the error introduced by the uncontrollable modes can be explicitly controlled. Furthermore, in challenging numerical examples exhibiting considerable deformation and nonlinear effects, it is demonstrated that the asymptotic error can be made insignificant (from a practical perspective) usingour NMPC scheme and excellent performance is obtained evenwhen applied to a highly resolved numerical simulation of thePDEs. We also present a generalisation of Kelvin-Voigt dampingto the intrinsic description of geometrically-exact beams. Finally,special emphasis is placed on constructing a framework suitablefor real-time NMPC control, where the particular structure ofthe underlying PDEs is exploited to obtain both efficient finite-dimensional models and numerical schemes.
Cea A, Palacios R, 2021, A non-intrusive geometrically nonlinear augmentation to generic linear aeroelastic models, Journal of Fluids and Structures, Vol: 101, ISSN: 0889-9746
A new approach to build geometrically-nonlinear dynamic aeroelastic models is proposed that only uses information typically available in linear aeroelastic analyses, namely a generic (linear) finite-element model and frequency-domain aerodynamic influence coefficient matrices (AICs). Good computational efficiency is achieved through a two-step process: Firstly, a geometric reduction of the structure is carried out through static or dynamic condensation on nodes along the main load paths of the vehicle. Secondly, manipulation of the resulting linear normal modes (LNMs), the condensed stiffness and mass matrices, and the nodal coordinates provides the modal coefficients of the intrinsic beam equations along these load paths. This preserves the LNMs of the original problem and augments them with the geometrically nonlinear terms of beam theory. The structural description is in material coordinates and modal AICs are thus naturally included as follower forces. Numerical examples include cantilever wings built using detailed models, for which effects such as nonlinear aeroelastic equilibrium, nonlinear dynamics and structural-driven limit-cycle oscillations are shown. Results demonstrate the ability of the methodology to seamlessly and efficiently incorporate critical nonlinear effects to (linear) arbitrarily large aeroelastic models of high aspect ratio wings.
Cea A, Palacios R, 2021, A non-intrusive nonlinear aeroelastic extension of loads packages with application to long range transport aircraft configuration, AIAA Scitech 2021 Forum, Publisher: American Institute of Aeronautics and Astronautics
A new method for constructing geometrically-nonlinear aeroelastic systems from standard linear models is applied to an industry-level aircraft configuration. The new approach seamlessly integrates with current aeroelastic load packages performing linear analysis based on generic finite-element models (FEMs) and aerodynamic influence coefficient matrices (AICs). We generalize the methodology to incorporate control inputs, find the trimmed aircraft state, or generate gusts disturbances, which can be employed separately or combined to obtain a simplified flight dynamics model for load analysis. An initial study of the aeroelastic response of a long range aircraft is presented. Linear and nonlinear results are introduced in static and dynamic computations of manoeuvres, trim, and gust disturbances. These are compared to commercial software calculations, showing the need for geometrically nonlinear analysis in the production environment of airplanes with ultra high aspect ratio wings.
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