Status and challenges of aeroshape design of future
aircrafts
Philippe Rostand+
Dassault Aviation, Saint Cloud, France
ECCOMAS, September 8th, 2006, The Netherlands
1. Introduction
Aeroshape design of aircrafts is a continuously evolving process; very significant progresses have been made in the last ten years and a high degree of maturity has been reached. Most of the elements of the arodynamics of conventional fighter aircrafts or business jets are modeled and understood to a degree, and so can be the object of reasonably deterministic design processes. Consequently innovative designs have been obtained which could not have been derived earlier, and large efficiency gains in the design process have been observed. Progresses have been obtained combining research efforts of industry, research centers and academia; a summary of recent results for business jet design is presented in [13].
However new challenges have appeared that call for further evolution of processes, on which current research efforts are focused. New challenges come primarily from:
• New environmental requirements, which call for significant changes in business jet design, with community noise reduction a first level objective if ACARE targets are to be met,
• Stealth requirements, to be fully integrated in combat manned and unmanned aircraft design processes. • New competitivity requirements, both on performances, in particular on aircraft speed, with the
Supersonic Business Jet, and on recurring and non-recurring costs,
• Risk reduction requirements, in particular in the much shortened design phases.
2. Status and challenges
ACARE has identified that a 20 dB community noise reduction is to be obtained before 2020; to meet that objective aircraft manufacturers will need to contribute:
• Integration of nacelles large enough to install very high bypass ratio engines and efficient noise absorbers
• Modification of airframes in order to contain jet noise
• Reduction of airframe noise and in particular of noise generated by flaps and slats.
Large changes in aircraft designs are induced by these requirements, as exemplified by the concepts below:
+ Dassault Aviation DGT BP300 78 Quai Marcel Dassault 92214 Saint Cloud France
Falcon concept with semi buried engine Falcon concept with noise shielding afterbody
Aeroshape design for such aircrafts have specific challenges; in particular linked to the assessment of propagation of distributed noise sources in presence of complex geometries, the design of longer and possibly bent ducted inlets, the effect of interaction between engine exhaust and empennage. Research activity is underway in a number of European and National programs to adapt and validate aerodynamic and acoustic design tools; obtaining full scale validation will be one of the targets of the Joint Technical Initiative of the 7th PCRD.
Propagation of sound waves in FALCON S-DUCT.
Stealth requirements have effects on general aircraft shapes, choice of control surfaces, in particular for tail-less aircrafts, and propulsion integration, with the necessity of preventing direct views from both front face and back face of engines. New concepts and new technologies are under development to meet these objectives; in particular passive and active flow control techniques are used to prevent flow separation, most notably in bent ducts, or to force it, most notably for thrust vectoring applications.
Stealth is also the motivation for internal carriage, which is itself a challenge in terms of aerodynamics during weapon separation, and in terms of aeroacoustics:
Validation of unsteady Navier Stokes calculations in a weapon bay: comparaison of experimental (red) and calculated Sound Pressure Levels.
Adaption and dedicated validation of "classical" aerodynamic and aeroacoustic methods and concepts for stealth combat aircrafts is the object of ongoing national (DGA) research activities associating Dassault Aviation and Onera, ranging from LES/DES development and validation for inlets, afterbodies and weapon bay flow predictions, to adaption of adjoint based optimization techniques, to flow control techniques, and to sub scale and full scale flight of test aircrafts.
Reduction of the cost and of the duration of design phases is a major trend; faster and more efficient designs are made possible by:
• More efficient design processes: adjoint based optimization procedures are operational for a number of applications and have brought performance improvements but also probably more importantly reductions in the time necessary to obtain correct designs, with two outcomes: a cost reduction, and the possibility in a given time frame to explore a wider range of solutions, resulting in better optimized choices. Extension of the applicability of shape optimization to more complex flow field, including flows with large viscous effects, up to flow separation, is the current challenge.
Application of aeroshape Navier Stokes optimization to the design of the after body of a Falcon: local isentropic Mach number before and after optimisation
• Validation of new technologies in realistic conditions (i.e. for aerodynamics at flight Reynolds numbers) prior to program utilization, in order to avoid last minute surprises: flight tests have been used in a number of research programs by different organization; Dassault Aviation conducted with DGAC support extensive flight tests to validate buffeting prediction methods which were then applied to the Falcon F7X program.
Alternatively or in complement the ETW can also be used to provide validation data at flight Reynolds number: Cmv20 Cxv Alpha Czv 276. 314.
Forme A ; Calcul NS ; Mach 0.80 ; Re= 16.0M
Voilure Forme A Comparaison ETW / CFD
Mach 0.80 Re= 16.0M ; Pi= 2.0 bar ; Ti= 220 K Re= 16.0M ; Pi= 1.2 bar ; Ti= 146 K
résultats ETW non corrigés
Falcon Model in ETW; comparison of calculated and measured drag polars
Comparison of measured and computed pressure distribution
The objective of cost reduction calls also for improvements on high lift systems, both to reduce the cost of design, which is still partly done through time consuming model adjustment in a wind tunnel, and to reduce recurring costs through the use of mechanically simpler flap and slats. To avoid aerodynamic performance degradation flow control can be used to prevent separation; European and national research programs are underway to developpe flow control based high lift systems; the flight demonstration of such systems will be one of the major objectives of the Joint Technical Initiative scheduled in the 7th PCRD.
AoA=21°
Grid generation is a key point in efficient flow simulation for high lift systems ([4]). Mach=0.8
Simulation of flow reattachment due to leading edge blowing
The requirement of risk reduction is also a major trend and challenge in aeroshape design; flight Reynolds number technological validations certainly provide solid elements in that direction; another requirement is a quantitative strategy for the evaluation of uncertainties.
• In that respect error estimates based on the utilization of the adjoint equations, in particular with respect to finite grid fineness, are a very promising path.
• Validation on the specific point of drag prediction are mandatory; specific efforts have been made by Dassault Aviation and Onera to cross check and validate numerical and experimental predictions of the drag break down of a Falcon business jet. Numerical predictions are obtained on unstructured meshes using Dassault's AETHER Navier Stokes code and Onera's FFD71 drag analysis procedure.
Cxi NS Cxi sondage recalé
calcul
essai
Cxi NS Cxi sondage recalé
calcul
essai calcul
essai
Falcon model in Onera's S2Ma wind tunnel with wake exploration device; comparison of measured and calculated induced drag, see [15]
3. Conclusion
The aeroshape design of future aircrafts, both civil and military, will be very challenging in a number of aspects, some of which have large scientific impact:
• Combined aero acoustic design
• Design of shapes with with unsteady / separated flow fields • Utilisation of flow control
• Generalized use of CFD/CAA based shape optimization, with known simulation uncertainties and prior to program high Reynolds number validations.
4. References
Details of the simulation methods used in Dassault Aviation can be found in the references below.
[1] F. Chalot, T. Fanion, M.Mallet, M.Ravachol and G.Rogé ; "Status and future challenges of CFD in a coupled simulation environment for aircraft design" in Notes on numerical fluid mechanics and multidisciplinary design volume 85, pp277-286, Springer, 2002.
[2] F. Chalot, M. Mallet, "The stabilized finite element method for compressible Navier Stokes simulations: review and application to aircraft design", in Finite Element Methods: 1970's and beyond. 2004.
[4] JP Figeac, "Intégration et utilisation de composants logiciels INRIA pour l'élaboration de maillages chez Dassaul Aviation", Rencontres INRIA Industrie, February 2005.
[5] F. Chalot, et al, “Recent progresses in the use of CFD methods for the design of hypersonic aircrafts”, ECCOMAS, Athens, 1998.
[6] T.J.R. Hughes and M. Mallet, "A New Finite Element Formulation For Computa tional Fluid Dynamics, III The Generalized Streamline Operator for Multidimensional Advective -Diffusive Systems ", Comp. Meth. in Applied Mech. and Eng., North-Holland, (1986).
[7] Y. Saad and M.H. Schultz :"GMRES, A generalized minimum residual algorithm for solving nonsymmetric linear systems" Research report YALEU/DCS/RR-254, Yale University, Department of Computer Science, New Haven, 1983.
[8] B.Aupoix,H.Bézard,S.Catris and T.Daris Towards a Calibration of the Length-Scale Equation In Fluid Dynamics and Aeronautics New Challenges, pp 327-348 CIMNE,Barcelona (Edit.Sept. 2003)
[9] F. Chalot, JM. Hasholder, M. Mallet, A. Naïm, P. Perrier, M. Ravachol, P. Rostand, B. Stoufflet, B.Oskam, R. Hagmeijer, K. de Cock, “Ground to flight transposition of the transonic characteristics of a proposed Crew Rescue / Crew Transfer Vehicle”, AIAA 97-2305, Atlanta, Ga.
[10] PL Georges et H Bourouchaki, "Delaunay Triangulation and meshing", Hermès ed, Paris, 1998. [11] PJ Frey et PL Georges, "Maillages. Applications aux éléments finis", Hermès ed, Paris, 1999. [12] M. Stojanowski, "Business jet type configurations @ ETW", AIAA 2005-460.
[13] P. Rostand, "New generation aerodynamic and multidisciplinary design methods for Falcon business jets. Application to the F7X", AIAA 2005-5083, Toronto, 2005.
[14] L. Daumas & al, " Aerodynamic design process of a supersonic business jet", AIAA 2006-xxxx, San Francisco, 2006.
[15] M. Meheut and D.Bailly, "Profile drag formulation and drag breakdown from wake measurements", AIAA 2006-3166, San Francisco, June 2006.
