Active Compressor Stability Management and Impact

on Engine Operability

Manuj Dhingra∗, Yedidia Neumeier† and J.V.R. Prasad‡

School of Aerospace Engineering,

Georgia Institute of Technology,

Atlanta, GA 30332

A new, innovative compressor stability management system is described. The systemcombines surge precursor detection using a single pressure sensor with a control schemeutilizing existing actuators. A digital simulation of a small turbo-shaft engine is usedto demonstrate the benefits of this system towards engine operability. Three differentactuators are evaluated. Both fuel and bleed actuation are found to be suitable for surgeavoidance. In contrast, guide vanes, which could be used for anticipatory surge marginenhancement, are found to be ineffective when used actively.

I. Introduction

The operational envelopes of current day compressors represent a difficult trade-off between safety andperformance because of the inherent aerodynamic instabilities introduced by compressor stall and surge.Once initiated, the oscillations in the mass flow rate triggered by the compressor surge, can destroy theengine. Various dynamic events such as flow distortions in the inlet, rotor blade tip clearance changes, rapidchanges in the operating conditions, etc., can trigger these instabilities. Therefore, engines are constrained tooperate below the surge line by what is known to be a safe surge margin. Since the surge line is not exact butrather represents an area where any distortions in flow may initiate the event, the surge margin is quite large.

Surge

Limit

(with Active Control)Expanded Limit

EfficiencyContours of Constant

Lines of ConstantRotational Speed

Pres

sure

rat

io

Mass Flow Rate

Current Limit

Figure 1. Expected performance en-hancement with active control.

A large surge margin restricts the compressor operation to lower thanoptimal performance (see Figure 1), degrading the overall efficiencyof the engine. Moreover, an acceleration scheduler typically usedduring transients associated with large increments in power or thrustrequired severely impacts the transient performance of the engine.

The subject of stall control gained significant attention with theformulation of the “Moore-Greitzer model” that captured the es-sentials of rotating stall and provided an analytical framework forcontrol.1,2 However, in spite of several stall control studies,3–8 ac-tive stall and surge control has not found it’s way to a productionengine. There are various reasons for this: (a) some of the actua-tors are cumbersome and heavy and multiple sensors are required forimplementation, (b) in many cases, the compressor can jump intosurge without a rotating stall phase, thus rendering the stall controlscheme ineffective , and (c) there is no need for steady operationsat and near the stall line since it represents a zone of low efficiency(see Figure 1).

This paper considers an innovative active stall management sys-tem that addresses some of the previously mentioned limitations. The proposed system (a) detects surgeprecursors using a single sensor located over the rotor of one of the compressor stages, and (b) makes use

∗Member AIAA, Graduate Student†Member AIAA, Senior Research Engineer‡Associate Fellow AIAA, Professor

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Copyright © 2004 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

of existing actuation mechanisms in the gas turbine engine. There are two operability improvements associ-ated with this approach: (a) At cruise conditions, the engine can be operated at the maximum compressorefficiency, and (b) The engine response during rapid acceleration transients is significantly improved. Theactive stability management allows the engine to accelerate at a faster rate, and the controller cuts in onlywhen the margin drops below a critical value and causes an alarm. It should be noted, that the precise valueof the critical margin that causes an alarm is not known a priori. Moreover, the minimum critical marginthat will allow surge avoidance depends on the dynamic response of the chosen actuator.

This work concentrates on the benefits to the transient response from active stability management control.Specifically, active stability management is used to obtain significant improvements in the response of aturboshaft engine when subjected to a step increment in demanded torque. This response is crucial, forexample, to the autorotation recovery of a helicopter. An available numerical engine simulation has beenextended and is used to study the dynamic response of the engine. The stability management schemesstudied are simple, and are by no means an attempt at optimal implementation of the proposed active controlschemes. Besides being proof-of-concept, they are intended to highlight certain parameters important to theproposed active stability management system.

II. Simulation Model

The digital simulation used for this work has been developed on the basis of a component-type model ofthe T700-GE-700 turboshaft gas turbine engine. This model is described in detail by Ballin,9 which formedthe basis for the real-time simulation studies conducted in the cited work. Only the pertinent differencesbetween the original model, hereafter referred to as the Ballin model, and the current implementation aredescribed in the following paragraphs. The interested reader is directed to the referenced report for thedetails of the Ballin model.

The emphasis of the original work was on real-time, pilot in-the-loop simulation performance. Thus inthe original implementation, a quasi-steady representation of certain dynamic states was used based on thevolume dynamics approximation. As part of this approximation, it is assumed that pressures and mass flowswithin mixing volumes are always in equilibrium. The present work does not require a real-time solution ofthe simulation. On the contrary, the goal is to capture the transient response of the engine as accuratelyas possible. The dynamic states are thus not neglected, and the respective non-linear governing equationsare used. A variable step Runge-Kutta solver is used to solve the resulting system of ordinary differentialequations.

The Ballin model neglected mass-flow dynamics, and thus did not model compressor surge. This simu-lation, however, is capable of exhibiting surge. The mass flow dynamics included in the model are based onthe incompressible flow assumption, similar to the original Greitzer compression system surge model.10

Ad−Hoc Extension Transition Region

Pres

sure

rat

io

Mass Flow

Figure 2. Compressor characteristicextension. A parabolic law is used forthe transition region.

The compressor characteristics used in the Ballin model end atthe surge line. For the simulation to be surge-capable, the compres-sor characteristics have to be extended beyond the surge line. This isaccomplished by dividing the extension into two parts. The transi-tion zone, as marked in Figure 2, is more critical for the present work.For a given transient, the transition zone determines whether thecompressor will surge or not. This extension has thus been based onknown behavior of stage characteristics in the transition zone. Theparabolic nature of the stage characteristics was tempered slightlyto account for a mix of stalled and unstalled stages. Beyond thetransition region, the characteristics were extended in an ad-hocmanner, simply aimed at not introducing any numerical artifacts.Consequently, the simulation is believed to be correct only up tosurge cycle initiation. The simulation may not accurately capturethe engine dynamics while in surge, may exhibit a single surge cy-cle instead of multiple, and generally could be unreliable for surgerecovery investigations. It is, however, considered to be sufficient for surge avoidance studies, which is thestated goal of this work.

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Guide Vanes, NominalGuide Vanes, Closed

Wscale

PRscale

Figure 3. Generic scaling of the com-pressor characteristics for guide vanesactuation.

As mentioned in the introduction, the candidates for stabilitymanagement control are limited to the existing actuation mecha-nisms. This necessitates the inclusion of the effect of guide vanesclosure on the compressor characteristics. As a first approximation,this is incorporated as a generic shift of the compressor characteris-tics, modeled as a linear function of the change in the guide vanesangle. This shift, as illustrated in Figure 3, moves the characteristicsleft and down, without modifying the original stall line. This is con-sidered to be an appropriate first approximation. Further, the shiftin the compressor characteristics is considered to be instantaneous.If so required, this assumption can be amended by suitably shapingthe commanded guide-vane closure. However, this is not consideredto be necessary at this point.

III. Achievable Improvements

A helicopter rotor is designed to be run at a fixed RPM for its entire operational regime. For the rotorRPM to remain nearly constant, any variation in aerodynamic loads has to be matched by the engine. Theaerodynamic torque loading is a function of flight conditions and pilot controls. If an increase in torquerequired, e.g. due to increased collective, is not met by the engine, the rotor RPM would go down. Thisdecrease is often referred to as the RPM droop. The transient performance of a helicopter propulsion systemcan thus be benchmarked in terms of it’s ability to minimize RPM droop.

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ue (l

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, ref

. to

engi

ne s

haft

QdmdQpt

Figure 4. Variations of de-manded and available torquewith time for the nominal case.

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Figure 5. The trajectory for thenominal engine with accelerationscheduler (nominal case).

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Nominal EngineAcceleration Scheduler Bypassed

Figure 6. Power Turbine RPMvariation with time.

The conventional stall prevention during acceleration transients, takes the form of a scheduler. This accel-eration scheduler, restricts the maximum allowable fuel flow, as a predetermined function of the compressorstate and it’s inlet conditions. In order to illustrate the impact of the acceleration scheduler, and implicitlycompressor stall, on engine transient response, the autorotation recovery maneuver has been analyzed. Thisspecific case has been chosen to illustrate both the need for the scheduler and the increased performancefor the ideal case when it is bypassed. The maneuver involves a stop-to-stop step command in collective,switching it from the lower to the upper stop. This collective command translates to a step change in thetorque demanded from the engine. The response of the nominal engine along with the torque demanded aregraphed in Figure 4. The corresponding variations in compressor pressure ratio and inlet mass flow rate areshown as a trajectory plot superimposed on the compressor map in Figure 5. As expected, the accelerationscheduler protects the system from surge. This protection, however, comes at the cost of a significant droopin power turbine RPM, and in turn a large droop in the rotor RPM, as seen in Figure 6 (nominal case).This RPM droop is attributed to the lag in the engine torque response. The undesirable consequences ofthe RPM droop are two fold. The loss in rotor RPM means a loss of thrust, which is especially undesirablegiven the requirements of the maneuver. Moreover, the loss of main rotor RPM is also accompanied witha proportional loss in tail rotor RPM. This reduces the pilot’s rudder authority, sometimes leading to theincorrect inference of a tail rotor failure. Together, or even individually, these factors seriously impact thesafety of flight.

If the acceleration scheduler is bypassed, there is a significant reduction in the RPM droop for the same

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Figure 7. The trajectory for the nomi-nal engine without acceleration sched-uler.

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Figure 8. Trajectory for the degradedengine without acceleration scheduler.

pilot command. This can be seen in Figure 6, which compares the power turbine RPM for the two cases.Moreover, due to a sufficient available surge margin, the compressor remains free of surge for the entiretransient, as seen in Figure 7.

The results so far show an incomplete picture. This dramatic gain in engine performance is valid onlyfor nominal inlet and compressor characteristics. Several factors, including inlet distortion, deteriorationwith age, and ambient conditions could degrade the compressor performance. To capture this deteriora-tion, the compressor characteristics have been shifted to the right. This has been implemented by scalingthe mass-flow by 5%, which roughly reduces the surge margin for a given operating point by 5 − 6%.

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Figure 9. Trajectory for the degradedengine with acceleration scheduler.

This degree of reduction is normal, and by no means exaggerated.If this slightly degraded engine is subjected to the same step

change in torque demand, and the scheduler is bypassed, the com-pressor enters a surge cycle. This can be seen in Figure 8, where thetrajectory is superimposed on the compressor map. On the otherhand, when the acceleration scheduler is retained, as with the nomi-nal engine, the degraded engine is free of any compressor instabilityfor the duration of the transient (Figure 9). In fact, as seen in Fig-ure 10, the degraded engine replicates the original performance.

Clearly some form of surge prevention is required as a part of theengine control system. A passive, predetermined scheduler severelyimpacts the engine transient response. An active stability manage-ment alternative is hence sought, which can save the engine duringa degraded scenario, while improving it’s transient response.

IV. Detection of Surge Precursors

The success of any compressor stability management scheme depends on the successful detection instabil-ity precursors. The uncertainties associated with the surge line preclude the use of conventional compressorstates as a viable surge margin estimator. However, recent work at the Georgia Institute of Technologyin the area of stall precursors has developed an innovative correlation based precursor detection scheme.11

This detection scheme has been successfully demonstrated in laboratory as well as full scale compressor rigs.Although, in general the correlation measure decreases with deceasing surge margin, a precise detectionof available margin is difficult, especially during transient operation. The correlation measure can reliablyprovide a boolean indicator of impending surge, which is sufficient for active stability management. For thepurpose of the following numerical experiments, it is assumed that such an indicator is available from theprecursor detection scheme presented in Ref. 11. The surge margin at which the precursor detection schemecan provide a consistent warning of an impending stall, is treated as a parameter in these studies, and isdesignated as SMtrig point.

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V. Numerical Experiments

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Figure 10. Comparison of the RPMresponse of the nominal and degradedengines with acceleration scheduler.

The goal of the current work is to improve engine performancewithout introducing any additional actuators. The candidate actu-ation mechanisms are thus limited to compressor variable geometry,fuel modulation and bleed. Various numerical experiments have beenperformed to evaluate the effectiveness of these actuation methods.Although it is possible to use a combination of the above mentionedactuators, only single actuation schemes have been analyzed here.

A. Guide Vanes

The guide vanes are already in use for surge margin enhancement inproduction engines. Currently their use is restricted to anticipatorycontrol, i.e. they are used when the engine controller can anticipatean upcoming rapid increase in power demand. Consequently, guidevanes actuation is investigated first. As a sanity check, the antici-patory guide vane actuation, where guide vanes are closed prior tothe step collective command, is first simulated. This has been usedto show that the simulation adequately, if not accurately, captures

the effect of guide vanes.The active control case is analyzed next. The amplitude of the guide vanes closure is kept fixed, and

equal to that of the anticipatory control case. Two parameters, the rate of guide vane closure and the SMtrig

point are varied. The rate at which the guide vanes are closed is varied from a step change to a ramp lastingfor two seconds. The resulting effect of the guide vane closure on the compressor force characteristics isassumed to be instantaneous.

B. Fuel Modulation

Wfmod

Wf

Fuel Metering ValveWFMVHMUSEL

PS3L

Figure 11. Modifications for Fuel Mod-ulation

Fuel is the strongest control available for the gas turbine engine. Fuelmodulation is included in the form of a multiplicative factor appliedto the fuel command at the input of the fuel metering valve. Thisensures that any fuel modulation introduced by the surge protectioncontroller does not bypass the transport delay associated with thefuel lines. If the pure time delay introduced due to the transportlines were to be neglected, fuel modulation could produce unrealisticresults. The lag of the actuator is already modeled as part of thefuel metering valve. A single parameter, the SMtrig point, is variedin fuel modulation experiments. The arrangement is sketched inFigure 11.

C. Bleed control

τ bleed *s + 1

1Active Bleed Pulse To Engine Dynamics

Figure 12. Modifications for BleedControl

Bleed is the most direct control, which explicitly enters the pressuredynamics of the compression system. High pressure flow is alreadybled from the engine for various purposes, e.g. to cool the gas gen-erator blades and shrouds. An additional bleed is introduced at theexit of the compressor, expressed as a fraction of compressor inletmass flow. A first order lag-filter is introduced to model the dynam-ics of associated actuator. The control arrangement is summarizedin Figure 12.

A sensitivity study is done to assess the trade-offs between the time constant of the lag-filter, τbleed, andthe surge margin at which control is triggered, SMtrig. Additionally, the impact of the fraction being bledis also investigated.

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VI. Results and Discussion

The effectiveness of a given actuation is evaluated based on the reduction in RPM variation while suc-cessfully preventing the engine from surge. All the actuation studies have been performed using the degradedengine, while overriding the limits imposed by the acceleration scheduler. Consequently, the degraded en-gine results are used as the reference for any comparison. Moreover, in all the cases presented here, the stepcommand in collective was initiated at 20.50s.

A. Guide Vanes

The guide vane angles are expressed in generic degrees, with the nominal position considered as 0◦. Forall the cases studied, the guide vanes are closed by 2◦. This corresponds to a generic shift of 8% in thecompressor characteristic.

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Figure 13. RPM response with anticipatory guidevane closure prior to the acceleration command.

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Figure 14. The trajectory for the degraded enginewithout acceleration scheduler, with guide vanesclosure prior to acceleration command.

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Figure 15. The effect of anticipatory guide vanesclosure on compressor speed.

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Sur

ge M

argi

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Figure 16. The effect of anticipatory guide vanesclosure on compressor surge margin.

The anticipatory guide vanes control results are shown in Figure 13. The corresponding trace on thecompressor map is presented in Figure 14. As expected, the anticipatory guide vanes control is effectivein surge avoidance, and in addition, it improves the transient response. Effectively, the vane closure shiftsthe steady state compressor operating point to a higher RPM (Figure 15), increases the steady state surgemargin (Figure 16), and eventually prevents compressor surge during the acceleration transient.

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Unfortunately, when the active guide vanes control during the transient is employed, it fails to keep thecompressor away from surge. This fact is captured in Figures 17. In fact, as discussed later, this actuationpushes the system into surge. In these results, control was initiated at SMtrig = 3% and the vanes wereclosed along a 10ms wide ramp. Similar results were observed for the SMtrig range from 1% to 6%, andvanes closure from a descrete step to 2s wide ramp.

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Figure 17. Trajectory for the degraded enginewithout acceleration scheduler, with active guidevanes control. SMtrig = 3%.

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Figure 18. The trajectory for the degraded enginewithout acceleration scheduler, with active fuelmodulation. SMtrig = 3%.

B. Fuel Modulation

The pressure versus mass flow trace, in Figure 18, illustrates that the fuel actuation is effective in surgeprevention. Although for the results presented here, control was activated at 3% surge margin, similar resultswere observed when SMtrig was varied from 1% to 6%. The resulting enhancement in transient performanceis shown in Figure 19. The oscillations seen on the compressor map correspond to the oscillations in the fuelflow rate, graphed in Figure 20. As previously mentioned, the control-laws used here are simple, and with acomprehensive design, such behavior can be minimized, if not completely eliminated.

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Figure 19. The comparison of RPM variationsin the degraded engine between the accelerationscheduler and active fuel modulation for surgeavoidance.

20.5 20.6 20.7 20.8 20.9 21 21.1 21.2 21.3 21.4 21.50.04

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Figure 20. Fuel modulation for surge avoidance.

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C. Bleed Control

The results from the bleed actuation studies are similar to those of fuel modulation. Three different bleedamplitudes, namely 0.5%, 1%, and 2%, have been analyzed. These amplitudes are referenced to the massflow at the compressor inlet. The 0.5% bleed was not able to prevent surge, but both 1 and 2% bleeds weresatisfactory. Figure 21 shows the pressure versus mass-flow trace for 1% bleed amplitude. The transientperformance, in terms of RPM droop, as seen in Figure 22 is slightly better than the fuel modulation case.For these results, the control was triggered at a Surge Margin of 3% with a τbleed of 10ms. The correspondingbleed control pulse is graphed in Figure 23.

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Figure 21. The trajectory for the degraded enginewithout acceleration scheduler, with active bleed.SMtrig = 3%.

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Figure 22. The comparison of RPM variationsin the degraded engine between the accelerationscheduler and active bleed for surge avoidance.

20.5 20.6 20.7 20.8 20.9 21 21.1 21.2 21.3 21.4 21.50

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Figure 23. Time trace of the bleed control.

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τ blee

d

Unsafe

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Figure 24. τbleed versus SMtrig, for active bleed.

The maximum τbleed tolerated, as a function of SMtrig is shown in Figure 24. As expected, the controlwith 2% bleed can tolerate larger lag before loosing it’s effectiveness.

The fundamental difference in the behavior of guide vanes and bleed is illustrated using Figure 25. Theimmediate effect of vane closure is to reduce the force provided by the compressor. As the pressure in thecombustor is unchanged, this acts to reduce the mass-flow rate through the compressor (Figure 26), pushingthe system towards the surge line. On the other hand, bleed at the compressor outlet acts to reduce the“back-pressure” seen by the compressor, hence accelerates the flow and keeps it away from the surge line.The reduction in fuel, by reducing the stagnation temperature, induces a similar reaction. In the anticipatoryvane closure, as evidenced in Figure 16, a sufficient initial margin allows the safe completion of associatedtransients. This accounts for the contradictory behavior observed between anticipatory guide vane closure

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Figure 25. A comparison of the initial trajectoryresponse due to active guide vanes control andactive bleed.

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Figure 26. Effect of guide vane closure on thecompressor inlet mass flow rate. SMtrig = 3%.

and active guide vane control.

VII. Conclusions

This paper addresses the benefits of active compressor stability management, when applied to transientresponse of a gas turbine engine. A simulation model of the GE T700 turbo-shaft engine has been extendedfor this purpose. The three different actuation mechanisms, available in existing gas turbine engines, havebeen evaluated in this respect. In particular, it is shown that:

• Fuel modulation and bleed can effectively perform surge avoidance during rapid engine acceleration,while improving the overall engine operability

• There exists a trade-off in Surge Margin at which control should be triggered and the latencies in theassociated stall precursor detection and control system. A faster system could allow for an operationmuch closer to the surge line. However, a threshold value of Surge Margin trigger may exist, belowwhich a practical system may not be able to prevent surge.

• Guide vanes, which may be an effective actuation mechanism in anticipatory surge margin enhance-ment, may yield undesirable results when used for active stability management.

The results obtained to-date are promising and warrant further investigations on active stability man-agement. The generic scaling of compressor characteristics with guide vane angle, though a sufficient firstapproximation, needs to be refined. The impact of model based surge prevention control-laws should be ex-plored. Finally, any dynamic interactions between the surge avoidance controller and the engine fuel controlsystem need to be addressed in detail.

VIII. Acknowledgments

This study was conducted under the NASA URETI on Aeropropulsion and Power Technology (UAPT)and the GE Aircraft Engines University Strategic Alliance (GEUSA) program at the Georgia Institute ofTechnology. Discussions with Mr. Peter Szucs of GEAE are gratefully acknowledged. Dr. Carlos J. Rivieraprovided invaluable help with numerical ODE solvers.

References

1Moore, F. K. and Greitzer, E. M., “Theory of Post-Stall Transients in Axial Compression Systems: Part I - Developmentof Equations.” Journal of Engineering for Gas Turbines and Power, Transactions of the ASME , Vol. 108, No. 1, Jan. 1986,pp. 68–76.

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2Moore, F. K. and Greitzer, E. M., “Theory of Post-Stall Transients in Axial Compression Systems: Part II - Application.”Journal of Engineering for Gas Turbines and Power, Transactions of the ASME , Vol. 108, No. 2, April 1986, pp. 231–239.

3Weigl, H., Paduano, J., Frechette, L., Epstein, A., Greitzer, E., Bright, M., and Strazisar, A., “Active stabilizationof rotating stall and surge in a transonic single-stage axial compressor,” Journal of Turbomachinery, Vol. 120, Oct. 1998,pp. 625–636.

4Epstein, A. H., Williams, J. E. F., and Greitzer, E. M., “Active Suppression of Aerodynamic Instabilities in Turboma-chines,” Journal of Propulsion and Power , Vol. 5, No. 2, March–April 1989, pp. 204–211.

5Badmus, O. O., Chowdhary, S., Eveker, K. M., Nett, C. N., and Rivera, C. J., “Simplified Approach for Control ofRotating Stall Part 1: Theoretical Development,” Journal of Propulsion and Power , Vol. 11, No. 6, Nov.–Dec. 1995, pp. 1195–1209.

6Badmus, O. O., Chowdhary, S., Eveker, K. M., Nett, C. N., and Rivera, C. J., “Simplified Approach for Control ofRotating Stall Part 2: Experimental Results,” Journal of Propulsion and Power , Vol. 11, No. 6, Nov.–Dec. 1995, pp. 1210–1223.

7Gysling, D. L. and Greitzer, E. M., “Dynamic Control of Rotating Stall in Axial Flow Compressors Using AeromechanicalFeedback,” ASME Paper 94-GT-292, 1994.

8D’Andrea, R., Behnken, R. L., and Murray, R. M., “Rotating Stall Control of an Axial Flow Compressor Using PulsedAir Injection,” Journal of Turbomachinery, Vol. 119, No. 4, Oct. 1997, pp. 742–752.

9Ballin, M. G., “A high fidelity real-time simulation of a small turboshaft engine,” NASA TM–100991, Ames ResearchCenter, Moffett Field, california, July 1988.

10Greitzer, E. M., “Surge and Rotating Stall in Axial Flow Compressors: Part I - Theoretical Compression System Model,”Journal of Engineering for Power, Transactions of the ASME , Vol. 98 Ser A, No. 2, April 1976, pp. 190–198.

11Dhingra, M., Neumeier, Y., Prasad, J. V. R., and Shin, H., “Stall and Surge Precursors in Axial compressors,” Proceedingsof the 39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, AIAA Paper 2003–4425, July 20–23, 2003.

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