Aero-elastic simulations of Counter-Rotating Open-Rotors (CROR)

10èmes Journées Des Doctorants MFE/ Nord de l’Onera

Benjamin FRANCOIS

2nd 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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IntroductionNew 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 CO2, NOx)

Counter Rotating Open Rotors (CROR) appears to be one suitable option ...

CROR concept

Example of aircraft powered by open rotorshttp://www.flightglobal.com

The unducted nature of the open-rotors lead to strong aerodynamic interactions with the aircraft and to coupled complex phenomena

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IntroductionSome 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

Vinflow

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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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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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Outline

1. Introduction/Context

2. Unsteady simulations with 360° configurations of CROR1. Mesh strategy2. Aerodynamic interactions3. Incidence effect4. Whirl flutter

3. Moderate-cost techniques for CROR computations1. Bibliography2. Spectral Method for CROR

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Isolated test-case : AI-PX7 configurationThe 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 effect3.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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Mesh strategy360° 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

rotorThis 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 Rblade

8 LCROR

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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

Ωcur/opp rotation speed of current/opposite stageNopp number of blades in the opposite stage

BPF = (Ωopp – Ωcur).Nopp Front Rotor Aft Rotor

Ω

ΩBPFFR = 231.5 Hz BPFAR = 291.5 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. 309-332, 1962

Aerodynamic interactions are high frequencies sources (BPFFR/AR , 2BPFFR/AR ,3BPFFR/AR … )

What are the impact on these interactions on aerodynamic loads?

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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

Vinflow

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Cruise conditions (M=0.73, alt=35 kft), incidence : Do we need to capture aerodynamic interactions ?

8% 2%

3%

0.5%

Y

FRONT ROTOR

Cy+Cz

Z

Cy

Cz

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 1st harmonic of aerodynamic interactions

Analysis of 1P-forces shows that accurate prediction of lateral force requires at least 1°/iteration

4°/it2°/it1°/it0.5°/it

Vinflow

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V0cos

V0sin

FRONT BLADE

Th

rust

(N

)

Cruise conditions (M=0.73, alt=35 kft), incidence : Impact on the blade thrust level

60%

r

V0sin sin

loc

V0cos

V

r

V0sin sin

loc

V0cos

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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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°

(t) msin(2ft)

1mHzf 5

Vinf

)(tCenter of rotation

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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 (frot = 13.25Hz ) Pitching effect (f = 5Hz)

There is no periodicity because pitching mode is not linked to the rotation speed !

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Outline

1. Introduction/Context

2. Unsteady simulations with 360° configurations of CROR1. Mesh strategy2. Aerodynamic interactions3. Incidence effect4. Whirl flutter

3. Moderate-cost techniques for CROR computations1. Bibliography2. Spectral Method for CROR

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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, 20032. J-L LIONS and al. , Résolution d’EDP par un schéma en temps pararéel, Comptes rendus de l'Académie des sciences. Série 1, Mathématique, 20013. 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 Dynamics4. 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 Meshes2010 - AIAA- 40th Fluid Dynamics Conference and Exhibit6. K.Ekici and K.C.Hall : Non-linear Analysis of Unsteady Flows in Multtistage Turbomachines Using the Harmonique Blance Technique. AIAA Journal, 45(5) : 1047-1057, 2007. AIAA Paer 2006-422

YesNo

Multi-timescale approaches

Multi-timescale approaches

Time–parallel approach1,2

Multi-size meshMulti-timestep3,4

Coupling methods

Coupling of two calculations solving its own timescale

Hybrid spectral approach coupled

with URANS5

Spectral approach

for Low Frequency

Timescales are spatially separated ?

Spectral methods(HBT)6

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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 2012 -- Volume 134, Issue 1, 011001 (13 pages) doi:10.1115/1.4003210

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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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ConclusionsAchievements 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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Publications and training ProceedingsFrançois, Dufour and Costes, Comparison of Chimera and Sliding Mesh Techniques for

Unsteady Simulations of Counter Rotating Open-Rotors, ISABE proceeding, Göteborg (Sweden), sept 2011

JournalOn-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, 3ème

year course, 2010 « Physique Général de l’avion », Airbus Training, 2011

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