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Aero-elastic simulations of Counter- Rotating Open-Rotors (CROR) 10 èmes Journées Des Doctorants MFE/ Nord de lOnera Benjamin FRANCOIS 2 nd year PhD Student (Airbus/Cerfacs) Funding : CIFRE Airbus Ph.D Supervisors: Michel COSTES (ONERA- DAAP/H2T) Guillaume DUFOUR (ISAE) Airbus Advisor : Florian BLANC (EGAMT2)

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2 Introduction New engine concepts for civil aircrafts are driven by the fuel burn reduction motivated by : Prediction of rise of oil prices due to the growth of air traffic Strong environmental concern (reduction of CO 2, NO x ) Counter Rotating Open Rotors (CROR) appears to be one suitable option... CROR concept Example of aircraft powered by open rotors The unducted nature of the open-rotors lead to strong aerodynamic interactions with the aircraft and to coupled complex phenomena

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3 Introduction Some challenges adressed by this Ph.D : 1P-forces Transverse forces due to non-homogeneous inflow (incidence, installation effect …) Essential for HTP and VTP sizing Whirl flutter phenomenon Unsteady phenomenon coupling pitching and yawing motion of the engine system (nacelle + rotors) 1P-forces contributes to whirl flutter Two major accidents referenced (1959,1960) on Lockheed Electra L188 Whirl flutter on aircraft 1P-forces α Incidence effect on a aircraft powered by Open-Rotors V inflow

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4 Scientific issue An aircraft powered by CROR contains a large spectrum of frequencies : High frequency (HF) sources : Front and rear interactions (wakes, potentials effects) driven by BPF ~ 250 Hz – 400 Hz (cruise – Take off) + harmonics Low Frequency (LF) sources : Interactions between the wake of the pylon and the two rotors ~ 13 Hz – 20 Hz Incidence effect due to rotation speed of CROR ~ 13 – 20 Hz Aircraft modes ~ 10 Hz Whirl flutter of the open rotors ~ 1-10 Hz Single timestep integration technique becomes too expensive as soon as the spectrum contains wide-separated frequencies The goal of this Ph.D is to develop moderate-cost numerical methods and tools able to capture phenomena with wide-separated frequency

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5 Consolidate experiences and best practices for unsteady simulations of isolated CROR with 360° configurations => Reference computations to be compared with moderate-cost techniques Investigation and development of alternative techniques to compute at moderate-cost unsteady phenomena with wide-separated frequency Contribution to improve the physical analysis of whirl flutter and its prediction tools Extend these methods and tools to installed configurations on aircraft Scientific objectives of the Ph.D

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6 Outline 1.Introduction/Context 2.Unsteady simulations with 360° configurations of CROR 1.Mesh strategy 2.Aerodynamic interactions 3.Incidence effect 4.Whirl flutter 3.Moderate-cost techniques for CROR computations 1.Bibliography 2.Spectral Method for CROR

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7 Isolated test-case : AI-PX7 configuration The isolated configuration AI-PX7 was selected as a test-case to assess numerical methods and tools in the frame work of this Ph.D AI-PX7 is the Airbus generic design used on European Research Platform Cleansky JTI-SFWA used to : Develop and validate dedicated methods and tools for the aerodynamic simulations of CROR Enhance the understanding of the complex aerodynamic flows of CROR Some features about the geometry: 11x9 bladed pushed configuration Front rotor diameter = 14 ft (4.2672m) Aft rotor blades are cropped by 10% Several flight conditions were simulated on this isolated CROR configuration with an URANS approach (flight point ; unsteadiness to capture) 1. Cruise conditions (M=0.73, alt=35 kft), no incidence; aerodynamic interactions 2. Cruise conditions (M=0.73, alt=35 kft), incidence ; aerodynamic interactions + incidence effect 3. Cruise conditions (M=0.73, alt=35 kft), pitching motion; aerodynamic interactions + incidence effect + pitching motion effect Airbus generic geometry (AI-PX7)

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8 Mesh strategy 360° configurations were chosen because suitable to model : - incidence effect - installation effect - whirl flutter motion Mesh Topology : Structured multi-blocks mesh Full 360° mesh (53M pts) Mesh is split into three domains : Background fixed mesh : Far-field and nacelle area Cylindrical shaped rotating meshes : Front rotor & Rear rotor This decomposition offers two advantages : Mesh strategy suitable for the modelling of installation effect No propagation of refined cell of rotor meshes into the far-field area Chimera/Sliding mesh interfaces btw domains Mesh domain for isolated CROR 13 R blade 8 L CROR

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9 Aerodynamic interactions around an open-rotors Flow-fields exhibits strong aerodynamic periodic phenomena called aerodynamic interactions : Pressure waves Propagation of acoustic waves upstream and downstream Wakes Deficit of velocity propagating downstream Ω cur/opp rotation speed of current/opposite stage N opp number of blades in the opposite stage BPF = (Ω opp – Ω cur ).N opp Front Rotor Aft Rotor Ω Ω BPF FR = HzBPF AR = Hz These phenomena are well-known in the literature and their frequencies Blade Passing Frequency (BPF) are deterministic : References : J.M. Tyler and T.G. Sofrin, "Axial Flow Compressor Noise Studies", Society of Automotive Engineers Transactions, vol. 70, p , 1962 Aerodynamic interactions are high frequencies sources (BPF FR/AR, 2BPF FR/AR,3BPF FR/AR … ) What are the impact on these interactions on aerodynamic loads?

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10 Cruise conditions (M=0.73, alt=35 kft), incidence : Computation of aerodynamic loads In case of non-homogeneous inflow (incidence for instance), 360° pressure field is necessary to compute 1P-forces Pressure field on rotating blade shows unsteady variations with wide-separated frequencies Incidence effect (Low-frequency) Aerodynamic interactions (High frequency) 1P-forces Unsteady pressure coefficient field on the rear blade intrados V inflow

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11 Cruise conditions (M=0.73, alt=35 kft), incidence : Do we need to capture aerodynamic interactions ? 8% 2% 3% 0.5% Y FRONT ROTOR C y +C z Z CyCy CzCz Several integration timesteps (0.5°/it, 1°/it, 2°/it, 4°/it) were tested for these computations for the prediction of 1P-forces Previous studies in this Ph.D (not shown here) have proved that maximum timestep of 0.5°/it is necessary to capture 1 st harmonic of aerodynamic interactions Analysis of 1P-forces shows that accurate prediction of lateral force requires at least 1°/iteration 4°/it 2°/it 1°/it 0.5°/it V inflow

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12 V 0 cos V 0 sin FRONT BLADE Thrust (N) Cruise conditions (M=0.73, alt=35 kft), incidence : Impact on the blade thrust level 60% r V 0 sin sin loc V 0 cos V r V 0 sin sin loc V 0 cos V Incidence effect leads to strong variation of thrust over one rotation These results have been compared to NLR results (different mesh and solver) and show good agreement

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13 Cruise conditions (M=0.73, alt=35 kft), pitching motion : Features Computation features Integration timestep : 0.5° of rotation per timestep 4 pitching cycles,10.6 rotations, 7632 iterations CPU time : 20 days using 128 cores on HPC3 Forced pitching motion Kinematics Rotation around Y-axis Center of rotation : (-2.15,0,0) => this point has been chosen to model the effect of a CROR cantilevered in the pylon (pusher configuration) Magnitude : Frequency : Equation Modelling Rigid motion No structure model -> aerodynamic forces Z X +1° -1° V inf Center of rotation

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14 Cruise conditions (M=0.73, alt=35 kft), pitching motion : Impact on blade thrust Unsteady phenomena involved : Rotor-rotor interactions (BPF = 231.5Hz, 291.5Hz, …) Incidence effect (f rot = 13.25Hz ) Pitching effect (f = 5Hz) There is no periodicity because pitching mode is not linked to the rotation speed !

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15 Outline 1.Introduction/Context 2.Unsteady simulations with 360° configurations of CROR 1.Mesh strategy 2.Aerodynamic interactions 3.Incidence effect 4.Whirl flutter 3.Moderate-cost techniques for CROR computations 1.Bibliography 2.Spectral Method for CROR

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16 References : 1.C.Farhat, M. Chandesris, Time-decomposed parallel time-integrators : theory and feasibility studies for fluid, structure, and fluid-structure applications, International Journal for numerical methods in engineering, J-L LIONS and al., Résolution dEDP par un schéma en temps pararéel, Comptes rendus de l'Académie des sciences. Série 1, Mathématique, C.K.W. Tam and K.A. Kurbatskii, Multi-Size-Mesh Multi-Time-Step Dispersion-Relation-Preserving Scheme for Multiple-Scales Aeroacoustics Problems, International Journal of Computational Fluid Dynamics 4. T. Le Garrec, X. Gloerfelt, C. Corre, Multi-Size-Mesh, Multi-Time-Step Algorithm for Noise Computation Around an Airfoil in Curvilinear Meshes, 13th AIAA/CEAS Aeroacoustics Conference (28th AIAA Aeroacoustics Conference) 5. Zhi Yang and Dimitri Mavriplis, Time Spectral Method for Periodic and Quasi-Periodic Unsteady Computations on Unstructured Meshes AIAA- 40th Fluid Dynamics Conference and Exhibit 6. K.Ekici and K.C.Hall : Non-linear Analysis of Unsteady Flows in Multtistage Turbomachines Using the Harmonique Blance Technique. AIAA Journal, 45(5) : , AIAA Paer YesNo Multi-timescale approaches Time–parallel approach 1,2 Multi-size mesh Multi-timestep 3,4 Coupling methods Coupling of two calculations solving its own timescale URANS URANS/Harmonic technique Hybrid spectral approach coupled with URANS 5 Spectral approach for Low Frequency Timescales are spatially separated ? Spectral methods (HBT) 6

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17 Time-Spectral Method (TSM) Spectral Approach Principle : Only one blade passage is meshed and computed Periodic conditions are applied on the boundaries of the domain CROR mesh suitable for TSM TSM solve directly the periodic flow without solving useless unsteady transient part CPU cost expected to be lower than URANS TSM (mono-frequency) can only applied to isolated CROR without incidence References : A Time-Domain Harmonic Balance Method for Rotor/Stator Interactions Frédéric Sicot, Guillaume Dufour and Nicolas Gourdain J. Turbomach. -- January Volume 134, Issue 1, (13 pages) doi: /

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18 Extension to multi-frequencies techniques : Harmonic Balance Technique Harmonic Balance Technique is based on the same principle as TSM but enables to solve periodic flows with multiples frequencies Extension to multi-frequencies methods (HBT) for simulations with incidence and whirl flutter Mesh for HBT One channel per rotor 360° Far-field mesh : incidence effect, installation effect Development of duplicating sector sliding interfaces to allow communication between 360° sliding interface and sector sliding interface HBT simulations without incidence, with incidence … sector sliding interface 360° sliding interface Mesh suitable for HBT References : "Non evenly spaced timelevels for multifrequential harmonic balance computations" T. Guédeney, A. Gomar, F. Sicot, G. Dufour, will be submitted in 2012 for AIAA Journal "Evaluation of two numerical approaches for the simulation of multi-frequential turbomachinery flows, T. Guédeney, N. Gourdain, L. Castillon, will be submitted in 2012 for AIAA Journal or Journal of Turbomachinery

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19 Conclusions Achievements 360° CROR simulations were performed and analysed for : Aerodynamic performances 1P-loads prediction Whirl flutter forced motion Deep unsteady analysis of wide-separated phenomena around CROR Development/Validation of moving grid techniques (chimera/sliding) for CROR=> Proceeding at ISABE 2011 Implementation of whirl flutter motion in elsA code Development and validation of TSM capabilities for CROR applications Future work TSM simulations currently running and to be compared with 360° config with URANS (no incidence) Extension to multi-frequencies applications with Harmonic Balance Technique

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20 Publications and training Proceedings François, Dufour and Costes, Comparison of Chimera and Sliding Mesh Techniques for Unsteady Simulations of Counter Rotating Open-Rotors, ISABE proceeding, Göteborg (Sweden), sept 2011 Journal On-going 2012 : B.Francois, M.Costes and G.Dufour,1P-loads prediction at cruise conditions on Counter-Rotating Open Rotors, AIAA Journal Training « Aérodynamique des hélices », Jean-Marc Bousquet, SUPAERO, 3 ème year course, 2010 « Physique Général de lavion », Airbus Training, 2011

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21 © AIRBUS Operations S.A.S. All rights reserved. Confidential and proprietary document. This document and all information contained herein is the sole property of AIRBUS Operations S.A.S. No intellectual property rights are granted by the delivery of this document or the disclosure of its content. This document shall not be reproduced or disclosed to a third party without the express written consent of AIRBUS Operations S.A.S. This document and its content shall not be used for any purpose other than that for which it is supplied. The statements made herein do not constitute an offer. They are based on the mentioned assumptions and are expressed in good faith. Where the supporting grounds for these statements are not shown, AIRBUS Operations S.A.S. will be pleased to explain the basis thereof. AIRBUS, its logo, A300, A310, A318, A319, A320, A321, A330, A340, A350, A380, A400M are registered trademarks. Thank you for your attention

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