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Aero Days 2011, Madrid .

FUTUREFlutter-Free Turbomachinery Blades

Torsten Fransson, KTH

Damian Vogt, KTH

2011-03-31

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RR Trent 1000

A Typical Turbomachine

Picture courtesy of RR

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What is it “flutter”?

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Blades oscillate in traveling wave modeNeighbor blades usually lead to instability

An isolated blade would not flutter

Turbomachinery Flutter

• Flutter denotes a self-excited and self-sustainedaeroelastic instabilityVery harmful unless properly damped

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Why do turbomachinery blades flutter?

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Underlying Mechanisms

• Flutter involves the interaction of fluid and structureUpon the motion of a component, the surrounding fluid will

respond with an aerodynamic forceThe direction and phase of this force will lead to having the

motion damped or augmented

In case of augmentation, flutter will establish

• The character of the fluid response depends on many factors such asGeometrical aspects (i.e. profile shape, blade size, blade count)Operating point (idle, take-off, cruise)Ambient conditions (air temperature, etc)Dynamics (engine acceleration, deceleration)

Flutter might establish only at very few of the above conditions. Due to its harmful character it must however be avoided at any cost

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How can we ensure “flutter-free

turbomachinery blades”?

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Flutter-Free Turbomachinery Blades

• A good design does not flutter

• How to ensure a ”good design”?Design for stability performing accurate predictions of the

unsteady behavior of the structural dynamics (FEM) and aerodynamics (CFD) in a turbomachine

Ensure large-enough stability limits (i.e. moderate changes in operating conditions, profile shape, etc will not directly lead to a flutter instability)

• A ”good design” must also be economically viableEngine development costs and timeFulfilling other objectives such as performance, weight,

manufacturing cost, maintainability etc

During component design, industry nowadays largely relies on numerical simulations at affordable analysis costs (model size and run time)

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How well are we to date doing on

aeroelastic predictions?

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Prediction Accuracy

• Test case: transonic compressorEach industry partner is using their own (trusted) aeroelastic

analysis tool to analyze the aeroelastic behaviorVariation of minimum aerodynamic damping with operating

point

π

mass flow

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Background

• Despite the high level of sophistication in today’s numerical prediction tools, it is not uncommon that we have to deal with an accuracy of +-40% of predicted minimum aerodynamic dampingIn the present test case: 2 out of 5 predict flutter, 3 do not

• Test cases exist but these do not fully cover the spectrum needed for modern turbomachine designsComponent types (blisks, bladed disks)Flow conditions (transonic flow, high loading, separations)Combinations of unsteady pressure and vibration data

This ”empty spot” shall be filled-in by the FUTURE projectEstablishing of new experimental test casesExtensive validation of state-of-the-art prediction tools

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Flutter-Free Turbomachinery Blades

www.future-project.eu

Presentation of FUTURE Project

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EU FP7 Project FUTURE

• Project aiming at the acquiring new sets of relevant validation data on turbomachinery aeroelasticity (compressor, turbine) and validating numerical tools

• Project coordinator: KTH, Prof Torsten Fransson

• Partners: 25 partners from industry, research institutes, academia

• Budget: 10.6M€

• Duration: July 2008 – June 2012

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FUTURE Project Partners

Industry Research Institutes

Academia

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Project Concept

Aeroelastic experimentsAeroelastic computationsSynthesis of experiments

and computations

x xx xx x

Fan Compressor

Turbine

Picture courtesy of RR

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Project Structure

• Two main streaks of validation test cases as followsTransonic compressorHigh subsonic Low-Pressure Turbine (LPT)

These test cases have been conceived within FUTURE

• Interconnected experimentsNon-rotating cascade tests, controlled blade oscillationRotating tests, multi-blade row, free and forced oscillationMechanical characterizations of components (blisk, bladed disks)Application of novel measurement techniques such as PSP

• Interconnected computationsPerformed by virtually all partners in the projectPre-test predictionsPost-test predictions

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Work Package Structure

• WP1: Turbine and compressor cascade flutterPaolo Calza, Avio

• WP2: LPT Rotating rig flutterRoque Corral, ITP

• WP3: Multi-row compressor flutterJan Östlund, Volvo Aero

• WP4: Synthesis of experiments and computationsDetlef Korte, MTU

• WP5: Project managementDamian Vogt, KTH

Shortcut to ”Benefits”

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Presentation of FUTURE Test Cases

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Transonic Compressor

• Design intentAeroelastic stable operation at design pointN 18’000rpm, φ ~ 0.6Reduction of positive aerodynamic damping as stall line is

approached

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Compressor Flow Field

ADP, Π 1.412

50% span

90% span

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Compressor - Overview of Tests

• Non-rotating tests (isolated blade row, EPFL)Detailed steady aerodynamicsAerodynamic damping (controlled oscillation, free oscillation)Data: inlet/outlet flow parameters, blade loading, time-resolved

blade surface pressure

• Rotating tests (1 ½ stage compressor, TUD)Detailed steady aerodynamics (blade loading, probe traverses)Mechanical characterization of rotor blisk (ECL)Damping measurements at various operating pointsData: inlet/outlet flow parameters, blade loading, time-resolved

blade surface pressure, blade vibration (tip-timing)

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Non-Rotating Compressor Test Facility (EPFL)

Annular cascade module

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Rotating Compressor Test Facility (TUD)

Rotor blisk

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High Subsonic LPT Rotor

• Design intentControlled aeroelastic instability at design point Limit Cycle

Oscillations (LCO)N 2’416rpm, M2 ~ 0.75Goal: measurable LCO amplitudes

displacement

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LPT Rotor Flow Field

Mach number50% span

Outlet ptot

SS PS

Surface oil flow

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LPT - Overview of Tests

• Non-rotating tests (isolated blade row sector, KTH)Detailed steady aerodynamicsAerodynamic damping (controlled oscillation influence

coefficients)Data: inlet/outlet flow parameters, blade loading, time-resolved

blade surface pressure

• Rotating tests (1 stage LPT, CTA)Detailed steady aerodynamics (probe traverses)Two test objects: 1) cantilever 2) interlockMechanical characterization of rotor bladed disks (AVIO)Damping measurements at various operating pointsData: inlet/outlet flow parameters, blade vibration (tip-timing)

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Non-Rotating LPT Test Facility (KTH)

Annular sector cascade module

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Cascade Flow Field

• Annular sector cascade5 blades, 6 passages70% span loading of rotating rig matched

Outlet Mach number distribution

Fig with midspan loading

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Rotating LPT Test Facility (CTA)

Assembled rotor bladesInterlock

configuration

Cantilever configuration

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What are the expected benefits of the

FUTURE project?

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Expected Benefits

• The FUTURE project shall contribute to making turbomachinery aeroelastic predictions more reliable

Numerical tools validated on new, relevant and uniqueaeroelastic test cases that shall lead to best practice guidelines

• Achieving this will …… help making turbomachinery blades flutter-free… make new aircraft engines more efficient… cut development costs and time frames

The FUTURE project will provide key enabling technologies towards a green, safe, reliable and affordable air transport of the future

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Dissemination

• Great attention is given to the dissemination of project findingsFeeding-back findings to education and life-long learning

• ExamplesSharing of audiovisual instruction material from industry

partners with universities

Development of e-learning tools

THRUST – TurbomacHinery AeRomechanical UniverSity TrainingThe world’s first Masters programme in turbomachinery

aeromechanics

• UpcomingTHRUST+ Joint PhD programme on aeromechanicsEXPLORE Aero World Virtual University

www.explorethrust.eu

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What do we envision after FUTURE?

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• Within the FUTURE project many questions will be answered but there might be unresolved topics at the end

• Having a strong project consortium and unique hardware in place, we envision research in the following directions

Control of flutter (active, mistuning, novel damping concepts)

Influence of flow distortion and impedance

Flutter in the presence of other unsteady aerodynamic phenomena

Development of new improved numerical models

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