Imperial College London

Dr Rafael Palacios

Faculty of EngineeringDepartment of Aeronautics

Reader in Aeronautics
 
 
 
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Contact

 

+44 (0)20 7594 5075r.palacios CV

 
 
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Location

 

355Roderic Hill BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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105 results found

Broughton-Venner JJ, Wynn A, Palacios R, 2017, Aeroservoelastic optimisation of an aerofoil with active compliant flap via reparametrisation and variable selection

© 2017, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. This paper presents an investigation on simultaneous optimisation strategies for flexible vehicles and their control systems. The aeroservoelastic system consists of a two-dimensional, potential flow over a deforming aerofoil; an actively controlled, but saturated compliant trailing edge; a dynamic observer that uses a series of pressure sensors on the aerofoil; and a heave/pitch linear spring model. Although computationally simple, the design allows for optimisation over multiple disciplines: the structure can be designed by varying the stiffness of the springs; the control architecture through weightings in a LQR controller; the observer by means of the placement of pressure sensors; and the aerodynamics via the shaping of the compliant trailing edge. Optimising the weight and a metric of performance over all disciplines simultaneously is compared to a sequential methodology of optimising the open-loop characteristics first and subsequently adding a closed-loop controller. We show that varying the parametrisation and number of design variables during the optimisation can lead to improvements in the final design, and present a procedure to automate this process. To accomplish this, a new basis for the design vector is created via Proper Orthogonal Decomposition (POD) using the trajectories of initial optimisation paths as a "training set". This parametrisation is shown to make the optimisation more robust with respect to the initial design, and facilitate an automated variable selection methodology. This variable selection allows for the dimension of the problem to be reduced temporarily and it is shown that this makes the optimisation more robust.

CONFERENCE PAPER

Buoso S, Palacios R, 2017, On-Demand Aerodynamics in Integrally Actuated Membranes with Feedback Control, AIAA JOURNAL, Vol: 55, Pages: 377-388, ISSN: 0001-1452

JOURNAL ARTICLE

González-Salcedo A, Aparicio-Sanchez M, Munduate X, Pires O, Méndez B, Palacios R, Graham JMRet al., 2017, A computationally-efficient panel code for unsteady airfoil modelling including dynamic stall

© 2017, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. A new approach based on inviscid panel methods has been developed for airfoils undergoing unsteady kinematics. The model is tailored to practical applications related to wind turbines, taking into account accuracy and computational cost considerations. It is able to represent rigid or deformable airfoils, attached and separated flow focusing also on unsteady cases including dynamic stall. An extensive validation has been carried out including steady and unsteady conditions, while simulations of airfoils with trailing edge flaps have also been performed.

CONFERENCE PAPER

Maraniello S, Palacios R, 2017, Optimal rolling maneuvers with very flexible wings, AIAA Journal, Vol: 55, Pages: 2964-2979, ISSN: 0001-1452

© Copyright 2017 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. The single-shooting method is used to identify optimal maneuvers in the lateral dynamics of a partially supported flexible wing. The aim is to identify efficient actuation strategies from a fully coupled nonlinear aeroelastic/flightdynamics model, which accounts for potentially large wing deflections, to improve vehicle maneuverability. The flexible vehicle dynamics is described using a geometrically exact composite beam on a body-attached frame and an unsteady vortex lattice with arbitrary kinematics of the lifting surfaces. Rolling maneuvers are obtained through optimal control. A flight-dynamics model based on elasticized stability derivatives is used as a reference, and it is shown to capture the relevant dynamics either under slow actuation or for stiff wings. Embedding the full aeroelastic description into the optimization framework expands the space of achievable maneuvers, such as quick wing response with low structural vibrations or large lateral forces with minimal lift losses. It is also seen to provide a general methodology to identify unconventional maneuvers that use large wing geometry changes to meet multiple simultaneous control objectives.

JOURNAL ARTICLE

Maraniello S, Palacios R, 2017, Geometrically-nonlinear effects in lateral manoeuvres with coupled flight dynamics and aeroelasticity

© 2017, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. An investigation on the aeroelastic effects in lateral manoeuvres with very flexible wings is presented. The aim is to identify efficient actuation strategies from fully coupled non- linear aeroelastic/flight dynamics models, which account for potential large wing deections to improve vehicle manoeuvrability. The flexible vehicle dynamics is described using geometrically-exact composite beams on a body-attached frame and an unsteady vortex lattice with arbitrary kinematics of the lifting surfaces, while rolling manoeuvres are identified through optimal control. A flight-dynamics model based on elastified stability derivatives is used as a reference, and it is observed to capture the relevant dynamics either under slow actuation or for stiff wings. Embedding the full aeroelastic description into an optimal control framework is shown to expand the space of achievable manoeuvres, such as quick wing response with low structural vibrations or large lateral forces with minimal lift losses. It is also seen to provide a general methodology to identify unconventional manoeuvres that utilize large wing geometry changes to meet multiple simultaneous control objectives.

CONFERENCE PAPER

Ng BF, New TH, Palacios R, 2017, Bio-inspired leading-edge tubercles to improve fatigue life in horizontal axis wind turbine blades

© 2017, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. Bio-inspired leading-edge tubercles are known to improve aerodynamic performances during stall but there could be additional advantages in other operating regimes and their effect on aeroelastic loadings is also less understood. In this study, the effect of leading-edge tubercles on fatigue loadings on wind turbine blades is investigated using an aeroelastic model that couples a composite beam to the unsteady vortex-lattice method. To accommodate the leading-edge tubercles, spanwise structural properties and aerodynamic geometries are varied, which resulted in a reduction in both torsional frequencies and aerodynamic lift despite keeping the same planform area. The reductions in structural frequencies and aerodynamics have counteracting effects on fatigue responses, with the former increasing and the latter reducing loads. On a turbine blade with tubercles (amplitude of 0.2c and wavelength of 0.5c) occupying 20% to 95% span of the leading-edge, flapwise root-bending moment was found to be 6% lower than the unmodified configuration, which can be further enhanced with a trough termination at the blade tip. Torsional moment was 17% lower due to the reduction in leading-edge suction along tubercle crests, which are further away from the elastic axis. In terms of tubercle positioning, having tubercles close to the blade tip enables performance enhancement during episodes of stall from large tip deflections and has significant contributions to root-bending moment due to a larger moment arm and higher relative flow speed. On the other hand, positioning tubercles close to the blade root may also be favoured as this region is prone to stall from low speeds, yet having little effect on fatigue responses.

CONFERENCE PAPER

Ng BF, Palacios R, Graham JMR, 2017, Model-based aeroelastic analysis and blade load alleviation of offshore wind turbines, INTERNATIONAL JOURNAL OF CONTROL, Vol: 90, Pages: 15-36, ISSN: 0020-7179

JOURNAL ARTICLE

Palacios R, Simpson RJS, Maraniello S, 2017, State-space realizations of potential-flow unsteady aerodynamics with arbitrary kinematics

© 2017, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. We introduce a nondimensional state-space formulation of the unsteady vortex-lattice method for time-domain aerodynamics. It deals with 2- or 3-dimensional geometries, re- solves frequencies up to a spatio-temporal Nyquist limit defined by the wake discretization, and has a convenient form for linearization, model reduction and coupling with structural dynamics models. No assumptions are made relating to the kinematics of the fluid-structure interface (inputs) and use of Joukowski's theorem to compute forces naturally resolves all components of the unsteady aerodynamic forcing (outputs). Linearized expressions are written about arbitrary non-zero reference geometries, velocities and loading distributions and as such yield models that are as general as possible given the assumptions in the un- derlying uid mechanics. The implementation is verified against classical solutions in the unsteady aerodynamics, and in aeroelastic stability analysis of cantilever wing configurations.

CONFERENCE PAPER

Qi P, Wang Y, Zhao X, Palacios R, Wynn Aet al., 2017, Trajectory control of a very flexible flying wing, Pages: 4480-4485, ISSN: 0743-1619

© 2017 American Automatic Control Council (AACC). This paper investigates the trajectory control of a very flexible flying wing model, which is derived from geometrically-nonlinear beam theory using intrinsic structural description in [1]. This model is coupled with structural dynamics, aeroelastic dynamics and flight dynamics. The control design is using a two-loop LADRC (linear active disturbance rejection control) and H ∞ scheme in both the longitudinal and lateral channels, based on a reduced-order linearised model. In each channel, the outer loop (position control) employs LADRC technique to track desired flight routes and generate attitude command to the inner loop, while the inner loop (attitude control) uses H ∞ control technique to track the attitude command generated from the outer loop and computes the control inputs to the corresponding control surfaces. The simulation tests are conducted in a reduced-order nonlinear model (the reduced-order linearised model with a quadratic nonlinear term). Simulation study shows that the trajectory control system achieves good robust and tracking performance. We mention that the simulation differences between the reduced-order and full-order nonlinear models are negligible in the case of trajectory control tests.

CONFERENCE PAPER

Sanchez R, Albring T, Palacios R, Gauger NR, Economon TD, Alonso JJet al., 2017, Coupled Adjoint-Based Sensitivities in Large-Displacement Fluid-Structure Interaction using Algorithmic Differentiation, International Journal for Numerical Methods in Engineering, ISSN: 0029-5981

JOURNAL ARTICLE

Sanchez R, Palacios R, Economon TD, Alonso JJ, Albring T, Gaugerk NRet al., 2017, Optimal actuation of dielectric membrane wings using high-fidelity fluid-structure modelling

© 2017, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. This paper describes a computational framework for the analysis and design of electromechanically actuated membrane wings operating at low flight speeds. A fluid-structure interaction formulation with large deformations and complex material behavior has been developed, which is suited for integrally-actuated wings built with dielectric elastomers. A coupled adjoint-based methodology based on algorithmic differentiation has also been developed to determine optimal actuation profiles. Coupled sensitivities are shown to be accurately computed on the wing response, both under constant pressure and immersed in a reattached laminar flow. A simple optimization problem on the equilibrium position of the membrane is finally solved to exemplify the proposed design methodology.

CONFERENCE PAPER

Bao Y, Palacios R, Graham M, Sherwin Set al., 2016, Generalized thick strip modelling for vortex-induced vibration of long flexible cylinders, JOURNAL OF COMPUTATIONAL PHYSICS, Vol: 321, Pages: 1079-1097, ISSN: 0021-9991

JOURNAL ARTICLE

Broughton-Venner JJ, Wynn A, Palacios R, 2016, Aeroservoelastic optimisation of an aerofoil with active compliant flap via reparametrisation and variable selection

© 2016, American Institute of Aeronautics and Astronautics. All right reserved. To aid in the investigation of new simultaneous optimisation strategies for exible vehicles and their control systems, a two-dimensional aerofoil optimisation which demands minimal computational effort is studied. The aeroservoelastic system consists of a two-dimensional, potential flow over a deforming aerofoil; an actively controlled, but saturated compliant trailing edge; a dynamic observer that uses a series of pressure sensors on the aerofoil; and a heave/pitch linear spring model. Although computationally simple, the design allows for optimisation over multiple disciplines: the structure can be designed by varying the stiffness of the springs; the control architecture through weightings in a LQR controller; the observer by means of the placement of pressure sensors; and the aerodynamics via the shaping of the compliant trailing edge. Optimising the weight and a metric of performance over all these fields simultaneously is compared to a sequential methodology of optimising the open-loop characteristics first and subsequently adding a closed-loop con-troller. Parametrisation of the design vector and variable selection often require user input and are fixed during optimisation. Our research aims to automate this process. Further-more, we investigate whether varying the parametrisation and number of design variables during the optimisation can lead to improvements in the final design. To accomplish this, a new basis for the design vector is created via Proper Orthogonal Decomposition (POD) using the trajectories of initial optimisation paths as a “training set". This parametrisation is shown to make the optimisation more robust with respect to the initial design, and facilitate an automated variable selection methodology. This variable selection allows for the dimension of the problem to be reduced temporarily and it is shown that this makes the optimisation more robust

CONFERENCE PAPER

Buoso S, Palacios R, 2016, Feedback control of integrally actuated membrane wings: A computational study

© 2015 by Stefano Buoso and Rafael Palacios. The paper is an investigation on computational modelling and control system design of integrally actuated membrane wings. A high-fidelity electro-aeromechanical model is used for the simulation of the dynamic fluid-structure interaction between a low-Reynolds- number flow and a dielectric elastomer wing. A reduced-order model is obtained coupling a modal structural description with a linearisation of the fluid equations based on the Proper Orthogonal Decomposition. The low-order system is then used for the design of Proportional-Integral-Derivative and Linear Quadratic Gaussian feedback schemes for the control of the wing lift coefficient. When implemented in the high-fidelity model closed- loop dynamics are in very good agreement with the reduced-order model, demonstrating the suitability of the approach. Finally, the designed controllers are used to track required aerodynamic performance and compensate for prescribed disturbances of the inlet flow conditions. The control laws selected in this work were found to be effective only for low- frequency disturbances due to the large phase delay introduced by the fluid convective time- scales but the numerical results demonstrates the potential for the aerodynamic control of membrane wings in outdoor flight using dielectric elastomers.

CONFERENCE PAPER

Buoso S, Palacios R, 2016, Viscoelastic effects in the aeromechanics of actuated elastomeric membrane wings, JOURNAL OF FLUIDS AND STRUCTURES, Vol: 63, Pages: 40-56, ISSN: 0889-9746

JOURNAL ARTICLE

Buoso S, Palacios R, 2016, High-fidelity simulation and reduced-order modelling of integrally-actuated membrane wings with feedback control, Active and Passive Smart Structures and Integrated Systems 2016, Publisher: SPIE-INT SOC OPTICAL ENGINEERING, ISSN: 0277-786X

CONFERENCE PAPER

Hesse H, Palacios R, 2016, Dynamic Load Alleviation in Wake Vortex Encounters, JOURNAL OF GUIDANCE CONTROL AND DYNAMICS, Vol: 39, Pages: 801-813, ISSN: 0731-5090

JOURNAL ARTICLE

Maraniello S, Palacios R, 2016, Optimal vibration control and co-design of very flexible actuated structures, JOURNAL OF SOUND AND VIBRATION, Vol: 377, Pages: 1-21, ISSN: 0022-460X

JOURNAL ARTICLE

Maraniello S, Simpson RJS, Palacios R, 2016, Optimal manoeuvres with very flexible wings

© 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. The single shooting method is used identify optimal manoeuvres in the lateral dynamics of partially-supported wings of very low stiffiness. The aim is to identify actuation strategies in the design of aircraft manoeuvres in which large wing deflections can substantially modify the vehicle structural and aerodynamic features. Preliminary studies are presented for a representative high-altitude long-endurance aircraft wing in hinged configuration. Nonlinear effects due to large deflections are captured coupling a geometrically exact beam model with an unsteady vortex lattice method for the aerodynamics. The optimal control problem is solved via a gradient-based algorithm. When lowering the wing stiffiness, the nonlinearities connected to the system - such as the fore-shortening effect due to large bending deflections - increase the wing lateral stability but at the same time they also reduce aileron authority. The single-shooting optimisation is shown to capture these features and to provide satisfactory results, not only when refining a predetermined actuation law but also when designing it from zero.

CONFERENCE PAPER

Ng BF, New TH, Palacios R, 2016, Effects of leading-edge tubercles on wing flutter speeds, BIOINSPIRATION & BIOMIMETICS, Vol: 11, ISSN: 1748-3182

JOURNAL ARTICLE

Ng BF, Palacios R, Kerrigan EC, Graham JMR, Hesse Het al., 2016, Aerodynamic load control in horizontal axis wind turbines with combined aeroelastic tailoring and trailing-edge flaps, WIND ENERGY, Vol: 19, Pages: 243-263, ISSN: 1095-4244

JOURNAL ARTICLE

Sanchez R, Kline HL, Thomas D, Variyar A, Righi M, Economon TD, Alonso JJ, Palacios R, Dimitriadis G, Terrapon Vet al., 2016, Assessment of the fluid-structure interaction capabilities for aeronautical applications of the open-source solver SU2, Pages: 1498-1529

We report on an international effort to develop an open-source computational environment for high-fidelity fluid-structure interaction analysis. In particular, we will focus on verification of the implementation for application in computational aeroelasticity. The capabilities of the SU2 code for aeroelastic analysis have been further enhanced both by developing natively embedded tools for the study of largely deformable solids, and by wrapping it using Python tools for an improved communication with external solvers. Both capabilities will be demonstrated on relevant test cases, including rigid-airfoil solutions with indicial functions, the Isogai Wing Section, test cases from the AIAA 2nd Aeroelastic Prediction Workshop, and the vortex-induced vibrations of a flexible cantilever in the wake of a square cylinder. Results show very good performance both in terms of accuracy and computational efficiency. The modularity and versatility of the baseline suite allows for a flexible framework for multidisciplinary computational analysis. The software libraries have been freely shared with the community to encourage further engagement in the improvement, validation and further development of this open-source project.

CONFERENCE PAPER

Wang Y, Wynn A, Palacios R, 2016, Model-predictive control of flexible aircraft using nonlinear reduced-order models

© 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. This paper introduces an optimization-based load control strategy suitable for flexible vehicles with very flexible wings. A model for the coupled flight dynamics/aeroelastic response of the vehicle is built from geometrically-nonlinear beams using an intrinsic de- scription and a simple linear 2-D unsteady aerodynamic with no spanwise couplings. The resulting model has only quadratic nonlinearities and a balanced projection of the dynamic equations is then practicable and results in a much smaller model that retains the relevant nonlinear couplings. This is finally used to drive a model-predictive control designed to suppress wing oscillations in the response to atmospheric gusts. Results are presented on high-aspect-ratio flying wing undergoing moderately large excursions in the presence of discrete gusts. They demonstrate the performance of the nonlinear MPC in comparison to equivalent linear control schemes.

CONFERENCE PAPER

Wang Y, Wynn A, Palacios R, 2016, Nonlinear Modal Aeroservoelastic Analysis Framework for Flexible Aircraft, AIAA JOURNAL, Vol: 54, Pages: 3075-3090, ISSN: 0001-1452

JOURNAL ARTICLE

Buoso S, Palacios R, 2015, Electro-aeromechanical modelling and feedback control of actuated membrane wings

© 2015, by the American Institute. This paper presents a numerical investigation on the potential for unsteady aerodynamic control on integrally-actuated membrane wings made of dielectric elastomers (DEs). They combine the advantages of membrane shape adaptability, which produces increased lift and delayed stall, with the benefits of simple, lightweight but high-authority control mechanism offered by integral actuation. High-fidelity numerical models have been developed to pre­dict their performance and include a fluid solver based on the direct numerical integration of the unsteady Navier-Stokes equations, an electromechanical constitutive material model and a non-linear three-dimensional membrane structural model. Numerical results show that harmonic actuation of 5.0 kV gives an overall increase in the aerodynamic efficiency a fixed wing configuration the wing of up to 6.0%, measured as the lift-to-drag ratio. In addition, the definition of a reduced order model based on POD modes of the complete high-fidelity system allows the synthesis of a feedback control system to obtain on-demand aerodynamic performance.

CONFERENCE PAPER

Buoso S, Palacios R, 2015, Reduced-order modelling and feedback control of integrally actuated membrane wings

This paper presents a numerical investigation on aerodynamic control of integrally-actuated membrane wings made of dielectric elastomers. They combine the advantages of membrane shape adaptability with the benefits of the simple, lightweight but high-authority control mechanism offered by integral actuation. For that purpose, high-fidelity numerical models have been developed to predict their performance. They include a fluid solver based on the direct numerical integration of the unsteady Navier-Stokes equations, an electromechanical constitutive material model and a non-linear three-dimensional membrane structural model. In addition, using the Eigensystem Realization Algorithm, it is obtained a very low order model description of the fully coupled aero-electromechanical system to aid the design of a simple PID control scheme for the feedback control of the wing. The resulting regulator is then implemented in the high-fidelity model and used for the mitigation of flow disturbances.

CONFERENCE PAPER

Buoso S, Palacios R, 2015, Electro-aeromechanical modelling of actuated membrane wings, JOURNAL OF FLUIDS AND STRUCTURES, Vol: 58, Pages: 188-202, ISSN: 0889-9746

JOURNAL ARTICLE

Hesse H, Palacios R, 2015, Dynamic load alleviation of flexible aircraft in wake vortex encounters

This paper introduces an integrated approach for flexible-aircraft time-domain aeroelastic simulation and controller design suitable for wake encounter situations. The dynamic response of the vehicle, which may be subject to large wing deformations in trimmed flight, is described by a geometrically-nonlinear composite-beam finite-element model. The aerodynamics is modeled using the unsteady vortex lattice method and includes the arbitrary time-domain downwash distributions of a wake encounter. A consistent linearization in the structural degrees of freedom enables the use of balancing methods to reduce the problem size while retaining the nonlinear terms in the rigid-body equations. Numerical studies on a very flexible aircraft demonstrate the reduced-order modeling approach for load calculations in wake vortex encounters over a large parameter space. Closed-loop results finally explore the potential of combining feedforward/feedback H ∞ control and conventional control surfaces for load alleviation.

CONFERENCE PAPER

Jaswantlal RJ, Marzocca P, Palacios R, 2015, Unsteady Aerodynamics of a 3D Wing Hosting Synthetic Jet Actuators

Copyright © 2015 SAE International. The implementation of Synthetic Jet Actuators (SJAs) on Unmanned Aerial Vehicles (UAVs) provides a safe test-bed for analysis of improved performance, in the hope of certification of this technology on commercial aircraft in the future. The use of high resolution numerical methods (i.e. CFD) to capture the details of the effects of SJAs on flows and on the hosting lifting surface are computationally expensive and time-consuming, which renders them ineffective for use in real-time flow control implementations. Suitable alternatives include the use of Reduced Order Models (ROMs) to capture the lower resolution overall effects of the jets on the flow and the hosting structure. This research paper analyses the effects of SJAs on aircraft wings using a ROM for the purpose of determining the unsteady aerodynamic forces modified by the presence of the SJAs. The model developed is a 3D unsteady panel code where the jets are represented by source panels. This code has the ability to model thick 3D trapezoidal wings with sweep and dihedral, accepting any aerofoil coordinate file. Multiple rows of SJAs can also be applied where desired. In order to validate the panel code, comparisons are made with past experimental data and other numerical methods like the vortex lattice method and CFD. There is avenue for further work in implementing a robust control architecture around SJA technologies, which in the future could serve as replacements for traditional hinged flight control surfaces and flutter suppression solutions.

CONFERENCE PAPER

Maraniello S, Palacios R, 2015, Co-design of very flexible actuated structures

A numerical investigation on open-loop control and combined structural and control design (co-design) of very flexible beams is presented. The objective is to allow for an efficient design of these systems by identifying design strategies that provide significant performance advantages with respect to conventional sequential design methods. The control vector parametrisation method, implemented for both a B-splines (local) and discrete sines (global) set of basis functions, is used in conjunction with a gradient based optimiser to solve first the open-loop control and then the co-design problems. Numerical results show the impact of the time-frequency resolution of the parametrisation on the outcome of the optimisation. Overall, B-splines can achieve higher performance as they better exploit the flexible, high frequency driven, behaviour of the structure, particularly as large deformations lead to changes in the natural frequencies of the system. The discrete sines based parametrisation, on the other hand, is found to be a more robust choice. The mutual influence between control and structural dynamics during the design process is showed and used to explain the ability of the optimiser to approach a global optimum. In particular, it was found that control and structural disciplines can freeze the design around specific characteristic frequencies (locking), limiting the advantages of a co-design approach based on gradient methods.

CONFERENCE PAPER

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