GT200 5-68600 - NASA · wake generator positioned upstream of the curved plate. ... be used for the design procedure of modern gas turbine ... which is a critical parameter for blade

Proceedings of GT2005ASME Turbo Expo 2005: Power for Land, Sea and Air

June 6-9, 2005, Reno-Tahoe, Nevada, USA

GT2005-68600

Effect of Reynolds Number and Periodic Unsteady Wake Flow Conditionon Boundary Layer Development, Separation, and Re-attachment along the Suction

Surface of a Low Pressure Turbine Blade

B. Öztürk, M. T. Schobeiri, David E. AshpisTurbomachinery Performance and Flow Research Laboratory National Aeronautics and Space Administration

Texas A&M University John H. Glenn Research Center at Lewis FieldCollege Station, Texas Cleveland, OH 44135-3191

ABSTRACT

The paper experimentally studies the effects ofperiodic unsteady wake flow and different Reynoldsnumbers on boundary layer development, separation andre-attachment along the suction surface of a lowpressure turbine blade. The experimental investigationswere performed on a large scale, subsonic unsteadyturbine cascade research facility at TurbomachineryPerformance and Flow Research Laboratory (TPFL) ofTexas A&M University. The experiments were carriedout at Reynolds numbers of 110,000 and 150,000(based on suction surface length and exit velocity). Onesteady and two different unsteady inlet flow conditionswith the corresponding passing frequencies, wakevelocities, and turbulence intensities were investigated.The reduced frequencies chosen cover the operatingrange of LP turbines. In addition to the unsteadyboundary layer measurements, surface pressuremeasurements were performed. The inception, onset,and the extent of the separation bubble informationcollected from the pressure measurements werecompared with the hot wire measurements.

The results presented in ensemble-averaged, and thecontour plot forms help to understand the physics of theseparation phenomenon under periodic unsteady wakeflow and different Reynolds number. It was found thatthe suction surface displayed a strong separation bubblefor these three different reduced frequencies. For eachcondition, the locations defining the separation bubblewere determined carefully analyzing and examining the

pressure and mean velocity profile data. The location ofthe boundary layer separation was dependent of theReynolds number. It is observed that starting point ofthe separation bubble and the re-attachment point movefurther downstream by increasing Reynolds numberfrom 110,000 to 150,000. Also, the size of theseparation bubble is smaller when compared to that forRe=110,000.

NOMENCLATURE

c blade chordcax axial chordCppressure coefficient, C. = P` - P"

dRrod diameter (Pr - PA.,H 12 shape factor, H 12=61/62

hmmaximum separation bubble heightLSS suction surface lengthM number of samplesN number of wake cyclespistatic pressure taps i=1,..,48p s, ptstatic and total pressure at the inletReLSS Reynolds number based Re = L.Vâr /vSBblade spacingSRrod spacings streamwise distance from the leading edge of

the bladesostreamwise distance from the leading edge to

the trailing edge of the blade

smd maximum separation bubble height at a

Copyright © 2005 by ASME

https://ntrs.nasa.gov/search.jsp?R=20100015672 2018-07-09T00:38:01+00:00Z

streamwise distance from blade leading edgesr re-attachment point of the separation bubble at

a streamwise distance from blade leading edges s starting point of the separation bubble at a

streamwise distance from blade leading edget timeTu turbulence intensityU belt translational velocityVax axial velocityVexit exit velocityV velocityv fluctuation velocityy blades stagger angleS boundary layer thicknessS 1 boundary layer displacement thicknessS2 boundary layer momentum thicknessS 3 boundary layer energy thicknessV kinematic viscositya cascade solidity, a = c/SB

T one wake-passing periodcp flow coefficient, cp = Vax/UrA Zweifel coefficient

*A = 2 gOU 2 (cot a2 - cot a O5B&C

S^ reduced frequency a = c U

= a $B

SR V. (P $R

INTRODUCTION

In recent years, gas turbine engine aerodynamicistshave focused their attention on improving the efficiencyand performance of the low pressure turbine (LPT)component. Research works from industry, researchcenters, and academia have shown that a reduction ofthe blade number can be achieved without substantiallysacrificing the efficiency of the LPT blading. Thisreduction contributes to an increase in thrust/weightratio, thus reducing the fuel consumption. Contrary tothe high pressure turbine (HPT) stage group thatoperates in a relatively high Reynolds numberenvironment, dependent on operation conditions, theLPT experiences a variation in Reynolds numberranging from 50,000 to 250,000. Since the majorportion of the boundary layer, particularly along thesuction surface is laminar, the low Reynolds number inconjunction with the local adverse pressure gradientmakes it susceptible to flow separation, thus increasingthe complexity of the LPT boundary layeraerodynamics. The periodic unsteady nature of theincoming flow associated with wakes that originate fromupstream blades substantially influences the boundarylayer development including the onset of the laminarseparation, the extent of the separation bubble, and its

turbulent re-attachment. Of particular relevance in thecontext of LPT aerodynamics is the interaction of thewake flow with the suction surface separation bubble.While the phenomenon of the unsteady boundary layerdevelopment and transition in the absence of theseparation bubbles has been the subject of intensiveresearch, that has led to better understanding thetransition phenomenon, comprehending the multipleeffects of mutually interacting parameters on the LPTboundary layer separation and their physics still requiresmore research.

The significance of the unsteady flow effect onefficiency and performance of compressor and turbinestages was recognized in the early seventies by severalresearchers. Fundamental studies by Pfeil and Herbst[1], Pfeil et al. [2], and Orth [3] studied and quantifiedthe effect of unsteady wake flow on the boundary layertransition along flat plates. Schobeiri and his co-workers[4], [5], [6], [7] experimentally investigated the effectsof the periodic unsteady wake flow and pressuregradient on the boundary layer transition and heattransfer along the concave surface of a constantcurvature plate. The measurements were systematicallyperformed under different pressure gradients andunsteady wake frequencies using a squirrel cage typewake generator positioned upstream of the curved plate.Liu and Rodi [8] carried out the boundary layer and heattransfer measurements on a turbine cascade, which wasinstalled downstream of a squirrel cage type wakegenerator mentioned previously.

Analyzing the velocity and the turbulence structureof the impinging wakes and their interaction with theboundary layer along the concave side of the curvedplate, Chakka and Schobeiri [7] developed anintermittency based unsteady boundary layer transitionmodel. The analysis revealed a universal pattern for therelative intermittency function for all the frequencies andpressure gradients investigated. However, the curvedplate investigations were not sufficient to draw anyconclusion with regard to an eventual universalcharacter of the relative intermittency function. Furtherdetailed investigations of the unsteady boundary layeron a high Reynolds number turbine cascade by Schobeiriet al. [9], [10] and its subsequent analysis [11] and [12]verified the universal character of the relativeintermittency function. For this purpose, Schobeiri et al.[9] utilized a conceptually different type wakegenerator, which is also used for the investigationpresented in this paper. Fottner and his coworkers [13],[14] and Schulte and Hodson [15] used the same wakegenerating concept for the investigations on theinfluence of the unsteady wake flow on the LPT-


boundary layer. Kaszeta, Simon, and Ashpis [16]experimentally investigated the laminar-turbulenttransition aspect within a channel with the side wallsresembling the suction and pressure surfaces of a LPTblade. Lou and Hourmouziadis [17] investigated themechanism of separation, transition, and re-attachment,and the effect of oscillating inlet flow conditions onlaminar boundary layer separation along a flat plateunder a strong negative pressure gradient which wassimilar to the LPT pressure gradient. This was simulatedby contouring the top wall. They studied the Reynoldsnumber effect on the transition region. Their resultsshowed that the higher Reynolds numbers cause anearlier transition and reduction of the transition length,while the separation point does not change its location.Using the top wall contouring, Volino and Hultgren[18] performed an experimental study and measured thedetailed velocity along a flat plate which was subjectedto a similar pressure gradient as the suction side of alow pressure turbine blade. They also stated that thelocation of the boundary layer separation does notstrongly depend on the Reynolds number or free-streamturbulence level, as long as the boundary layer remainsnon-turbulent before separation occurs. Furthermore,they showed that the extent of the transition is stronglydependent on the Reynolds number and turbulenceintensity.

Using the surface mounted hot film measurementtechnique, Fottner and his coworkers [13] and [14],Schröder [19], and Haueisen, Hennecke, and Schröder[20] documented strong interaction between the wakesand the suction surface separation bubble on the LPTblades, both in the wind tunnel cascade tests and in aturbine rig. Furthermore, they investigated theboundary layer transition under the influence of theperiodic wakes along the LPT surface and found thatthe interaction of the wake with the boundary layergreatly affects the loss generation. Shyne et al. [21]performed an experimental study on a simulated lowpressure turbine. The experiments were carried out atReynolds numbers of 100,000 and 250,000 with threelevels of free-stream turbulence. They indicated that thetransition onset and the length are strongly dependenton the free-stream turbulence. As the free-streamturbulence increases, the onset location and the lengthof the transition are decreased. Treuren et al. [22]performed an experimental study along a LPT surface atthe very low Reynolds number of 25,000 and 50,000with different free-stream turbulence levels. Theyshowed that a massive separation at the very lowReynolds number of 25,000 is persistent, in spite of anelevated free stream turbulence intensity. However, at

the higher Reynolds number of 50,000, there was astrong separation on the suction side for the low free-stream turbulence level. The separation bubble waseliminated for the higher free-stream turbulence level of8-9%. The investigations by Halstead et al. [23] on alarge scale LP turbine uses surface mounted hot films toacquire detailed information about the quasi-shear stressdirectly on the blade surface. Investigations byCardamone et al. [14] and Schröder [19] indicate thatthe benefit of the wake-boundary layer interaction canbe used for the design procedure of modern gas turbineengines with a reduced LPT blade number withoutaltering the stage efficiency.

Most of the studies mentioned above on LP turbinecascade aerodynamics have largely concentrated on themeasurement of the signals stemming from hot filmsmounted on the suction and pressure surfaces of theblades under investigation. Although this technique isqualitatively reflecting the interaction of the unsteadywake with the boundary layer, because of the lack of anappropriate calibration method, it is not capable ofquantifying the surface properties such as the wall shearstress. The few boundary layer measurements are notcomprehensive enough to provide any conclusiveevidence for interpretation of the boundary layertransition and separation processes and their directimpact on profile loss, which is a critical parameter forblade design. Furthermore, the numerical simulation ofthe unsteady LPT blade aerodynamics usingconventional turbulence and transition models fails if itis applied to low Reynolds number cases. Recent workpresented by Cardamone et al. [14] shows that in thesteady state case at Re= 60,000, the separation iscaptured, however, for the unsteady case, the separationbubble is not reproduced.

A recent experimental study by Schobeiri andÖztürk [24],[25] investigated the physics of theinception, onset and extent of the separation bubblealong a low pressure turbine blade which was the firstpart of a series of investigations carried out at TPFL. Adetailed experimental study on the behavior of theseparation bubble on the suction surface of a highlyloaded LPT blade under a periodic unsteady wake flowwas presented in [24]. Surface pressure measurementswere performed at Re= 50,000, 75,000, 100,000,125,000. Increasing the Reynolds number has resultedin no major changes to the surface pressure distribution.They concluded that the unsteady wake flow with itshighly turbulent vortical core over the separation regioncaused a periodic contraction and expansion of theseparation bubble. It was proposed that, in conjunctionwith the pressure gradient and periodic wakes, the


temporal gradient of the turbulence fluctuation, or moreprecisely the fluctuation acceleration avrms /atprovides a higher momentum and energy transfer intothe boundary layer, energizing the separation bubble andcausing it to partially or entirely disappear. They foundthat for /at> , the separation bubble starts tocontract whereas for , it graduallyassumes the shape before the contraction. They arguedthat not only the existence of higher turbulencefluctuations expressed in terms of higher turbulenceintensity influences the flow separation, but also itsgradient is of crucial importance in suppressing orpreventing the onset and the extent of the separationbubble. They stated that the fluctuation gradient is aninherent feature of the incoming periodic wake flow anddoes not exist in a statistically steady flow that mighthave a high turbulence intensity. They also stated that,unsteady wake flow with its highly turbulent vorticalcore over the separation region, caused a periodiccontraction and extension of the separation bubble anda reduction of the separation bubble height. Increasingthe passing frequency associated with a higherturbulence intensity further reduced the separationbubble height [25].

The objective of the present study dealing with thespecific issues of the LPT boundary layer aerodynamicsis to provide a detailed unsteady boundary flowinformation to understand the underlying physics of theinception, onset, and extension of the separation bubblefor different Reynolds numbers. Furthermore, theunsteady boundary layer data from the present andplanned experimental investigations will serve to extendthe intermittency unsteady boundary layer transitionmodel developed by Schobeiri and his coworkers [7, 11,12] to the LPT cases, where a massive separation occurson the suction surface at a low Reynolds number at thedesign and off-design incidence. Furthermore, theexperimental results are intended to serve as benchmarkdata for a comparison with numerical computation usingDNS or RANS-codes.

It is well known that the boundary layermeasurement is one of the most time consumingaerodynamic measurements. Any attempt to increase thenumber of parameters to be studied would inevitablyresult in substantial increase of the measurement time.Considering this fact, the research facility described in[9] and [10] with state-of-the-art instrumentation hasbeen substantially modified to study systematically andefficiently the influence of the periodic unsteady andhighly turbulent flow on the LPT cascade aerodynamicsat the design and off-design incidence angles, where theReynolds number, wake impingement frequency, free-

stream turbulence, and the blade solidity can be variedindependently.

EXPERIMENTAL RESEARCH FACILITY

To investigate the effect of unsteady wake flow onturbine and compressor cascade aerodynamics,particularly on unsteady boundary layer transition, amulti-purpose large-scale, subsonic research facility wasdesigned and has been taken into operation since 1993.Since the facility in its original configuration is describedin [9], [10] and [24] only a brief description of themodifications and the main components is given below.The research facility consists of a large centrifugalcompressor, a diffuser, a settling chamber, a nozzle, anunsteady wake generator, and a turbine cascade testsection as shown in Figure 1. The compressor with avolumetric flow rate of 15 m 3/s is capable of generatinga maximum mean velocity of 100 m/s at the test sectioninlet. The settling chamber consists of five screens andone honeycomb flow straightener to control theuniformity of the flow.

Two-dimensional periodic unsteady inlet flow issimulated by the translational motion of an unsteadywake generator (see Figure 2), with a series ofcylindrical rods attached to two parallel operating timingbelts driven by an electric motor. To simulate the wakewidth and spacing that stem from the trailing edge ofrotor blades, the diameter and number of rods can bevaried. The rod diameter, its distance from the LPTblade leading edge, the wake width and thecorresponding drag coefficient is chosen according tothe criteria outlined by Schobeiri et al. [26]. The belt-pulley system is driven by an electric motor and afrequency controller. The wake-passing frequency ismonitored by a fiber-optic sensor. The sensor alsoserves as the triggering mechanism for data transfer andits initialization, which is required for ensemble-averaging. This type of wake generator produces clean2-dimensional wakes, whose turbulence structure, decayand development is, to a great extent, predictable [26].The unsteady boundary layer transition and heat transferinvestigations [9-12] performed on this facility serve asthe bench mark data for validation of turbulence models,transition models, and general code assessments.

To account for a high flow deflection of the LPTcascade, the entire wake generator and test section unitincluding the traversing system, was modified to allowa precise angle adjustment of the cascade relative to theincoming flow. This is done by a hydraulic platform,which simultaneously lifts and rotates the wakegenerator and test section unit. The unit is then attached


OStatic pressure blade © Timing belts. rod attachments O Inlet nossle © Large silence chamber with

O2 Blade with hot film sensors © Transition duct 10 Hydraulic cylinders 14 honcyope and rive screens

Telesc

O3 Wake generating rods O Straight duct 1© Pivot point 16 Honeycomb flow straightener

® Wake Traversing systemenerator © Wake generator a—motor 1® Traversing slotsg O

Figure 1. Turbine cascade research facility with the components and the adjustabletest section

6.5

6.250=3.18 y

S3= 80 mmE=- 6

'5.75

5.5725 750

t(MS) 775 800

6.5

6.25H

0=1 .59 E6S2=160 mm

X5.75

5.5425 450

t(MS) 475 500

6.5

6.250=0.0 6S1= ∞

5.575 100

t(m/S) 125 150

Figure 2. Wake Generator


Table 1: Parameters of turbine cascade test section

Parameters Values Parameters Values

Inlet velocity Vin = 4 m/s Inlet turbulence intensity Tuin = 1.9 %

Rod translational speed U = 5.0 m/s Blade Re-number Re = 110,000

Nozzle width W = 200.0 mm Blade height hB = 200 mm

Blade chord c = 203.44 mm Cascade solidity a = 1.248

Blade axial chord cax = 182.85mm Zweifel coefficient arA = 1.254

Blade suction surface length LSS = 270.32 mm Cascade angle cp = 55°

Cascade flow coefficient (D = 0.80 Cascade spacing SB = 163 mm

Inlet air angle to the cascade " 1 = 0° Exit air angle from the cascade "2 = 90°

Rod diameter DR = 2.0 mm Rod distance to lead. edge LR = 122 mm

Cluster 1 (no rod, steady) SR = — mm parameter steady case 0.0

Cluster 2 rod spacing SR = 160.0 mm parameter for cluster 1 1.59

Cluster 3 rod spacing SR = 80.0 mm parameter for cluster 2 3.18

to the tunnel exit nozzle with an angular accuracy lessthan 0.05 o, which is measured electronically.

The special design of the facility and the length ofthe belts (Lbelt = 4,960 mm) enables a considerablereduction of the measurement time. For the presentinvestigation, two clusters of rods with constantdiameter of 2 mm are attached to the belts as shown inFigure 2. The two clusters with spacings S R = 160 mmand SR = 80 mm are separated by a distance which doesnot have any rods, thus simulating steady state case (S R

= —). Thus, it is possible to measure sequentially theeffect of three different spacings at a single boundarylayer point. To clearly define the influence domain ofeach individual cluster with the other one, the clustersare arranged with a certain distance between each other.Using the triggering system mentioned above and acontinuous data acquisition, the buffer zones betweenthe data clusters can be readily identified.

The data analysis program cuts the buffer zones andevaluates the data pertaining to each cluster.Comprehensive preliminary measurements were carriedout to make sure that the data were exactly identical tothose, when the entire belt length was attached withrods of constant spacing, which corresponded to each

individual cluster spacing. The cascade test sectionshown in Figure 1, located downstream of the wakegenerator, includes 5 LPT blades with a height of 200.0mm and the chord of 203.44 mm. For boundary layerinvestigations, five identical “Pak B” airfoils designed byPratt & Whitney were implemented whose cascadegeometry is given in Table 1.

The blade geometry resembles the essential featuresuch as the laminar boundary layer separation that isinherent to typical LPT blades. The blade geometry wasmade available to NASA researchers and academia tostudy the specific problems of LPT flow separation, itspassive and active control and its prevention. As shownin [9], this blade number is necessary and sufficient tosecure a spatial periodicity for the cascade flow. Theperiodicity is noticed in the pressure distribution for thesecond and fourth blade for steady and unsteady flowconditions, and is shown in Figure 1. These blades werespecially manufactured for measurement of pressure andshowed identical pressure distributions.

A computer controlled traversing system is used tomeasure the inlet velocities and turbulence intensities, aswell as the boundary layers on suction and pressuresurfaces. The traversing system (see Figure 3) was


Figure 3. Turbine cascade research facility with 3-axis traversing system

modified to allow the probe to reach all streamwisepositions along the suction und pressure surfaces. Thethree axis traversing system is vertically mounted on theplexiglass side wall. Each axis is connected to a DC-stepper motor with an encoder and decoder. The opticalencoder provides a continuous feedback to the steppermotor for accurate positioning of the probes. Thesystem is capable of traversing along the suction andpressure surfaces in small steps up to 1 µm, and thethird axis is capable of rotating with an angular accuracyless then 0.05 o, which is specifically required forboundary layer investigations where the measurement ofthe laminar sublayer is of particular interest.

INSTRUMENTATION, DATA ACQUISITION,AND DATA REDUCTION

The data acquisition system is controlled by apersonal computer that includes a 16 channel, 12-bitanalog-digital (A/D) board. Time dependent velocitysignals are obtained by using a commercial 3-channel(TSI), constant temperature hot-wire anemometersystem that has a signal conditioner with a variable lowpass filter and adjustable gain. A Prandtl probe, placedupstream of the diffuser, monitors the reference velocity

at a fixed location. The pneumatic probes are connectedto high precision differential pressure transducers fordigital readout. Several calibrated thermocouples areplaced downstream of the test section to constantlymonitor the flow temperature. The wake generatorspeed and the passing frequency signals of the rods aretransmitted by a fiber-optic trigger sensor. The passagesignals of the rods are detected by the sensor using asilver-coated reflective paint on one of the belts. Thissensor gives an accurate readout of the speed of thewake generator and the passing frequency of the rods.The signals of the pressure transducers, thermocouples,and trigger sensors are transmitted to the A/D board andare sampled by the computer. To ensure the cascadeperiodicity, the second and fourth blades areinstrumented each with 48 static pressure taps. Twoadjacent blades are used for boundary layermeasurement. The taps are connected to a scanivalve,which sequentially transferred the pressure signals toone of the transducers that was connected to the A/Dboard.

The unsteady data are taken by calibrated, customdesigned miniature, single hot wire probes. At eachboundary layer position, samples were taken at a rate of20kHz for each of 100 revolutions of the wakegenerator. The data were ensemble-averaged withrespect to the rotational period of the wake generator.Before final data were taken, the number of samples perrevolution and the total number of revolutions werevaried to determine the optimum settings forconvergence of the ensemble-average.

For the steady state case, the instantaneous velocitycomponents are calculated from the temperaturecompensated instantaneous voltages by using thecalibration coefficients. The instantaneous velocity canbe represented in the following form:

V= V+ v (1)

where is the mean (time-averaged) velocity and v isthe turbulent fluctuation component. The mean velocity,also known as the time-average, is given by:

_MV = 1 E V. (2)

M pi r

where M is the total number of samples at one boundary


layer location. The root mean square value of theturbulent velocity fluctuation is:

ns _

V = 1 (V. — V)2 (3)M J=1

i

and the local turbulence intensity is defined as:

Tu,= 1. 100 = 1 1 (V.- y)2x 100 (4)V V MI-1

For unsteady cases, the ensemble-averaged velocity,fluctuation velocity, and the turbulence intensity werecalculated from the instantaneous velocity samples by:

VP) _ <V(t,)' = 1 E yo (5)NJ=1

v kte) ° <v'#)> = V

1 L LV__(tl)-< i(ti)>]2 (6)

`^ Ni=1r

<V'#)>t(t^> _ <Tut <v,

(tt)' = x100 (7)Tu<V(tt)>

where N= 100 is the total number of wake generatorperiods and M the number of samples taken per period.<Vi (ti)> is the reference ensemble averaged velocity forthe particular boundary layer traverse.

EXPERIMENTAL RESULTS AND DISCUSSION

To investigate the influence of the unsteady wakeflow on the boundary layer development along thesuction and pressure surfaces of the LPT blade and,particularly, its impact on the inception and onset of theseparation bubble, the detailed surface pressure andboundary layer measurements were performed at aReynolds number of 110,000 and 150,000. ThisReynolds numbers, which pertain to a typical cruiseoperation , exhibit a representative value within LPToperating range between 75,000 and 400,000 asdiscussed by Hourmouziadis [27]. Furthermore, itproduces separation bubbles that can be accuratelymeasured by miniature hot wire probes. For theReynolds number of 110,000 and 150,000, threedifferent reduced frequencies were examined. For

generation of the unsteady wakes, cylindrical rods withthe diameter dR = 2mm were chosen to fulfill thecriterion that requires the generation of a dragcoefficient CD that is approximately equal to the CD ofthe turbine blade with the chord and spacing given inTable 1 (for details look for the studies in [26] and[28]).

To accurately account for the unsteadiness causedby the frequency of the individual wakes and theirspacings, the flow velocity, and the cascade parameters,a reduced frequency 0 is defined that includes thecascade solidity a, the flow coefficient cp, the bladespacing SB, and the rod spacing SR. Many researchershave used Strouhal number as the unsteady flowparameter, which only includes the speed of the wakegenerator and the inlet velocity. However, the currentlydefined reduced frequency Q is an extension of Strouhalnumber in the sense that it incorporates the rod spacingSR and the blade spacing SB in addition to the inletvelocity and wake generator speed. For surfacepressure measurement rods with uniform spacings asspecified in Table 1 were attached over the entire beltlength. For boundary layer measurement, however,clusters of rods were attached, as mentioned previously.

Surface Pressure Distributions

Detailed pneumatic surface pressure measurementswere taken at Re = 110,000, and 150,000. For eachReynolds number three different reduced frequencies,namely Q = 0.0, 1.59, and 3.18 are applied thatcorrespond to the rod spacings S R = 80 mm, 160 mm,and — mm. The pressure distributions in Figure 4 showthe results of the steady case and two unsteady cases.The pressure signals inherently signify the time-averagedpressure because of the internal pneumatic dampingeffect of the connecting pipes to the transducer. Thenoticeable deviation in pressure distribution between thesteady and unsteady cases, especially on the suctionsurface, is due to the drag forces caused by the movingrods. The drag forces are imposed on the main streamand cause momentum deficiency that lead to a reductionof the total and static pressure.

The time-averaged pressure coefficients along thepressure and suction surfaces are plotted in Figure 4.The suction surface (upper portion), exhibits a strongnegative pressure gradient. The flow accelerates at arelatively steep rate and reaches its maximum surfacevelocity that corresponds to the minimum C p= -4.0 ats/so= 0.42. Passing through the minimum pressure, thefluid particles within the boundary layer encounter, apositive pressure gradient that causes a sharp


U -1

Re=110,000

s/so

Re=150,000

(b)

9It

M C14

®a®®

e 0 0W W

n no rod

0 160 mm a0 80 mm

8 8V

0.2 0.4 0.6 0.8s/so

Figure 4. Static pressure distributions at two different Re-numbers and reduced frequencies Q=0, 1.59, 3.18(no rod, 160 mm, 80 mm), SS=Separation start, SE= Separation end

deceleration until s/s o= 0.55 has been reached. Thispoint signifies the beginning of the laminar boundarylayer separation and the onset of a separation bubble. Asseen in the subsequent boundary layer discussion, theseparation bubble characterized by a constant C p

-plateau extends up to s/so= 0.746, thus occupying morethan 19% of the suction surface and constituting amassive separation. Passing the plateau, the flow firstexperiences a second sharp deceleration indicative of aprocess of re-attachment followed by a furtherdeceleration at a moderate rate. On the pressure surface,the flow accelerates at a very slow rate, reaches aminimum pressure coefficient at s/so= 0.42 andcontinues to accelerate until the trailing edge has beenreached. Unlike the suction surface, the pressure surfaceboundary layer does not encounter any adverse positivepressure gradient that triggers separation. However,close to the leading edge, a small plateau extending froms/so= 0.08 to 0.16 indicates the existence of a small sizeseparation bubble that might be attributed to a minorinlet flow incident angle.

Considering the unsteady case with the reducedfrequency Q = 1.59 corresponding to a rod spacing ofSR= 160 mm, Figure 4 exhibits a slight difference in thepressure distribution between the steady and unsteadycases. As mentioned above, this deviation is attributedto the momentum deficiency that leads to a reduction ofthe total and static pressure. For Re=110,000, the wakeshave a reducing impact on the streamwise extent of theseparation plateau. As seen in Figure 4 ( a), the trailingedge of the plateau has shifted from s/s o= 0.74 to s/so=0.702. This shift reduced the streamwise extent of the

separation plateau from 19% to 15% of the suctionsurface length which is in this particular case, 21% ofreduction in streamwise extent of the separation.Increasing the reduced frequency to Q=3.18 byreducing the rod spacing to SR= 80 mm causes a slightshift of the Cp- distribution compared with Q=1.59 case.One should bear in mind that pneumatically measuredsurface pressure distribution represents a time integralof the pressure events only.

Increasing the Reynolds number to Re=150,000,has brought major changes in steady state C p

-distribution. The combination of higher Re-number withunsteady wakes reveals the noticeable deviation on thestreamwise extent of the separation plateau. As seen inFigure 4 (b), the trailing edge of the plateau has shiftedfrom s/so= 0.74 to s/s o= 0.702 for Reynolds number of150,000. The combination of higher Reynolds numberwith high unsteady wakes introduce fluctuation kineticenergy into the boundary layer which tends to inhibit theseparation tendency. Cp- distribution clearly shows thatthe wake impingement with higher Reynolds numbershortens the streamwise extent of the separation zonecompared to the steady case. Also, the combination ofhigher Re-number with unsteady wakes reveals that thenoticeable deviation in pressure distribution between thesteady and unsteady cases discussed above isdiminishing with increasing the Re-number as shown inFigure 4(a, b). Two counteracting factors arecontributing to this deviation. The first factor isattributed to the momentum deficiency and theassociated total pressure losses caused by movingwakes, as discussed above. The second factor pertains


to the energizing effect of the impinging wakes on theboundary layer. Although the impinging wakes causevelocity and momentum deficits, their high turbulenceintensity vortical cores provide an intensive exchangeand transfer of mass, momentum, and energy to theblade surface, thus energizing the low energeticboundary layer. In conjunction with the surface pressuredistribution, the kinetic energy of the normal velocityfluctuation component plays a crucial role. In case of alow Re-number flow, the strong damping effect of thewall shear stress has the tendency to reduce the normalcontribution of turbulence kinetic energy, therebydiminishing its surface pressure augmenting effect.Increasing the Reynolds number results in a decrease ofthe damping effect of the wall shear stress, allowing thekinetic energy of the normal velocity fluctuationcomponent to increase the surface pressure, thusoffsetting the wake deficit effects on the pressuredistribution. This fact is clearly shown in Figure 4(a, b),where the pressure distributions of unsteady flow casesat Q=1.59 and Q=3.18 systematically approach thesteady state cases at Re=150,000.

Detailed information regarding the structure of theseparation bubble is delivered by means of a detailedunsteady boundary layer or surface pressuremeasurement by fast response probes, as will bediscussed in the subsequent sections.

Time Averaged Velocity Distributions

Following the surface pressure investigations that mainlyaddressed the onset and extent of the separation zonediscussed previously, comprehensive boundary layermeasurements were performed to identify thestreamwise and normal extent as well as the deformationof the separation zone under unsteady wake flow. Thesteady state case serves as the reference configuration.

Consistent with the surface pressure distributionwhich is discussed above, the effect of the wakefrequency on the time-averaged velocity profiles andfluctuation velocity distribution are presented for onesteady and two unsteady inlet flow conditions on thesuction surface along 31 streamwise locations for theReynolds number of 110,000 and 41 streamwiselocations for the Reynolds number of 150,000. Aftercompleting the velocity measurements, the boundarylayer coordinates were transformed into a bladeorthogonal coordinate system. Velocities at bladenormal positions were obtained by interpolating theirtransformed values. The results showed almost nodifference between the interpolated and non-interpolatedvelocity data. Experimental investigations were

performed for three different values of Q = 0.0, 1.59,and 3.18. These values cover the reduced frequencyrange encountered in LPT-design and off-designoperation conditions.

The effect of wake frequency on time averagedvelocity and velocity fluctuation distributions is shownin Figures 5 to 8 at 6 representative streamwiselocations for Re=110,000 and Re=150,000. Upstream ofthe separation bubble at s/s o= 0.52 and also at itsimmediate proximity s/s o= 0.588, the velocitydistributions inside the boundary layer experience aslight decrease with increasing the reduced frequency.Inside the separation bubble at s/s 0= 0.705, a substantialinfluence of the wake frequency is observed. The higherwake frequency introduces a fluctuation kinetic energyinto the boundary layer trying to reverse the separationtendency. As it can be seen from the velocitydistribution profiles wake impingement shortens thestreamwise extent of the separation zone and decreasesthe bubble height, compared to the steady case,however, the onset of the separation bubble is notchanged. This shows that the flow does not have thecapability to suppress the separation bubble. It onlyreduces the separation bubble height. In the downstreamof the separation bubble, where the flow is fullyreattached, s/s o=0.951, the impact of the wake on theboundary layer is reduced. This effect is clearly shownin the velocity distribution at s/so=0.951. According tothe previous investigations by Schobeiri et al. [10] on aHP-turbine cascade, an increased wake frequency causesturbulence fluctuations to rise insideand outside the boundary layer as shown in Figure 7 and8. However, in the LPT case with the boundary layerseparation once the boundary layer is re-attached andthe velocity distribution assumes a fully turbulentprofile, no major changes are observed either in thevelocity or in the fluctuation velocity distribution.Although Re=150,000 shows the same phenomenon forthe velocity and the fluctuation velocity distribution, itis observed that starting point of the separation bubbleand the re-attachment point move further downstreamto s/so= 0.56 and s/s o= 0.788 respectively. Also, the sizeof the separation bubble is smaller when compared tothat for Re=110,000.

Temporal Behavior of the Separation Zone UnderUnsteady Wake Flow

Velocity distributions on the suction surface withtime as the parameter are plotted in Figure 9 to 12 forRe = 110,000 and 150,000 and Q = 1.59, 3.18. Thenondimensional time (t/ti) values are chosen so that they

10 Copyright © 2005 by ASME

e=110,000, s/so=

norodo 160mm '+

80 mm

e=110,000, s/so=0.502

a0ppnorod o160mm a80mm p

eee Ee 5

Re=11 0,000, s/so=0.520 e9 g08 o 1w. o7 80mm O06 p

O5 pe

4 CO

3

2

1

002 4 6 8V(m/s)

Re=110,000, s/so=0.705 Re=11 0,000, s/so=0.767 Re=110,000, s/so=0.95120 30

18

16 orod25

norodo 160mm O 160mm

14 v 80mm v 80mm20

12

10 15

8r

6 ° °^ Oo vOV 10

° °O

oOv4 Ov

52

0 2 4 6 8 002 4 6 8

V(m/s) V(m/s)

Figure 5. Distribution of time-averaged velocity along the suctionsurface for steady case Q=0 (S R= 4) and unsteady cases Q=1.59(SR= 160 mm) and Q=3.18 (SR= 80 mm) at Re=110,000

Re=150,000, s/s0 =0.53 Re=150,000, s/s 0=0.56 Re=150,000, s/s0=0.72010 1

pp

8 no rodo

0 160 mm pe 80 mm p

p6 p

E o EX e X

4 pp0

464 6

V(m/s)

=150,000, s/s0=0

8

6 norod0 160 mm4 e 80 mm

2

0

8

6

4 oae g e°g 82

00 2 4 6 8

20

18

V(m/s)

Re=150,000, s/s 0=0.77030

V(m/s)

Re=150,000, s/s0=

16 norod25

no rod

140 160 mm o 160 ma 80 mm 20 0 80

12

E 10 E 15X

8X

6 0 ° °°Q O810

4°

° n0 oe° 5

2

00 2 4 6 8 00 2 4 6V(m/s) V(m/s) V(m/s)

Figure 6. Distribution of time-averaged velocity along the suctionsurface for steady case Q=0 (SR=—) and unsteady cases Q=1.59(SR= 160 mm) and Q=3.18 (SR= 80 mm) at Re=150,000


109 0

oov

ppv°8 ° ov ° norod

° pvo

60mm7 0 0 v wmm

o6 oOo

5 0V

ovoov

4 °ov0 w

3 0°d

2

15

0 0 0.1 0.2v(m/s)

v0.3 0.4

0

9 0 0 °v8 0 ov . no rod°ov o 160mm7 o 0

o

80mm

6 0 ovv

5 °° o v

4 o0v°

3Ov

2

1,yy,vv

9"^^

0 0.1 0.2 03v(m/s)

0.4 0.5

norod0 160 mm

80mm

8

10,000,1,000,

no rodp 160mmv 80mm

noroO 160mmv 80mm

0

8

6 norod

4 0160 m80 mm

2

0 AO8 °e p6 8 0° o °

°o4

^p

2

00 0.5 1 1.5

norod

n160 mm80 mm

0,000, s/s o=0.502 Re =110,000, s/s o=0.52 Re=110,000,

0,000,s20

18

16 no rodO 160mm

14 v 80mm

12

10

8

6 00^_G4 ° 0

0002

O v

0 0 0.5v(m/s) 1 v(m/s) v(m/s)

Figure 7. Distribution of time-averaged fluctuation rms velocity alongthe suction surface for steady case Q=0 (S R=—) and unsteady casesQ=1.59 (SR=1 60 mm) and Q=3.18 (SR=80 mm) at Re=110,000

Re=150,000, s/s 0 =0.53 Re=150,000, s/s 0=0.56 Re=150,000, s/s0=0.720

V(m/s) V(m/s) V(m/s

,000, s/s 0 =0.747 Re=150,000, s/s 0=0.770 Re=150,000, s

V(m/s) V(m/s) V(m/s)

Figure 8. Distribution of time-averaged fluctuation rms velocity alongthe suction surface for steady case Q=0 (S R=—) and unsteady casesQ=1.59 (SR=1 60 mm) and Q=3.18 (SR= 80 mm) at Re=150,000


represent the temporal states within one full period ofwake passing. For Re = 110,000 Figures 9(a) to 9(e)show, the velocity distributions inside and outside theboundary layer at fixed s/s o- locations experiencemoderate to pronounced changes. Figure 9(a) representsthe instantaneous velocity distribution upstream of theseparation zone followed by Figures. 9(b,c,d,e) whichrepresent the velocity distributions inside the separationzone. The last Figure 9(f) exhibits the instantaneousvelocity distribution downstream of the separation zone.In discussing the following results, we simultaneouslyrefer to the wake distribution as well as the turbulencefluctuation results.

Figure 9(a) exhibits the velocity distribution on thesuction surface at s/s o = 0.402. At this streamwiseposition, the laminar boundary layer is subjected to astrong negative pressure gradient. The boundary layerdistributions at different (t/ti) experience changes inmagnitude that reflect the corresponding changes of theimpinging periodic wake velocity. It is worth noting,that despite the injection of turbulence kinetic energy bythe impinging wakes, no local instantaneous boundarylayer transition occurs. This is because of the strongnegative pressure gradient that prevents the boundarylayer from becoming instantaneously transitional.Instantaneous velocity distributions inside the separationzone are shown in Figures 9(b,c,d,e,f).

As a representative case, the results plotted inFigure 9(e) at s/s o = 0.674 is discussed. During the timeinterval from t/ti close to 0.5 (1.5, 2.5, etc.) to about t/ ti= 0.75 (1.75, 2.75 etc.), the separation zone is exposedto the wake external flow which is under the influenceof relatively lower turbulence. This flow does not havethe capability to suppress the separation zone. Thus theseparation region is clearly shown by the velocitydistributions at t/ ti = 0.5 and t/ti = 0.75. As the wakepasses over the blade at s/s o = 0.674 introducing highturbulence kinetic energy into the boundary layer, theboundary layer is energized causing the separation zoneto partially reduce or disappear. This leads to aninstantaneous re-attachment. This time intervalcorresponds to the case where the flow is completelyunder the influence of wake and correspondingly thereattached velocity distribution assumes a turbulentprofile characterized by the curves at t/ ti = 1.0, t/ti =0.05, and t/ ti = 0.25 shown in Figure 9(e). To emphasizethis statement, the steady state velocity distribution atthe same streamwise position is also plotted in Figure9(e) using full circles. It shows clearly the separatednature of the boundary layer which coincides with theinstantaneous velocity profile at t/ti = 0.5. Intermediatetimes reflect the gradual change between the separation

and re-attachment as the flow is undergoing theinfluence of the oncoming wake. Moving to the trailingedge of the separation zone, at s/s o = 0.705, Figure 9(f),a partial reduction in boundary layer thickness as theresult of wake impingement is visible, however, theseparation zone does not seem to disappear.

Figure 10 (a) to (f) show the velocity distributionsincident outside the boundary layer at fixed s/s o

-locations experience moderate to pronounced changesat Q = 3.18. Similar instantaneous velocity distributionis observed when operating at a reduced frequency of Q= 3.18. Compared with Q = 1.59, a strongersuppression zone is noticed at Q = 3.18. When the wakepassing frequency is increased over the blade, it is alsoincreasing high turbulence kinetic energy. Therefore,boundary layer is energized comparatively more than thereduced frequency of Q = 1.59 and it causes theseparation bubble partially reduce more.

As it is seen in Figure 11 and 12, increasing theReynolds number to 150,000 causes starting point of theseparation bubble and the re-attachment point movefurther downstream.. It is also observed that increasingthe Reynolds number reduces the size of the separationbubble.

Behavior of Separation Bubble Under Wake Flow

The effect of the periodic unsteady wakes on theonset and extent of the separation bubble is shown inFigure 13 to 16 for Re=110,000 and 150,000 and fortwo different frequencies, namely Q=1.59 and Q=3.18.These figures display the full extent of the separationbubble and its behavior under a periodic wake flowimpingement at different t/ti . The wake propagation forQ=1.59 and Q=3.18 is analyzed, and the value of t/ticorresponds to the point in the cycle at which the dataacquisition system is triggered. During a rod passingperiod, the wake flow and the separation bubbleundergo a sequence of flow states which are notnoticeably different when the unsteady data are time-averaged. The temporal changes of the spatial positionof the separation bubble, is compared with the time-averaged separation bubble. Starting with a reducedfrequency of Q=1.59 at t/ti=0.25, the separation bubbleis under a full influence of the wake. The wake passingover the blade introduces a high turbulence kineticenergy into the boundary layer. The energized boundarylayer bubble is partially reduced or disappearedcompared to the time-averaged separation bubble sizeshown in Figure 13(a). As the wake passes, t/ti=0.50,the height of the separation bubble reaches its maximumsize at s/so= 0.705. The contraction starts again that


(f) Re=q=3.1

t/s=0.05e t/s=0.25

t/s=0.500 t/s=0.750 t/s=1

30

25

20

E15

10

5

(a) Re=110,000 evaq=1.59,s/s o=0.402 nvW

nvW

t/s=0.05 evmnaw

n t/s=0.25 nvmt/s=0.50 ^®mm

0 t/s=0.75 a m0 t/s=1 am E

♦ W Ea m .Z♦ mam♦m♦ Wamaw

(b) Re =110,000 vmeI q=1.59,s/so=0.52 vmn

ownI t/s=0.05 oowoe

n t/s=0.25 ocont/s=0.50 voon

0 t/s=0.75 vôoe0 t/s=1 voon

10(c) Re =110,000

9 q=1.59,s/so=0.588

8 t/s=0.05

7n t/s=0.25

t/s=0.50

6 0 t/s=0.75E a t/s=1,EE 5

4

3

2

1

(d) Re=110,000 aW eq=1.59,s/so=0.651 °W e

vao neoe

t/s=0.05 upone t/s=0.25 voo n

t/s=0.50 mo n0 t/s=0.75 0 o eO t/s=1 •oe

14 (e) Re =110,000q=1.59,s/so=0.674

12t/s=0.05

e t/s=0.2510 t/s=0.50

0 t/s=0.75

E8 O t/s=1

T • norod

30(f) Re=110,000

q=1.59,s/so=0.70525

t/s=0.05e t/s=0.25

20 t/s=0.50n 0 t/s=0.75

E 0 t/s=1e 15

10 v ° 0 0v vg° vmg^°ôo°

5 ^° o

Figure 9. Distribution of the ensemble-averaged velocity developmentalong the suction surface for different s/s o with time t/ ti as parameterfor Q=1.59 (S R= 160 mm) and Re=110,000

10 10(b) Re=110,000 vecm

9 q=3.18,s/so=0.52 9

8arom

t/s=0.05 ^ 8

7n t/s=0.25 êooa 7

t/s=0.50 voo6 0 =0.75voo 6E o

tft7s=1 voo E

5 5

4 00om o 4

3vao

3

2 2

1

(e) Re=110,000 MmI q=3.18,s/so= 0.674

^m

I an Wt/s=0.05 e•®

e t/s-.25 ,a• wt/s .0 v w

0 t/s=0.75 0.O t/s=1 •o m

norod

(a) Re=110,000q=3.18,s/s o=0.402 mn

t/s=0.05 mmmm

n t/s=0.25 mrt/r--0 oe.50 00

0 t/s=0.75 000 t/s=1 me

osw

(d) Re=110,000 °wo

q=3.18,s/s o=0.651 °°monanoevma

t/s=0.05e t/s=0.25 4b

t/s=0.50 ro0 t/s=0.75 aO t/s=1 a m

(c) Re=110,000q=3.18,s/so=0.588

t/s=0.05n t/s=0.25

t/s=0.500 t/s=0.75a t/s=1

Figure 10. Distribution of the ensemble-averaged velocitydevelopment along the suction surface for different s/s o with time t/tias parameter for Q=3.18 (S R= 80 mm) and Re=110,000


0 10(b) Re=150,000

9 q=3.18,s/so=0.56 orov 9

8wov

t/s=0.05 owv 8

7 t/s=0.25 7t/s=0.50

6 0 t/s=0.75 60 t/s=1 E

5 5X

4 m0

4

3 0 3

2 2

1 1

0 0.0 2V(n/s)

6 8

(a) Re=150,000 °-0Oq=3.18,s/s o=0.55 vo

voot/s=0.05 voo

° t/s=0.25 vot/s=0.50 a•o

0 t/s=0.75 po0 t/s=1

(c) Re=150,000 gym®q=3.18,s/so=0.6 m

o®t/s=0.05 0

° t/s=0.25 • mt/s=0.50

0 t/s=0.75a t/s=1

(a) Re=150,000 rv0oq=1.59,s/s o=0.55 mom

oomt/s=0.05 too

° t/s=0.25 e00t/s=0.50 too

o t/s=0.75 oom0 t/s=1 m

1(b) Re=150,000 -0

I q=1.59,s/so=0.56 msvoI ^^

t/s=0.05 mv0° t/s=0.25 mvo

t/s=0.50 mo t/s=0.75 my0 t/s=1 ,,,,, c

(c) Re=150,000 ° ®'q=1.59,s/so=0.6

o aet/s=0.05 O ma

° t/s=0.25 0 oat/s=0.50 0

o t/s=0.75 0a t/s=1 O

00o meoam

(d) Re=150,000 O o^ vq=1.59, s/so=0.720 o me o

o ^ vt/s=0.05 0 om

° t/s=0.25 o m+t/s=0.50 0

o t/s=0.75 0 w_iO t/s=1 0 ^^

a m1o O 40

v O e mv • 00

o m•o

I (e) Re=150,000 o m• v 1q=1.59, s/so=0.746 o m• v

1

om•v

t/s=0.05 m • v

° t/s=0.25 o m • vt/s=0.50 0* IM 1

o t/s=0.75 owf

O t/s=1 o m•

• rotl 0no •w o E0 ome

b v oo .l• m OI

v O eODI • •O ° m

• P m•O

(f) Re=150,000 m`q=1.59, s/so=0.770 me

me

t/s=0.05 ® em e •pa° t/s=0.25 o e cv

t/s=0.50 m eo vt/s=0.75 0° •

me0 t/s=1 e• norod m°

•® o0 vp0°

•O m V.p OAA Ot

•4A° e•060 °


(d) Re=150,000q=3.18,s/s o=0.720 m w

m omt/s=0.05 w ow

° t/s=0.25 w ot/s=0.50 veo

0 t/s=0.75 pe0O t/s=1 mo

4 ap

e ® P° vm0

I (e) Re=150,000 ô ® 1q=3.18,s/so=0.746 o ^

o ^ 1

t/s=0.05 of aro

° t/s=0.25 alI t/s=0.50 a ® 1

0 t/s=0.75 •

O t/s=1 • •

• no rod p0° Eve O

• • O O• e-0

et 00I • • e vD 00

ev °dp^ D O

(f) Re=150,000 °iO1q=3.18,s/so=0.770 o ml

Om1oml

t/s=0.05 00° t/s=0.25 00.1

t/s=0.50 0®0 t/s=0.75 o a

omst/s=1 amm•no rod omte•

OHO00•e

O ^°Cq e°

••.

O 00 •e^b OSe



reduces the size of the separation bubble at t/ti=0.75. Ats/so=0.705 the core region has slightly moved towardsto the leading edge. At t/ti=1, the full effect of the wakeon the boundary layer can be seen before another wakeappears and the bubble moves back to the originalposition. Once the wake starts to penetrate into theseparation bubble, the turbulent spot produced in thewake paths causes a total suppression at somestreamwise positions. Similar results are observed whenoperating at a reduced frequency of Q=3.18 andReynolds number of 150,000. It is observed thatincreasing Reynolds number to 150,000 moves theseparation bubble further downstream and the wakefrequency exerts no influence on the position of theseparation bubble. However, doubling the reducedfrequency and increasing the Reynolds number isassociated with the higher turbulence intensity that leadsto stronger suppression of the separation bubble asshown in Figure 14 to 16.

CONCLUSIONS

A detailed experimental study on the behavior ofthe separation bubble on the suction surface of a highlyloaded LPT blade under periodic unsteady wake flowwere investigated. The measurements were carriedutilizing a custom designed hot-wire probe. One steadyand two different unsteady inlet wake flow conditionswith the corresponding passing frequencies, the wakevelocity and the turbulence intensities were investigatedby utilizing a large-scale, subsonic research facility.Translational motion of the timing belt created theunsteady wake flow. It is found that the turbulent spotsgenerated by the unsteady wake flow were effective insuppressing or reducing the size of the separationbubble. The results of the unsteady boundary layermeasurements were presented in the ensemble-averaged,and contour plot forms. Surface pressure measurementswere performed at Re= 110,000 and Re=150,000. Ateach Reynolds number, one steady, and two periodicunsteady inlet flow measurements were performed.Slight changes of the pressure distribution occurred,while operating at the unsteady flow conditions.Increasing the Reynolds number to Re=150,000, hasbrought major changes in steady state Cp- distribution.The combination of higher Reynolds number with highunsteady wakes introduce fluctuation kinetic energy intothe boundary layer which tends to reverse the separationtendency. Cp- distribution clearly shows that the wakeimpingement with higher Reynolds number shortens thestreamwise extent of the separation zone compared tothe steady case. Detailed unsteady boundary layer

measurement identified the onset and extension of theseparation bubble as well as its behavior under theunsteady wake flow. Passing the wake flow with itshighly turbulent vortical core over the separation region,caused a periodic contraction and expansion of theseparation bubble and a reduction of the separationbubble height. Increasing the passing frequencyassociated with a higher turbulence intensity furtherreduced the separation bubble height. It is observed thatstarting point of the separation bubble and the re-attachment point move further downstream to s/s o= 0.56and s/s o= 0.788 respectively by increasing the Reynoldsnumber to 150,000. Also, the size of the separationbubble is smaller when compared to that forRe=110,000.

UNCERTAINTY ANALYSIS

The Kline and McClintock [29] uncertainty analysismethod was used to determine the uncertainty in thevelocity after calibration and data reduction for thesingle-wire probe. The Kline and McClintock methoddetermines the uncertainty with a 95% confidence level.The uncertainty in the velocity for the single-wire probeafter the data reduction is given in Table 3. As shown,the uncertainty in the velocity increases as the flowvelocity decreases. This is due to the pneumatic pressuretransducer having a large uncertainty during calibration.

Table2: Uncertainty in velocity measurement for hot-wire probe.

U (m/s) 3 5 12

G)U/U (%)red 5.78 2.41 1.40

ACKNOWLEDGMENTS

The presented study is a part of an ongoing LPT-aerodynamics project executed by the NASA GlennResearch Center. The authors were supported by NASACooperative Agreement NCC3-793 monitored by Dr.David Ashpis. The support and the permission forpublication is gratefully acknowledged. The authors alsogratefully acknowledge Pratt&Whitney for providingthe research community with the blade coordinates.

16


(b)10

9V(m/s) 88.007.507.00 76.506.005.50 65.004.50 54.00 X3.503.00 42.502.00 31.501.00

2

1

(a)10

9

8

7

6

E 5

3

2

1

®=1.59, t/s=0.25

(b)10

9

V(m/s) 88.007.507.00 76.506.005.50 65.00 E4.50__ E 5

3

2

1

(a)10

9

8

7

6

E 5

3

2

1

®=3.18, t/s=0.25 ®=3.18, t/s=0.50

V(m/s)8.007.507.006.506.005.505.004.504.003.503.002.502.001.501.00

®=1.59, t/s=0.50

Y III ( .

1

s/so

0.5 0.6 0.7 0.8s/s

o

(c) ®=1.59, t/s=0.75 (d) ®=1.59, t/s=1.010 10

9 9

8V(m/s) 8

V(m/s)

8.00 8.00

7.50 7.50

7 +7.00 7 7.00

6.50 6.50

6.00 6.00

6 5.50 6 5.50

5.00EE 5.00

5 ^ \ 4.50

r 54.00 4.00

3.50 3.50

4 3.00 4 3.00

2.50 2.50

3 3 1.50

1.00 1.00

2 2

1 1

0.5 0.6 s/so

0.7 0.8 0.5 0.6 s/ 0.7 0.8

Figure 13. Ensemble-averaged velocity contours along the suction surface for different s/s 0 withtime t/ti as parameter for Q=1.59 (S R= 160 mm), Re=110,000

V(m/s)8.007.507.006.506.005.505.004.504.003.503.002.502.001.501.00

s/so s/so

(c) ®=3.18, t/s=0.75 (d) ®=3.18, t/s=1.0

10 10

9 9

V(m/s) V(m/s)

8 88.00 8.00

7.50 7.50

7 7.00 7 7.00

6.50 6.50

6 '•6.00

5.50 6 '

5.50

E ^. 5.00 E 5.00

I 5 • • r 54.00

3.50,N 3.50

4 3.00 4 3.00

2.50 2.50

3 \ 2.00 9 200

1.50 1 1.50

,r 1.00 Y 1.00

2 2

1 1

0.5 0.60.7 0.6 - 0.5 ae 0.7 0.8s/so s/s

o

Figure 14. Ensemble-averaged velocity contours along the suction surface for different s/s 0 withtime t/ti as parameter for Q=3.18 (S R= 80 mm), Re=110,000


(b)10

9V(m/s) 89.508.898.29 77.687.076.46 65.86

E5.25 54.64 X4.043.43 42.822.21 31.611.00

2

1

(a)10

9

8

7

6

E 5X4

3

2

1

®=1.59, t/s=0.25 ®=1.59, t/s=0.50

V(m/s)9.508.898.297.687.076.465.865.254.644.043.432.822.211.611.00

(d)10

9V(m/s) 89.508.898.29 77.687.076.46 65.86 E5.25

EZ 54.644.043.43 42.822.21 31.611.00

2

1

(c)10

9

8

7

6EEZ 5

4

3

2

1

s/s o

®=1.59, t/s=0.75

s/so

®=1.59, t/s=1.0

V(m/s)9.508.898.297.687.076.465.865.254.644.043.432.822.211.611.00

(b)10

9V(m/s) 89.508.898.29 77.687.076.46 65.86

E5.25 54.64 X4.043.43 42.822.21 31.611.00

2

1

(a)10

9

8

7

6

E 5X4

3

2

1

®=3.18, t/s=0.25 ®=3.18, t/s=0.50

V(m/s)9.508.898.297.687.076.465.865.254.644.043.432.822.211.611.00

(d)10

9V(m/s) 89.508.898.29 77.687.076.46 65.86 E5.25

EZ 54.644.043.43 42.211.61 3

2

1

(c)10

9

8

7

6EEZ 5

4

3

2

1

s/s o

®=3.18, t/s=0.75

s/so

®=3.18, t/s=1.0

V(m/s)9.508.898.297.687.076.465.865.254.644.043.432.822.211.611.00

s/s o s/s o

Figure 15. Ensemble-averaged velocity contours along the suction surface for different s/s 0 withtime t/ti as parameter for Q=1.59 (SR= 160 mm), Re=150,000

s/s o s/s o

Figure 16. Ensemble-averaged velocity contours along the suction surface for different s/s 0 withtime t/ti as parameter for Q=3.18 (SR= 80 mm), Re=150,000


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[28]Eifler, J., 1975, “Zur Frage der freien turbulentenStrömungen, insbesondere hinter ruhenden undbewegten Zylindern,” Dissertation D-17,Technische Hochschule Darmstadt, Germany.

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