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Aeroelasticity of Turbomachines
Damian Vogt, formerly KTH (now University of Stuttgart)
Torsten Fransson, KTH
2013-09-18
MUSAF II Colloquium, Toulouse, France
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(Giles, 1991)
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Content of Today‘s Talk
• „Crash course” in turbomachinery flutter
• How well are we presently doing in predicting flutter?
• How can we validate our flutter prediction tools?
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Motivation
• The accurate and reliable prediction of flutter is of paramount interest to turbomachinery manufacturers- Increased accuracy and reliability of predictions allow
pushing limits further such as to make turbomachines„greener“
• There is a clear context between the greening potential of new turbomachine generations and aeroelastic phenomena- More efficient engines come along with less components,
more aggressive operating conditions and consequently greater aerodynamic loading
- In case of stationary turbomachines, greening is much related to operational flexibility
- Both aspects can lead to aeroelastic phenomena
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A ”Crash Course” in Turbomachinery Flutter
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General Aspects of Flutter
• Flutter is a self-excited and self-sustained vibration phenomenon
- The vibration starts by itself if the fluid-structure coupled system becomes unstable
- Unless we provide proper damping, vibration amplitudes rapidly escalate
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Vibration scenarios
• Positively damped- Preferred
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No flutter
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Vibration scenarios
• Self-excited (negatively damped)- Must be avoided
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Flutter
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Vibration scenarios
• Self-excited, attaining Limit Cycle Oscillations (LCO)- Can be tolerated
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Flutter
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What Happens if Blades Vibrate?
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What Happens if Blades Vibrate?
Structural part Aerodynamic part
x: deformation coordinate modal coordinate
F(t) : aerodynamic force
Structure and fluid )(tFkxxcxm
motion
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What about F(t)?
• F(t) is due to flow and the motion of the blade
• F(t) has an arbitrary direction- But it is only the component in direction of the mode that
matters
• F(t) is most probably not in phase with the motion
motion
F(t)
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Aerodynamic Force F(t)
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• It is the out-of-phase component that gives us the aerodynamic damping
- Of importance: the mode, the magnitude and the phase of the unsteady aerodynamics
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Blade Row Aeroelasticity
flow
motionFigure shows pressure perturbation
Single blade oscillating
Aerodynamic influence
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Blade Row Aeroelasticity
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Traveling Wave Mode Stability
• In a blade row, in which all blades oscillate, the influence of the individual blades is superimposed
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2
2
,,
,, ),(ˆ),(ˆ
Nn
Nn
nimnicAp
mtwmAp eyxcyxc Linear superposition
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Aeroelastic Stability Curve
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=180deg
ND 12 FT
=15deg
ND 1 FT
=-90deg
ND 6 BT
ND nodal diameter
FT forward traveling
BT backwards traveling
Least stable mode
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Aerolasticity „Crash Course“ Essentials
• Predicting flutter is about …- Predicting the aerodynamic damping for different
vibration modes (mode direction, nodal diameter pattern)
- Putting the aerodynamic damping against the structural damping at these modes
- Evaluating the resulting type of vibration
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How well are we presently doing in predicting flutter?
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Blind-Test Prediction Accuracy
• The results shown below have been obtained in the EU FP7 collaborative Research Project FUTURE (Flutter-free Turbomachinery Blades)
• They have been obtained as blind-test predictions- In other words, test data were first available at a much
later stage
• Test case- Transonic compressor, blisk type, 1 ½ stage- Well-defined geometry and boundary conditions have
been provided to several partners
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Flutter Prediction Accuracy Test Case
• Predict the minimum aerodynamic damping of the transonic compressor test case at various operating points on a speedline
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mass flow
Min damping
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Comparison of Predicted Aero Damping
Ref: FUTURE Project
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Analysis Aero Damping Predictions
0.25%
0.8%
-0.3%
The variance is of the order ofmagnitude of the predicted damping
Ref: FUTURE Project
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How can we validate our flutter prediction tools?
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Validation of Flutter Prediction Tools
• The validation must be performed on relevant test cases, on which high quality experimental data are available
• Ideally, the validation is carried out at various levels of complexity- Rotating rig tests: near-engine environment, total
damping measurements, expensive and complex
- Stationary cascade test: well-controlled conditions but real conditions are only assimilated, aero damping measurements, detailed, simpler and more affordable
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The FUTURE Project
• The FUTURE project was a collaborative research project that has been carried out within the EU FP7 framework during 2008-2013- 25 partners from industry, academia and research
institutes- Lead by KTH (Prof. T. Fransson and Doc. D. Vogt)- Total budget 10.6M€
• The goal of the project was to set-up new and relevant test cases in turbomachinery aeroelasticityand to validate state-of-the-art flutter prediction tools against these- Derivation of best practice guidelines for flutter analyses
and testing
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FUTURE Project Consortium
Industry Research Institutes
Academia
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Lead (Fransson, Vogt)
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FUTURE Project Concept
• Two main streaks of new validation test cases- Transonic compressor- High subsonic aero Low-Pressure Turbine (LPT)
• Interconnected experiments- Non-rotating cascade tests, controlled blade oscillation- Rotating tests, multi-blade row, free and forced
oscillation- Mechanical characterizations of components (blisk,
bladed disks)
• Numerical analyses performed by several partners
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FUTURE Transonic Compressor Tests
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Steady aero (probes, rakes)Blade tip timingBlade-mounted KulitesWall-mounted KulitesBlade excitation system
Transonic compressor rig at TUD Darmstadt, Germany
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FUTURE Compressor Excitation System
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Air jet exciter ringRotating excitation pattern (ND)
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FUTURE Results and Findings
• Results and findings from the FUTURE project have among others been (and will be) published at the following events:
- ISUAAAT 2009, 2012- IFASD 2011- Aero Days 2011- ETC 2011, 2013- AIAA Symposium 2013- ASME TE 2014
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Summary
• A brief introduction into aeroelasticity of turbomachineswith the focus on flutter has been given- In order to predict flutter, the aerodynamic and the
structural damping needs to be assessed for a variety of modes
• By means of a blind test case, it has been demonstrated that the accuracy of predicting aerodynamic damping in turbomachines is not within single digits %- Validation of flutter prediction codes on new and relevant
test cases is therefore of great importance- Acquiring relevant test data is extremely challenging
• Having accurate and reliable flutter prediction tools opens up for exploring the greening potential of next generations of turbomachines
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Thank you for your attention
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