The purpose of this report is to provide a description of the overall thrust, past relevant accomplishments and future work for research that was performed on the titled project. The purpose of this work was to perform applied research at the NASA Lewis Research Center in the area of the aeroelastic behavior of turbomachinery used in air-breathing propulsion. This activity was undertaken in close cooperation with the Structural Dynamics Branch of the Structures Division at NASA Lewis

Keith, Theo G., Jr.

Murthy, Durbha V.

English

NASA Technical Reports Server

NASA-CR-202643
,/.,! C >
Final Technical Report
for
Aeroelastic Analysis of Turbomachinery for
Space Propulsion
NASA Grant Number
NAG3-1449
Grant Duration
January 13, 1993 to February 23, 1996
Theo G. Keith, Jr.
Distinguished University Professor
&
Durbha V. Murthy
Senior Research Associate
Department of Mechanical, Industrial and Manufacturing Engineering
University of Toledo
Toledo, Ohio 43606
November 1996
https://ntrs.nasa.gov/search.jsp?R=19970007016 2020-06-16T02:50:30+00:00Z
Aeroelastic Analysis of Turbomachinery for Space Propulsion
SUMMARY
The purpose of this report is to provide a description of the overall thrust, past relevant
accomplishments and future work for research that was performed on the titled project.
The purpose of this work was to perform applied research at the NASA Lewis Research
Center in the area of the aeroelastic behavior of turbomachinery used in air-breathing
propulsion. This activity was undertaken in close cooperation with the Structural
Dynamics Branch of the Structures Division at NASA Lewis.
INTRODUCTION
Blade vibration phenomena such as flutter and forced response are important safety
and reliability considerations in the design of turbines, fans and compressors. Blade
fatigue induced by forced response of fan, compressor and turbine blades has caused
serious engine failures. Meanwhile, reduced weight/size requirements and improved
cycle efflciencies are leading to designs with higher risk associated with flutter and
forced response.
The activities to be described were aimed at developing analytical methods to
understand the above phenomena, efficient computational models to accurately and
efficiently predict them, validation of these models using experimental and test data
and dissemination of the developed methods and models to the industry.
PERSONNEL
This project was directed by Dr. Theo G. Keith, the Principal Investigator. The bulk of
the research work was carried out by Dr. Durbha V. Murthy, a Senior Research
Associate in the Mechanical, Industrial and Manufacturing Engineering Department at
the University of Toledo. Close collaborators included George Stefko, and Tony Kurkov
of NASA Lewis Research Center, and industrial collaborators from Allied-Signal, Pratt
& Whitney and General Electric Aircraft Engines.
THE FREPS SYSTEM
The focus for much of the work carried out is the Forced Response Prediction System (FREPS)
currently under development at NASA Lewis Research Center. This program is motivated by the
paramount importance of accurately predicting theaeroelastic behavior of new generation air-
breathing propulsion turbomachinery systems. The FREPS program is aimed at providing this
capability in an analytical/computational tool that does not need excessive computational resources
and is thus suitable for design environment. The objective of the FREPS system is to automatically
and efficiently integrate blade structural dynamic, steady and unsteady aerodynamic analyses to
calculate the forced vibration response amplitudes in turbomachinery blades to a variety of
excitations. FREPS system can reduce the subjective element in the blade design process and
improve the development cycle time for turbomachinery.
The basic objective in developing the FREPS system is to provide a state of the art
quantitative forced response prediction tool that enables turbomachinery designers to improve the
fatigue characteristics of the blades in a systematic manner. We expect that FREPS will lead to less
stringent safety factors and margins and shorter development and testing periods and turbomachinery
with higher performance. The distinctive characteristics of FREPS are: 1) it integrates advanced
numerical procedures from the two disciplines of structures and aerodynamics and 2) it is
computationally efficient. The actual flow path geometry and material properties can be used because
of the availability of advanced structural and aerodynamic models which account for the effects of
thickness, camber and incidence. The calculation of the effects of the distortion of the exciting gust
through the blade passage is an important capability. In addition, the computational efficiency of the
FREPS system allows the examination of the forced vibratory characteristics for various design
choices and the conduct of parametric studies that can be used to improve blade designs.
To advance the stated goal of quick technology transfer to industry, a workshop was
conducted at NASA Lewis on Forced Response in Turbomachinery on August 11, 1993. The
principal investigator, Durbha V. Murthy, played a major role in organizing and conducting the
workshop. It provided a forum to transfer technology to industry and to obtain industry and
university input for future work. About 35 people attended representing 6 rocket and aircraft engine
industries and 6 universities. Eight presentations were made describing the results of NASA funded
research in the areas of aeroelastic response prediction, steady and unsteady fluid dynamics,
mistuning, and associated experiments. Two of the presentations were made by the principal
investigator. This work will lead to increased engine reliability and lower engine fuel consumption.
The attendees participated in lively question and answer sessions at the end of each presentation. An
open discussion session at the end of the workshop was devoted to the evaluation of the work by
industry participants and suggestions for further work that would be of large benefit to engine
industry. The workshop ended with a demonstration of the Forced Response Prediction System
(FREPS) code, developed partially under this grant. In addition to fulfilling the technology transfer
objectives of NASA, this workshop provided an opportunity for industry representatives to make
recommendations for future NASA research work in this important technology area.
To demonstrate the capabilities and computational efficiency of the FREPS system, the effect
of blade thickness on the aeroelastic forced response ofturbomachinery rotors was investigated. This
study was motivated by a question that was recently posed on whether a thicker blade necessarily has
a reduced vibratory response. Test rotors with different blade thicknesses were analyzed. It was
demonstrated that, for some nodal diameter modes, aerodynamic damping reduced with increased
blade thickness and could possibly lead to higher dynamic stress amplitudes in thicker blades, counter
to intuition. For the cases examined where thicker blades experienced higher stresses, an analysis of
the aerodynamic damping and excitation forces showed that the higher stresses were a result of higher
aerodynamic excitation and that the effect of reduced aerodynamic damping is small. Results of this
investigation were documented in a paper (Murthy, Hoyniak and Morel, 1994).
A definite consensus has emerged at the Workshop that FREPS will be relatively less useful
to the industry if it needs a Cray-YMP computer to run. A workstation-class computer is preferred
by the industry. The system has recently been ported to a workstation and enhanced with graphical
postprocessing. This will also allow a more fiiendly interface to be implemented, propulsion systems.
The code was demonstrated to key aeromechanics engineers and managers in the U.S. engine industry
at a 1993 workshop. The aerodynamic modules of the FREPS system have been successfully
compared to test results from General Electric.
Oneofthe primarygoalsof this work hasbeento quicklytransferthetechnologydeveloped
to privateindustry.Towardsthisend,asubstantialportion of theeffortsrelevanto thisproposalis
directedto establishingacloseworkingrelationshipandrapportwith personnelfrom privateindustry
who aretrying to solvepracticalproblemsin theareaof aeroelasticityofturbomachinery.As part
oftheSpaceActAgreementbetweenNASALewisResearchCenterandtheGeneralElectricAircraft
EnginesCompany,theFREPScodewassuccessfullyinstalledatGEAE. In orderto facilitatethis,
the proprietarysteadyaerodynamicodethat wasbeingusedinFREPSwasreplacedby a public-
domainsteadyaerodynamiccodecalledSFLOW developedby Dan Hoyniak of NASA Lewis
ResearchCenter.TheFREPScodeinstalledat GEAEwill beusedfor cooperativeresearchprojects
betweenNASA LewisResearchCenterandthe GeneralElectricAircratt EnginesCompany. It is
expectedthattheFREPSsystemwill contributesubstantiallyto futuredesigneffortsof thiscompany.
In addition,a draft of aUser'sManualfor the codeis readyandisgoingto bepublishedsoon. A
shorterversionof theUser'sManual,whichdescribestheuseandcapabilitiesof theFREPSsystem
hasbeenpublishedandpresented(Murthy andMorel, 1993).
Anotheraspectof forcedresponseofturbomachineryin spacepropulsionis the influence of
small, unavoidable blade-to-blade variations in rotors, known as mistuning. Potentially damaging
dynamic behavior of mistuned systems has attracted great attention in the recent past, even though
most of the work has neglected the effect of unsteady aerodynamic forces. Some efforts by personnel
associated with this proposal have demonstrated some of the consequences of the presence of
mistuning in single rotor systems, including the effect of unsteady aerodynamic forces. Pierre and
Murthy (1991) demonstrated for the first time the drastic changes in aeroelastic characteristics of
single rotor systems, such as the loss of aeroelastic eigenstructure and the localization of aeroelastic
mode shapes. Localization of aeroelastic mode shapes results in only a few blades absorbing all the
energy of external excitation and suffering much more fatigue compared to the predictions by
conventional methods that usually ignore mistuning. It was shown that these phenomena could occur
for even small values of random mistuning, of the order that is normally present in realistic rotors.
A perturbation method, that yielded insight into the qualitative behavior of mistuned aeroelastic
systems, was developed. The perturbation method also provides a quick procedure to check if large
changes can be expected by considering a given level of mistuning.
ModeLocalizationin SSMEHPOTPTurbine
To demonstratethemodelocalizationphenomenain anexistingenginestructure,theHigh
PressureOxygenTurboPump(HPOTP)turbineof theSpaceShuttleMainEngine(SSME),which
hadthestructuralandaerodynamiccharacteristicstypicalof thenewgenerationhigh-energyturbines
for spacepropulsion,wasstudiedbyPierre,SmithandMurthy (1991). Theaeroelasticbehaviorof
the HPOTPturbinebladeswasobtainedusinga finite elementstructuralmodelandanadvanced
aerodynamicmodelthat accountsfor theeffectsof camber,incidenceandthickness.Theextreme
sensitivityto mistuningandtheoccurrenceof localizedvibrationsweredemonstratedfor thisclass
of turbines. A secondmajorcontributionhasbeenthe preliminaryinvestigationinto a powerful
sensitivitymeasurethat couldallow theglobalpredictionof mistuningeffectsbasedsolelyon the
knowledgeof thetunedor thenominalsystembehavior.Thissensitivitymeasurehadthepotential
to helpavoidenginesthatarepathologicallysensitiveto mistuningandwasparticularlysuitedfor the
designenvironment.Furtherwork regardingthepracticalutilizationof thedemonstratedpotential
of thissensitivitymeasurewasdocumentedbyMurthy andPierre(1992)andis describedin thenext
paragraph.
MistuningSensitivityMeasureandApplicationto OptimalDesign
While the phenomenon of mode localizaton can be extremely dangerous when it occurs, most
analysis and design procedures routinely ignore mistuning in practice. Though this could earlier have
been attn'buted to the lack of pertinent information on the mode localization phenomenon, mistuning
is ignored in present design and analysis procedures mainly because (a) tremendous computational
and analytical complexities are involved in dropping the assumption of tuned rotors (that is, those
having identical blades) and more importantly, (b) the actual mistuning pattern and its standard
deviation are generally not available until the design and manufacture of the rotor is complete.
Although the tuned rotor assumption is extremely useful for the analysis of complex rotor systems
such as high-performance turbomachinery, mistuned rotors may exhibit dynamic characteristics that
are vastly different from those of tuned rotors due to phenomena such as mode localization. Murthy
and Pierre (1992) and Murthy, Pierre and Ottarsson (1992) addressed the need for an easy-to-
implementprocedureto account for the effects of mistuning in the current analysis and design
procedures.
A stochastic measure of sensitivity was developed by Murthy and Pierre (1992) in order to
predict the effects of small random blade mistuning on the dynamic aeroelastic response of
turbomachinery blade assemblies. This sensitivity measure is based solely on the nominal system
design (i.e., on tuned system information), which makes it extremely easy and inexpensive to
calculate. Though structural coupling was not accounted for, the measure could become a valuable
design tool that will enable designers to evaluate mistuning effects at a preliminary design stage. The
predictive capability of the sensitivity measure is illustrated by examining the effects of mistuning on
the aeroelastic modes of the first stage of the oxidizer turbopump in the Space Shuttle Main Engine.
Results from a full analysis ofmistuned systems confirm that the simple stochastic sensitivity measure
predicts consistently the drastic changes due to mistuning and the localization of aeroelastic vibration
to a few blades.
Murthy, Pierre and Ottarsson (1992) presented a strategy to develop a constraint based on
a similar approach that can be used for optimal design of an engine rotor. The approach accounted
for structural coupling (but not aerodynamic coupling) and was illutsrated by applying it to a model
popularly used to analyze bladed-disk vibrations. The proposed constraint is again dependent on the
properties of only the tuned system, so a mistuned system analysis is not needed. The principal
difficulty in modeling mistuning was overcome by employing a statistical model, which does not
require knowledge of the actual mistuning pattern. This is practical because the statistics of expected
mistuning may be more readily obtained or estimated than the actual mistuning pattern. The complex
and time-consuming task ofa mistuned system analysis is avoided by employing perturbation theory
in deriving the constraint. As a result, the proposed constraint is very inexpensive to compute. This
constraint is also easy to differentiate, sensitivity analysis of the constraint imposes no additional
burden, provided a sensitivity analysis of the tuned assembly is already available. Thus, it is suitable
for the computation-intensive applications of optimization of engine rotors.
Further work on mistuning in the context of design optimization has been initiated in
cooperationwith Brian Watson,adoctoralstudentandProfessorManohar Kamat of the Georgia
Institute of Technology. Unlike previous attempts in mistuning optimization, which considered
mistuning as a design parameter and thus failed to be of practical utility, in this work, mistuned blades
are considered to be given and the pattern of arrangement of blades around the rotor is optimized for
the least response to excitation at a given frequency. This is a more practical approach than earlier
attempts at mistuning optimization because, in practice, mistuning arises out of manufacturing
tolerances and is thus usually out of the designer's control. Work on this problem was guided by the
principal investigator serving on the dissertation committee of Brian Watson. The study was
documented in the dissertation and a series of papers derived therefrom. (E.g., Watson, Kamat and
Murthy, 1993).
Bibliography
. Murthy, D. V. and Pierre, C., "Stochastic Sensitivity Measure for Mistuned High-
Performance Turbines", submitted for publication in the Journal of Engineering for Gas
Turbines and Power.
. Murthy, D. V., Pierre, C. and Ottarsson, G., "Efficient Design Constraint Accounting for
Mistuning Effects in Engine Rotors", AIAA Journal, 33 (5), May 1995, pp. 960-962.
. Pierre, C., Smith, T. E. and Murthy, D. V., "Localization of Aeroelastic Modes in Mistuned
High-Energy Turbines", Journal of Propulsion and Power, 10 (3), May-June 1994, pp. 318-
328.
.
.
Pierre, C. and Murthy, D. V., "Aeroelastic Modal Characteristics of Mistuned Blade
Assemblies: Mode Localization and Loss ofEigenstructure", A/AA Journal, 30 (10), October
1992, pp. 2483-2496.
Belegundu, A. D., Murthy, D. V., Salagame, R. R. and Constance, F. W., "Multiobjective
Optimization of Laminated Ceramic Composites Using Genetic Algorithms', presented at
the5th AIAA/USAF/NASA/OAI Symposium on Multidisciplinary Analysis and Optimization,
Panama City, Florida, September 1994.
. Murthy, D. V. and Stefko, G. L., "Forced Respon_ Prediction System for Turbomachinery",
presented at the 1994 Conference on Advanced Earth-To-Orbit Propulsion Technology and
Exhibit, Huntsville, Alabama, May 1994.
. Murthy, D. V., Hoyniak, D. and Morel, M. R., "The Effect of Thickness on the Aeroelastic
Forced Response of Turbomachinery Blading", presented at the AIAA/ASME/ASCE/-
AHS/ASC 35th Structures, Structural Dynamics and Materials Conference, Hilton Head,
South Carolina, April 18-21, 1994.
. Murthy, D. V. and Morel, M. R., "Turbine Blade Forced Response Prediction using
FREPS", presented at the 1993 SAE Aerospace Atlantic Conference & Exposition, Dayton,
Ohio, April 20-23, 1993.
. Watson, B. C., Kamat, M. P. and Murthy, D. V., "Forced Response of Mistuned Bladed Disk
Assemblies", presented at the AIAA/ASME/ASCE/AHS/ASC 34th Structures, Structural
Dynamics and Materials Conference, La Jolla, California, April 18-21, 1993.
10. Pierre, C. and Murthy, D. V., "Aeroelastic Dynamics of Mistuned Blade Assemblies with
Closely Spaced Modes", presented at the AIAA/ASME/ASCE/AHS/ASC 34th Structures,
Structural Dynamics and Materials Conference, La Jolla, California, April 18-21, 1993.
11. Murthy, D. V. , Pierre, C. and Ottarsson, G., "An Efficient Constraint to Account for
Mistuning Effects in the Optimal Design of Engine Rotors", presented at the
AIAA/USAF/NASA/OAI Symposium on Multidisciplinary Analysis and Optimization,
Cleveland, Ohio, September 21-23, 1992 (AIAA Paper 92-4711).
12. Murthy, D. V. and Stefko, G. L., "FREPS - A Forced Response Prediction System for
TurbomachineryRot0r_", presented at the AIAA/SAE/ASME 28th Joint Propulsion
Conference, Nashville, Tennessee, July 1992 (AIAA Paper 92-3072).
13. Murthy, D. V. and Stefko, G. L., "Forced Response Prediction System (Current Status)",
presented at the 1992 Conference on Advanced Earth-To-Orbit Propulsion Technology,
Marshall Space Flight Center, Alabama, May 19-21, 1992. (NASA CP-3174 Vol. I, Eds: R.
J. Richmond & S. T. Wu, pp. 569-577)
14. Murthy, D. V. and Pierre, C., "Stochastic Sensitivity Measure for Mistuned High-
Performance Turbines", presented at the 4th International Symposium on Transport
Phenomena and Dynamics of Rotating Machinery, Honolulu, Hawaii, April 5-9, 1992.
15. Pierre, C., Smith, T. E. and Murthy, D. V., "Localization of Aeroelastic Modes in Mistuned
High-Energy Turbines", presented at the AIAA/ASME/SAE 27th Joint Propulsion
Conference and Exhibit, Sacramento, CA, June 1991. (AIAA Paper 91-3379)
16. Smith, T. E., Pierre, C. and Murthy, D. V., "Localization of Aeroelastic Modes in Mistuned
High-Energy Turbines", presented at the 1991 Conference on Structural Integrity and
Durability of Reusable Space Propulsion Systems, NASA Lewis Research Center, Cleveland,
Ohio, May 14-15, 1991.
17. Pierre, C. and Murthy, D. V., "Aeroelastic Modal Characteristics of Mistuned Blade
Assemblies: Mode Localization and Loss of Eigenstructure", presented at the AIAA/ASME/-
ASCE/AHS/ASC 32nd Structures, Structural Dynamics and Materials Conference,
Baltimore, Maryland, April 8-10, 1991. (AIAA Paper 91-1218)


Aeroelastic Analysis of Turbomachinery for Space Propulsion

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