Gérald Carrier

Aerostructural Adjoint Method for Flexible Wing Optimization

This paper presents the current developments at ONERA on wing optimization via the aero-structural adjoint method. The aero-structural adjoint is the extension of the aero-elastic adjoint already used in aerodynamic function optimization.1 The aero-structural adjoint method allows the improvement of both aerodynamic and structural functions in the same design space. The internal structural element thicknesses (spar webs, caps, skins), the structural characteristics (flexibility) and the planform parameters are all variable in the aero-structural adjoint-based design process. A module for structural modelling, wing weight estimation and adjoint-compatible structural sensitivities computation is presented. The material stresses are aggregated into the Kreisselmeier-Steinhauser function to reduce the high number of design structural constraints. The structural module is integrated with the existing aero-elastic environment for adjoint-based optimizations in order to perform drag and weight optimizations.

Numerical and Experimental Aerodynamic Investigations of Boundary Layer Ingestion for Improving Propulsion Efficiency of Future Air Transport

This paper presents the recent investigations of the Boundary Layer Ingestion (BLI) concept conducted at ONERA. Both numerical and experimental investigations of a BLI propulsion system are conducted synergistically with the objectives of preparing and validating both the numerical simulations tools and the experimental methodologies that will be necessary for the design of a future transport aircraft incorporating a BLI propulsive system. A test rig dedicated to the investigation of BLI has been specifically designed manufactured and tested in the ONERA L1 wind tunnel in Lille in low-speed conditions. This experimental rig embeds an electrically powered fan and several measurements capabilities including separate measurements of the forces acting on the different model components, thus allowing a in depth investigations and quantification of the mechanisms of BLI. Numerical simulations of this test rig, based on RANS CFD calculations using the Actuator Disk (AD) approach to model the effect of the fan absorbing a boundary layer flow have been used, first to help defining the experimental test rig and then, using the experimental results to establish and validate a numerical simulation approach which could be used in the future to facilitate the integration of a BLI propulsion system on a future transport aircraft.

Cooperation and Competition Strategies in Multi-objective Shape Optimization - Application to Low-boom/Low-drag Supersonic Business Jet

Cooperation and competition are natural laws that regulate the interactions between agents in numerous physical, or social phenomena. By analogy, we transpose these laws to devise e cient multi-objective algorithms applied to shape optimization problems involving two or more disciplines. Two e cient strategies are presented in this paper: a multiple gradient descent algorithm (MGDA) and a Nash game strategy based on an original split of territories between disciplines. MGDA is a multi-objective extension of the steepest descent method. The use of a gradient-based algorithm that exploits the cooperation principle aims at reducing the number of iterations required for classical multi-objective evolutionary algorithms to converge to a Pareto optimal design. On the other hand side, the Nash game strategy is well adapted to typical aeronautical optimization problems, when, after having optimized a preponderant or fragile discipline (e.g. aerodynamics), by the minimization of a primary objective-function, one then wishes to reduce a secondary objective-function, representative of another discipline, in a process that avoids degrading excessively the original optimum. Presently, the combination of the two approaches is exploited, in a method that explores the entire Pareto front. Both algorithms are rst analyzed on analytical test cases to demonstrate their main features and then applied to the optimum-shape design of a low-boom/low-drag supersonic business jet design problem. Indeed, sonic boom is one of the main limiting factors to the development of civil supersonic transportation. As the driving design for low-boom is not compliant with the low-drag one, our goal is to provide a trade-o between aerodynamics and acoustics. Thus Nash games are adopted to de ne a low-boom con guration close to aerodynamic optimality w.r.t. wave drag.

Concept Study for a Mach 6 Transport Aircraft

A conceptual study is here presented and discusscd on the possibility to transport 200 passengers over a distance of about 7000km in a nominal point-to-point mission through the Atlantic (either London-New York or London-Rio) at a cruise Mach number of 6 and an altitude abont 30km. The aim of the study is not to design a specific airplane but to explore todays state of the art technology limits to realize such kind of concept, i.e. to identify if such a mission could succeed today. Because of the challenge the mission poses, its is being optimised with the major disciplines involved by means of Multi-Disciplinary Optimisation (MDO) tools as a way to realize an optimum integrated airframe/propulsion aircraft. The environmental impact is being analysed in terms of the resulting sonic boom. No experimental data but CFD results by means of independent assessments has been generated. The study indicates that today the available technology provides with sufficient maturity to accomplish with the mission in areas like aerodynamic and thermal resistance materials but in others like sonic boom mitigation it is required a deeper insight in the physics. Finally while the present investigation clear identify that complex designs involving large amount of variables from different disciplines could be only possible via MDO/MDA strategies, today such processes still suffer on lack of robustness of the involved tools.

Sensitivity Analysis of a Strongly Coupled Aero-Structural System Using Direct and Adjoint Methods

In this paper, we perform the sensitivity analysis of an aeroelastic system in steady-state conditions with respect to design parameters of the jig-shape of the system. The gradient of coupled system characteristic functions are computed analytically using either the discrete direct differentiation method or the discrete adjoint vector method, which make the designer potentially able to use analytical gradients taking fluid-structure coupling into account during the local shape optimization process of a coupled system. The influence coefficients matrix approach and beam theory are applied to calculate the structural displacements due to the aerodynamic loads and the nonlinear Euler equations or Reynoldsaveraged Navier-Stokes equations can be used to evaluate the aerodynamic loads, which are then transfered to the structural model through a consistent and conservative process. The discrete direct and adjoint systems of equations of both disciplines are derived and a lagged fixed-point-like iterative scheme is employed to solve for the coupled set of equations. Finally, the coupled direct and adjoint methods are applied to three different three-dimensional configurations, and the successful implementation of these methods is verified.

ZEHST: environmental challenges for hypersonic passenger transport

The ZEHST hypersonic transport airplane, derived from Astrium Space Plane concept, and having roots in a French-Japanese cooperation, was unveiled by EADS at Paris Air Show 2011. In the frame of a DGAC (French Civil Aviation Authority) sponsoring, EADS Innovation Works together with MBDA, Astrium and Onera launched a feasibility and system study to set up the detailed requirements, assess the technologies envisioned and perform a first design loop, based on shortterm availability of technologies. The 3-propulsion-system concept comprises turbofans for subsonic operations, rocket engines to cross the transonic regime and ramjet engines for efficient high-speed cruise. ZEHST stands for Zero Emission High Speed Technologies, thus emphasizing the will to take into account environmental footprint while designing the vehicle and its trajectory. The aim of the activities on environmental impact within the ZEHST program, for which Onera is responsible, is to set up a methodology highlighting the phenomenology to be taken into account, selecting the appropriate models to compute significant metrics, and conducting trade studies and assessments to provide design guidelines to the conceptual design team. This paper will illustrate this methodology with selected examples among sonic boom, rocket noise and particle emissions assessments.

Two-Dimensional Aerodynamic Optimization Using the Discrete Adjoint Method with or without Parameterization

An optimization method based on the use of the derivatives of functional outputs with respect to (w.r.t.) solid body mesh nodes is presented. These derivatives are obtained by a discrete adjoint method that first computes the derivatives of functional outputs w.r.t. all volume mesh nodes. They are smoothed before being used in a numerical optimization algorithm. The procedure is demonstrated for a 2D flow governed by the compressible Reynolds-Averaged Navier-Stokes equations (RANS) completed by the Spalart-Allmaras turbulence model. Discrete derivatives are computed with or without making the frozen eddy-viscosity assumption. The design algorithm is compared with a more classical one using design variables related to B-splines on the four test cases introduced by Kim et al. 1

Sensitivity analysis of a strongly coupled aero-structural system using the discrete direct and adjoint methods

This paper is dedicated to the sensitivity analysis of a static aeroelastic system with respect to design parameters governing its jig-shape. The gradients of interest are computed using either the discrete direct differentiation or adjoint vector methods. The aerodynamic load is predicted by the nonlinear Euler equations and transferred to the structure through a consistent and conservative procedure. The induced structural displacement field is computed using beam theory and the matrix of the aerodynamic influence coefficients. Finally, a threedimensional wing-body case is used to verify the successful implementation of the analytical sensitivity analysis methods.

Garteur AD/AG-52: Surrogate-Based Global Optimization Methods in Preliminary Aerodynamic Design

Aerodynamic design, with particular reference to its early stages, requires to explore the design space in a global sense in order to locate the optimal candidate. Global optimization methods (e.g., evolutionary algorithms) can meet this requirement as they have the ability to work with noisy objective functions without assumptions on continuity and with a high potential to find the optimum of complex problems. However, they involve a vast number of evaluations even for a small number of design variables. As each evaluation requires a CFD complete analysis, this would make the method unfeasible, in terms of computational cost. Some papers in the literature [1, 2] try to overcome this drawback by introducing surrogate evaluation models to calculate the fitness of the candidate solutions, thus replacing the timedemanding CFD tool. Therefore, a low-cost evaluation approach seems appropriate to handle complex CFD-based design problems with global optimization methods. Recently, a GARTEUR Action Group (www.garteur.org) has been established to explore these approaches. The main objective of the AG [3] is, by means of a European collaborative research, to make a deep evaluation and assessment of surrogate-based global optimization methods for aerodynamic shape design, dealing with the main challenges as the curse of dimensionality, reduction of the design space and error metrics for validation, amongst others. This mini-symposium aims to present the partial results of the AG, as well as other possible external contributions, which will enrich the collaborative network on this topic.