Flexible Engineering Toward Green Aircraft

Aeroelastic wind tunnel tests on a half-wing model have been performed at the University of Naples “Federico II” to acquire data about aerodynamic forces, section pressure coefficient, stress, strain, and model displacement, to validate high fidelity Fluid-Structure Interaction approaches based on Reynolds-Averaged Navier-Stokes and Finite Element Method solutions investigated at the University of Rome “Tor Vergata”. Most of the available experimental databases of aeroelastic measurements, performed on aircraft wings, model full scale systems, focusing primarily on aerodynamic aspects rather than on structural similitudes. To investigate flow regimes that replicate realistic operating conditions, wind tunnel test campaigns involve the generation of relative high loads on models whose safe dimensioning force the adoption of structural configurations that lose any similitude with typical wing box topologies. The objective of this work is to generate a database of loads, pressure, stress, and deformation measurements that is significant for a realistic aeronautical design problem. At this aim, a wind tunnel model of a half-wing that replicates a typical metallic wing box structure and instrumented with pressure taps and strain gages has been investigated. All experimental data and numerical models are freely available to the scientific community at the website www.​ribes-project.​eu.

The aim of this paper is to compare and validate two computational methods to study typical aircrafts aeroelastic problems. The first one is the very well-established 2-way coupling approach, which envisages the use of a mesh morphing tool to update the CFD mesh according to the computed displacements in combination with a mapping algorithm to transfer the loads onto the FEM mesh. The second one is based on the embedding of structural modes, computed in advance by a FEM solver, directly into the CFD solver. It requires a morphing tool to deform the CFD mesh according to FEA computed modal shapes and a surface integration algorithm that allows to evaluate the modal forces acting on the CFD mesh. Modes embedding makes the CFD model intrinsically aeroelastic and thus capable to self-adapt its shape in the respect of the actual deformation, removing all the complexities related to the data exchange between solvers. Both methods were validated against a literature benchmark test case consisting in the prediction of the static aeroelastic equilibrium of the HIRENASD model using two of the meshes available in the “NASA First Aeroelasticity Workshop” database. The fidelity of both methods has been successfully validated, achieving a satisfactory agreement with experimental data.

Current aircraft design leads to increased flexibility of the airframe as a result of modern materials application or aerodynamically efficient slender wings. The airframe flexibility influences the aerodynamic performance and it might significantly impact the aeroelastic effects, which can be more easily excited by rigid body motions than in case of stiffer structures. The potential aeroelastic phenomena can occur in large range of speeds involving transonic regime, where the non-linear flow effects significantly influence the flutter speed. Common aeroelastic analysis tools are mostly based on the linear theories for aerodynamic predictions, thus they fail to predict mentioned non-linear effect. This paper presents the first step in the design of high-fidelity aeroelastic simulation tool. Currently, it allows to perform static aeroelastic simulations by coupling Computational Fluid Dynamics solver with Matlab based Finite Element solver. The structural solver is a linear elasticity solver which is able to solve either models consisting of beam elements or arbitrary models using stiffness and mass matrices exported from Nastran solver. The aeroelastic interface is based on the Radial Basic Functions. The test case studied in this work is a static aeroelastic simulation of the Common Research Model in the transonic conditions. The structural models tested are a wing-box finite element model and a beam stick model which is statically equivalent to the wing-box model. The comparison of results using respective structural models shows good agreement in aerodynamic properties of the model wing at static equilibrium state.

More then 10 years ago a large investment was made in extending the NSMB Navier Stokes Multi Block (NSMB) Computational Fluid Dynamics (CFD) towards Fluid Structure Interaction (FSI) simulations (Guillaume et al. in Fluid structure interaction simulation on the F/A-18 vertical tail, 2010 [1], Guillaume et al. in Aeronaut J 115:285–294, 2011 [2]). At that time a segregated approach was adopted using a loosely coupled approach. More recently NSMB was coupled to the open-source Finite Element Analysis environment B2000++ (http://​www.​smr.​ch/​products/​b2000/​ [3]) in a strongly coupled approach. This has led to the possibility to perform both static and dynamic FSI simulations using either a modal or a FEM approach without the need to interrupt the simulation. Results of aero-elastic simulations for the MDO-aircraft, the AGARD445.6 wing and for a Strut Braced Wing configuration will be presented.

This chapter outlines the semi-analytical methodology that was developed over the past decade and a half to model transient fluid-structure interaction phenomena for thin-walled structures submerged in and/or filled with fluid. The theoretical framework of the methodology based on the use of the classical apparatus of mathematical physics is exposed first. Then, a demonstration of some of the capabilities of the methodology is presented as it is applied to an industrially relevant fluid-structure interaction problem. Specifically, the response of a submerged cylindrical shell to a double-front shock wave is considered, with the emphasis on the existence of certain resonance-like phenomena which result in a considerable increase of the maximum stress induced in the structure by such a loading. The outcomes of the modeling using both the 2D and 3D versions of the methodology are presented, and the differences between the results produced by these two approaches, a lower-fidelity one and a higher-fidelity one, are highlighted.

This paper presents a numerical approach for high fidelity modelling of ground excited vibrations of a structure interacting with surrounding fluid flow. The motion of the structure is represented directly on the CFD model mesh by embedding the structural modes using radial basis functions mesh morphing. Modal forces integrals are computed on the CFD mesh enabling a time marching FSI solution based on the weak approach. Ground vibration is represented by adding a rigid movement and related inertial loads using modal participation factors. The approach is validated by studying a cantilever beam vibrating in air excited by a transversal sine motion applied to the clamped end that is relevant for the design of flapping devices. Numerical results are successfully validated by comparing the coupled and uncoupled response computed according to the proposed approach with the analytic one and to a standard FEA solver.

The present paper is addressed to the numerical analysis of fluid-structure instabilities in a flexible tubes bundle subjected to the loads induced by a water turbulent crossflow, using the arrangement presented in Weaver and Abd-Rabbo (J Fluids Eng, 1985 [1]) as benchmark. The physical phenomena involved by the water turbulent crossflow raise strong interest from the scientific community. The nuclear industry is particularly concerned as the design of reliable large-scale exchangers is of primary importance to ensure good performance of nuclear plants. As a matter of fact, their detailed simulation is characterised by challenging traits such as the large amplitude of the tubes vibrations, the strong coupling between water and tubes, the need for an accurate evaluation of the fluid damping and critical flow velocity which vibration instabilities arise at, as well as the complex transition of the fluid-structure behaviour. To tackle these challenges in an effective way, unsteady Fluid-Structure Interaction (FSI) studies were performed applying the mode-superposition approach by means of a mesh morphing technique founded on the mathematical framework of Radial Basis Functions (RBF). In particular, the computational outputs were gained by employing a combined use of ANSYS^® Fluent^®, ANSYS^® Mechanical™ and RBF Morph™ software. The two-equation realizable κ-ε turbulence model was adopted to run the U-RANS simulations on high-fidelity structured hexahedral meshes. The achieved numerical results were compared with well-documented experimental data, and a satisfying agreement was finally attained. Furthermore, the operative crossflow velocity guaranteeing the stable functioning of the tubes array was also identified. We demonstrated that the proposed modal approach, in combination with mesh morphing, allows designers to set-up an effective workflow to predict unsteady FSI problems that can be widely adopted for industrial applications under the hypothesis of linear structural behaviour.

A Robust Design Optimization (RDO) method based on the use of Conditional Value-at-Risk (CVaR) risk measure is briefly described and applied to an aerodynamic shape design problem. The technique leads to optimal design solutions resilient to production tolerances and operating conditions instabilities. The approach is illustrated through the application to an airfoil section design optimization in low transonic conditions with the flow field modeled using an Euler plus boundary layer interactive approach. The results of the robust design are compared to those obtained with a classical deterministic method, and mutual advantages and disadvantages of the two approaches are discussed.

Aircraft, and in particular military aircraft, are complex systems and the demand for high-performance flying platforms is constantly growing both for civil and  military purposes.  The development of aircraft is inherently  multidisciplinary and the exploitation of the interaction between the disciplines driving the design opens the door for new (unconventional) aircraft designs, and consequently, for novel aircraft having increased performance. In modern aircraft development processes and procedures, it is crucial to enable the engineers accessing complex design spaces, especially in the conceptual design phase where key configuration decisions are made and frozen for later development phases. Pushing more MDO and numerical analysis capabilities into the early design phase will support the decision-making process through reliable physical information for very large design spaces which can hardly be grasped and explored by humans without the support of automated numerical analysis capabilities. Therefore, from the start of the aircraft development, process computer simulations play a major role in the prediction of the physical properties and behavior of the aircraft. Recent advances in computational performance and simulation capabilities provide sophisticated physics based models, which can deliver disciplinary analysis data in a time effective manner, even for unconventional configurations. However, a major challenge arises in aircraft design as the properties from different disciplines (aerodynamics, structures, stability and control, etc.) are in constant interaction with each other. This challenge is even greater when specialized competences are provided by several multidisciplinary teams distributed among different organizations. It is therefore important to connect not only the simulation models between organizations, but also the corresponding experts to combine all competences and accelerate the design process to find the best possible solution. A multi-disciplinary study of an unmanned aerial vehicle (UAV), presented in this article, was performed by eight different partners all over Europe to show the advances during the Horizon 2020 project Aircraft 3rd Generation  MDO for Innovative Collaboration of Heterogeneous Teams of Experts (AGILE).