Discrete adjoint-based simultaneous analysis and design approach for conceptual aerodynamic optimization

In this paper, a simultaneous analysis and design method is derived and applied for a non-linear constrained aerodynamic optimization problem. The method is based on the approach of defining a Lagrange functional based on the objective function and the aerodynamic model’s equations, using two sets of multipliers. A fully-coupled, non-linear system of equations is derived by requiring that the Gateaux variation of the Lagrange functional vanishes for arbitrary variations of the aerodynamic model’s dependent variables and design parameters. The optimization problem is approached using a one-shot technique, by solving the non-linear system in which all sensitivities and problem constraints are included. The computational efficiency of the method is compared against a gradient-based optimization algorithm using adjoint-provided gradient. A conceptual-stage aerodynamic optimization problem is solved, based on a non-linear numerical lifting-line method with viscous corrections

Nonlinear lifting-line model using a vector formulation of the unsteady Kutta-Joukowski theorem

In this paper, a vector form of the unsteady Kutta-Joukowski theorem is derived and then used in the formulation of a general Lifting-Line Model capable of analysing a wide range of engineering problems of interest. The model is applicable to investigating lifting surfaces having low to moderate sweep, dihedral, out-of-plane features such as winglets, in both steady-state and unsteady cases. It features corrections of the span-wise circulation distribution based on available two-dimensional aerofoil experimental data, and stable wake relaxation through fictitious time marching. Potential applications include the conceptual and initial design of low-speed Unmanned Aerial Vehicles, the study of flapping flight or Wind Turbine blade design and analysis. Several verification and validation cases are presented, showing good agreement with experimental data and widely-used computational methods

Parameterised non-intrusive reduced-order model for general unsteady flow problems using artificial neural networks

A non-intrusive reduced-order model for nonlinear parametric flow problems is developed. It is based on extracting a reduced-order basis from high-order snapshots via proper orthogonal decomposition and using multi-layered feedforward artificial neural networks to approximate the reduced-order coefficients. The model is a generic and efficient approach for the reduction of time-dependent parametric systems, including those described by partial differential equations. Since it is non-intrusive, it is independent of the high-order computational method and can be used together with black-box solvers. Numerical studies are presented for steadystate isentropic nozzle flow with geometric parameterisation and unsteady parameterised viscous Burgers equation. An adaptive sampling strategy is proposed to increase the quality of the neural network approximation while minimising the required number of parameter samples and, as a direct consequence, the number of high-order snapshots and the size of the network training set. Results confirm the accuracy of the non-intrusive approach as well as the speed-up achieved compared with intrusive hyper reduced-order approaches

Validation of morphine wing methodologies on an unmanned aerial system and a wind tunnel technology demonstrator

To increase the aerodynamic efficiency of aircraft, in order to reduce the fuel consumption, a novel morphing wing concept has been developed. It consists in replacing a part of the wing upper and lower surfaces with a flexible skin whose shape can be modified using an actuation system placed inside the wing structure. Numerical studies in two and three dimensions were performed in order to determine the gains the morphing system achieves for the case of an Unmanned Aerial System and for a morphing technology demonstrator based on the wing tip of a transport aircraft. To obtain the optimal wing skin shapes in function of the flight condition, different global optimization algorithms were implemented, such as the Genetic Algorithm and the Artificial Bee Colony Algorithm. To reduce calculation times, a hybrid method was created by coupling the population-based algorithm with a fast, gradient-based local search method. Validations were performed with commercial state-of-the-art optimization tools and demonstrated the efficiency of the proposed methods. For accurately determining the aerodynamic characteristics of the morphing wing, two new methods were developed, a nonlinear lifting line method and a nonlinear vortex lattice method. Both use strip analysis of the span-wise wing section to account for the airfoil shape modifications induced by the flexible skin, and can provide accurate results for the wing drag coefficient. The methods do not require the generation of a complex mesh around the wing and are suitable for coupling with optimization algorithms due to the computational time several orders of magnitude smaller than traditional three-dimensional Computational Fluid Dynamics methods. Two-dimensional and three-dimensional optimizations of the Unmanned Aerial System wing equipped with the morphing skin were performed, with the objective of improving its performances for an extended range of flight conditions. The chordwise positions of the internal actuators, the spanwise number of actuation stations as well as the displacement limits were established. The performance improvements obtained and the limitations of the morphing wing concept were studied. To verify the optimization results, high-fidelity Computational Fluid Dynamics simulations were also performed, giving very accurate indications of the obtained gains. For the morphing model based on an aircraft wing tip, the skin shapes were optimized in order to control laminar flow on the upper surface. An automated structured mesh generation procedure was developed and implemented. To accurately capture the shape of the skin, a precision scanning procedure was done and its results were included in the numerical model. High-fidelity simulations were performed to determine the upper surface transition region and the numerical results were validated using experimental wind tunnel data

Fast and accurate quasi-3D aerodynamic methods for aircraft conceptual design studies

In this paper, recent developments in quasi-3D aerodynamic methods are presented. At their core, these methods are based on the Lifting-Line Theory and Vortex Lattice Method, but with a relaxed set of hypotheses, while also considering the effect of viscosity (to a certain degree) by introducing a strong non-linear coupling with two-dimensional viscous aerofoil aerodynamics. These methods can provide more accurate results compared to their inviscid classical counterparts and have an extended range of applicability with respect to the lifting surface geometry. Verification results are presented for both steady-state and unsteady flows, as well as case studies related to their integration into aerodynamic shape optimisation tools. The good accuracy achieved for relatively low computational time requirement makes quasi3D methods a solid choice for conducting conceptual level design and optimisation of lifting surfaces

Optimization and design of an aircraft's morphing wing-tip demonstrator for drag reduction at low speeds, Part II - Experimental validation using Infra-Red transition measurement from Wind Tunnel tests

In the present paper, an ‘in-house’ genetic algorithm was numerically and experimentally validated. The genetic algorithm was applied to an optimization problem for improving the aerodynamic performances of an aircraft wing tip through upper surface morphing. The optimization was performed for 16 flight cases expressed in terms of various combinations of speeds, angles of attack and aileron deflections. The displacements resulted from the optimization were used during the wind tunnel tests of the wing tip demonstrator for the actuators control to change the upper surface shape of the wing. The results of the optimization of the flow behavior for the airfoil morphing upper-surface problem were validated with wind tunnel experimental transition results obtained with infra-red Thermography on the wing-tip demonstrator. The validation proved that the 2D numerical optimization using the ‘in-house’ genetic algorithm was an appropriate tool in improving various aspects of a wing’s aerodynamic performances

An Optimisation of a Chordwise Slot to Enhance Lateral Flow Control on a UCAV

This research aims to optimise a chordwise slot so that lateral flow control of a flying wing configuration can be enhanced. This was achieved by maximising the airflow rate over the trailing edge control surfaces of the wing. A higher rate of airflow over the trailing edge control surfaces will increase the operationality of control surfaces, resulting in enhanced lateral control of the air vehicle at medium to high angles of attack. Four variables describing the chordwise slot are identified for numerical optimisation. They are: location, width, length, and angle of trajectory of chordwise slot relative to freestream. The results of the airflow rate for the wing with an optimised chordwise slot are compared with a baseline clean configuration. The flying wing configuration with a chordwise slot has shown a higher mass flow rate over the control surfaces of the wing in comparison to the baseline clean configuration, demonstrating that an optimised chordwise slot can be implemented on a flying wing configuration as it successfully controls the lateral flow by maximising the flow rate over the trailing edge control surfaces

Optimization and design of an aircraft’s morphing wing-tip demonstrator for drag reduction at low speed, Part I – Aerodynamic optimization using genetic, bee colony and gradient descent algorithms

In this paper, an ‘in-house’ genetic algorithm is described and applied to an optimization problem for improving the aerodynamic performances of an aircraft wing tip through upper surface morphing. The algorithm’s performances were studied from the convergence point of view, in accordance with design conditions. The algorithm was compared to two other optimization methods, namely the artificial bee colony and a gradient method, for two optimization objectives, and the results of the optimizations with each of the three methods were plotted on response surfaces obtained with the Monte Carlo method, to show that they were situated in the global optimum region. The optimization results for 16 wind tunnel test cases and 2 objective functions were presented. The 16 cases used for the optimizations were included in the experimental test plan for the morphing wing-tip demonstrator, and the results obtained using the displacements given by the optimizations were evaluated

Flutter analysis of a morphing wing technology demonstrator : numerical simulation and wind tunnel testing

As part of a morphing wing technology project, the flutter analysis of two finite element models and the experimental results of a morphing wing demonstrator equipped with aileron are presented. The finite element models are representing a wing section situated at the tip of the wing; the first model corresponds to a traditional aluminium upper surface skin of constant thickness and the second model corresponds to a composite optimized upper surface skin for morphing capabilities. The two models were analyzed for flutter occurrence and effects on the aeroelastic behaviour of the wing were studied by replacing the aluminium upper surface skin of the wing with a specially developed composite version. The morphing wing model with composite upper surface was manufactured and fitted with three accelerometers to record the amplitudes and frequencies during tests at the subsonic wind tunnel facility at the National Research Council. The results presented showed that no aeroelastic phenomenon occurred at the speeds, angles of attack and aileron deflections studied in the wind tunnel and confirmed the prediction of the flutter analysis on the frequencies and modal displacements