LOW-FREQUENCY FLOWFIELD UNSTEADINESS DURING AIRFOIL STALL
AND THE INFLUENCE OF STALL TYPE
Andy P. Broeren* and Michael B. Braggt
University of Illinois at Urbana-ChampaignUrbana, Illinois 61801
The flow past stalled airfoils is generally unsteadyand can result in large force fluctuations. This paperaddresses the relationship between airfoil stall type andthe level of large-scale unsteady flow at stall. A totalof 12 airfoils, encompassing different stallingcharacteristics were tested over a large angle of attackrange at a Reynolds number of 300,000. Time-dependent lift data, wake hot-film data and flowvisualization data were acquired. The time-dependentlift data were low-pass filtered with a 20 Hz cut-off toremove unwanted contributions to the fluctuating lift(Clrms) from model and lift-balance resonances. Theresults show that airfoils having trailing-edge stallexperience minimal lift fluctuations at stall (Clrms <0.04). The fluctuating lift for leading-edge stall airfoilsincreases sharply (to C,rms ~ 0.04) with the abrupt lossof lift associated with this stall type. For thin-airfoilstall types, the fluctuating lift gradually increases tohigh levels (0.04 < C,rms < 0.08) as maximum lift isattained. Finally, for airfoils having a combination ofthin-airfoil and trailing-edge stall the lift fluctuations atmaximum mean lift were very high (Clnns > 0.08). Thefluctuating lift spectra for the latter two stall typescontained distinct low-frequency peaks, indicating thelarge-scale unsteadiness associated with these stalltypes.
Introduction
The flow past airfoils inclined at stalling angles ofattack can be dominated by large-scale unsteady flow,which develops over the airfoil despite steady free-stream and surface-boundary conditions. Thisflowfield unsteadiness can cause large force changes on
airfoil models, wings, rotor blades and the like. Indeed,as early as the 1930's, B. Melvill Jones1 observed"violent fluctuations" of lift and drag on airfoil modelsnear stalling conditions. Mabey2 suggests that low-frequency force changes on airfoils are a likely cause ofwing buffet. Zaman, McKinzie and Rumsey3 reporteda low-frequency, quasi-periodic oscillation of the flowover an airfoil near stall and argue that the frequencyand large lift oscillations may be responsible forinstigating stall flutter of wings and blades. A betterunderstanding of the unsteady flows past stalled airfoilsis therefore required to avoid potential damage toaircraft or machinery and improve safety.
Airfoil stall can be classified into three basic typesbased upon the time-averaged characteristics of theflowfield. Following the work of Jones,1 McCulloughand Gault4 conducted more detailed stall testing andestablished the presently accepted definitions of airfoilstall type. Trailing-edge stall is preceded by movementof the turbulent boundary-layer separation pointforward from the trailing edge with increasing angle ofattack. Leading-edge stall has abrupt flow separationnear the leading edge generally without subsequentreattachment. The "abrupt" separation usually resultsfrom a small laminar separation bubble which "bursts"at stall and usually causes a sharp decrease in lift.Thin-airfoil stall is preceded by flow separation at theleading edge with reattachment (laminar separationbubble) at a point which moves progressivelydownstream with increasing angle of attack. Airfoilstall type is a function of several variables such asReynolds number, surface roughness or free-streamturbulence. Therefore, any particular airfoil mayexhibit a combination of stall types, or its stall typemay change when flow conditions are changed.
* Graduate Research Assistant, Department of Mechanical and Industrial Engineering, Member AIAAT Professor, Department of Aeronautical and Astronautical Engineering, Associate Fellow AIAA
Airfoil stall is a complex fluid flow probleminvolving separated flow, and some level ofunsteadiness is expected. There is evidence in theliterature that some stall types exhibit more intenseunsteadiness than others, but there are conflictingconclusions. For example, Jones1 reported that the"violent fluctuations" of lift and drag occurred fortrailing-edge and thin-airfoil stall airfoils, but not forleading-edge stall airfoils. On the other hand,McCullough and Gault4 state that an NACA 63!-012airfoil, which has a leading-edge stall type, experienced"violent buffeting" at angles of attack just beyond themaximum lift coefficient (C/max). Also, Gault5 observedunsteady flow past a leading-edge stall NACA 63-009at Clmax. There is some agreement for thin-airfoil stalltype, as McCullough and Gault6 reported low-frequency flow unsteadiness for an NACA 64A006airfoil at angles of attack near stall. This airfoil has athin-airfoil stall type, and these observations areconsistent with Jones.1 Mabey2 reviews some existingdata in an attempt to predict the level of root-mean-square normal force fluctuations at stall, but he doesnot address airfoil stall type. These studies did notcontain detailed data about the frequency content of theunsteady flows, which is important in connecting theforce changes to buffet or stall flutter.
As mentioned above, Zaman et al.3 conducted adetailed study of unsteady flow past an LRN(1)-1007airfoil near stall and reported Strouhal numbers of 0.02for the flow oscillation at angles of attack near stall.Here, the Strouhal number is defined as: St = fcsina./t/w, where / is the dimensional frequency, c is thechord and (/„ is the free-stream speed. The value of St= 0.02 is considered very low as it is an order ofmagnitude lower than that of bluff-body shedding,which typically has frequencies of St = 0.2. For thisreason, this behavior has simply been called the "thelow-frequency flow oscillation" to distinguish it frombluff-body shedding. The data indicated that theunsteady oscillation was very intense and involved aperiodic switching of the airfoil flowfield betweenstalled and unstalled conditions. The correspondingforce fluctuations were very large, up to 50% of themean lift coefficient. The authors classified theLRN(1)-1007 airfoil as having a combination of thin-airfoil and trailing-edge stall types.
Research into this low-frequency oscillation onthe LRN(1)-1007 airfoil was subsequently performedby others7"10 and the features of unsteady flowfield arewell known. For example, Bragg et al.9 present flowvisualization data which clearly shows the laminarseparation bubble near the leading edge increase inchordwise extent (characteristic of thin-airfoil stall) andturbulent boundary-layer separation (characteristic of
trailing-edge stall) as the angle of attack is increased tomaximum lift. Broeren11 performed two-componentlaser-Doppler velocimeter measurements for theunsteady flowfield on the LRN(1)-1007. These resultsshowed an interaction between the separation bubblereattachment and the turbulent boundary-layerseparation as the airfoil stalls and unstalls. Thus, theremay be a relationship between the airfoil stallingcharacteristics and the low-frequency oscillation.Further, various reports show little fundamental changein the character of the low-frequency oscillation over aReynolds number range from 75,000 to 1,400,000.7'9Very similar low-frequency unsteady stall behavior hasbeen documented for a variety of Reynolds numbers inother studies2' 12~14 and the reported frequencies convertto Strouhal numbers less than or approximately equal to0.02.
In an effort to better understand what factorscontribute to unsteady flow near stall, this studyfocuses on relating the level of flowfield unsteadinessto airfoil stall type. The purpose of this paper is toresolve the apparent disagreement over which airfoilstall types contain large-scale unsteady flow near stalland determine if there is a relationship between stalltype and the low-frequency flow oscillation assuggested above. To accomplish these objectives,time-dependent lift measurements, wake hot-filmvelocity measurements and flow visualization werecarried out for a total of 12 different airfoils,encompassing different stalling characteristics. Thetwo-dimensional airfoil models were tested over anangle of attack range from 0° to 25° and at a chordReynolds number (Re) of 300,000. The level ofunsteadiness was determined from the root-mean-square of the fluctuating lift coefficient (Clrms) and thefrequency content was determined from spectralanalysis of the lift and wake hot-film data. The liftcurves and surface-oil-flow visualization data wereused to determine the stall type of each airfoil tested.For the airfoils tested here, the results showed that thin-airfoil and combination thin-airfoil and trailing-edgestall types had the most intense low-frequency liftfluctuations, occurring very close to C/max. The trailing-edge and leading-edge stall types did not exhibit largefluctuations until after the angle of attack had beenincreased above maximum lift. These results arediscussed in concert with previously published results.
Experimental Method and Apparatus
Wind-Tunnel Facility and Ancillary Equipment
All measurements were carried out in theSubsonic Aerodynamics Laboratory at the University
197American Institute of Aeronautics and Astronautics
of Illinois, utilizing the low-speed, low-turbulence windtunnel. The wind tunnel is a conventional indraft open-return type which has a four inch thick honeycombflow straightener followed by four anti-turbulencescreens. The turbulence levels in the empty 3 foot by 4foot test section are less than 0.1% at all operatingspeeds.
The general experimental apparatus is shown inFig. 1. The airfoil models (12-inch chord by 33.625-inch span) were mounted horizontally between 3/8-inchthick Plexiglas splitter plates to isolate the ends of themodel from the tunnel side-wall boundary layers andthe support hardware. The gap between each end of themodels and the splitter plates was nominally 0.05inches. One end of the airfoil model (far side of Fig. 1)was actuated to adjust and measure the angle of attack.The angle of attack was measured using a Bournsmodel 6574 precision rotary potentiometer. As shownin Fig. 2, the opposite end of the model is connected tothe lift carriage which contains linear ball bearings andspherical bearings to minimize frictional effects invertical translation on a precision ground shaft. The liftforce was measured directly via a connecting rod fromthe lift carriage to an Interface Inc. SM-25 strain gaugeload cell. A compression spring was used to supportthe weight of the lift carriage and model. For thepresent experiments, the load cell was routinelycalibrated in its measurement position to determine theeffects of mechanical hysteresis and/or othernonlinearities. The free-stream dynamic pressure wasmeasured upstream of the model between the splitterplates with a single pitot-static probe.
The frequency content of the flow past the airfoilswas measured in the wake. A TSI 1210-20 hot-filmprobe was mounted in a single location on a rigid strutat 0.47 chords downstream and 0.20 chords above theairfoil trailing edge at a = 0° (see Fig. 2). This probelocation corresponded to previous wake frequencymeasurements and the phase-averaged LDV data ofBroeren and Bragg.10 The hot-film sensor was notcalibrated to compute velocity as only the voltage wasrequired to provide frequency information.
A total of 12 airfoil sections were tested in thisstudy and are shown in Fig. 3. The airfoils wereselected based upon their expected stall type.References 15-17 provide the coordinates, moreperformance data (including drag) and contouraccuracy data for these models.
Data Acquisition
All of the ambient conditions and wind-tunneldata were acquired using an IBM-compatiblemicrocomputer equipped with an analog-to-digital
Fig. 1 General experimental apparatus (splitterplates not shown for clarity).
Strain-GaugeLoad Cell
Fig. 2 Side-view of the test-section apparatus.
MB253515 NACA64A010
NACA 0009
NACA2414 MA409
FX63-137
CLARK-Y
E374
E387
Fig. 3 The airfoils tested.
198American Institute of Aeronautics and Astronautics
(A/D) conversion board. All data were acquired at Re =300,000 and for a = 0° to 25°. The lift and hot-filmsignals were sampled for 5 seconds at a rate of 1,000samples/sec (5,000 samples) and all of the time-dependent voltages were written to disk. The lift andhot-film signals were (analog) low-pass filtered with a500 Hz cut-off. Detailed data were acquired for airfoilsexhibiting unsteady stalling characteristics. Powerspectra of both the lift and hot-film signals wereacquired where appropriate using a Wavetek model5830A Digital Signal Analyzer which interfaced withthe data acquisition computer via GPIB.
Flow Visualization
Surface-oil-flow visualization was also performedfor each of the airfoils tested. A light coat of oilcontaining fluorescent dye was sprayed on the surfaceof the model. The oil was allowed to flow for 20 to 30minutes with the tunnel on. The resulting flow patternsin the oil gave information regarding time-averagedboundary-layer separation, reattachment and transition.These features were recorded for each airfoil as theangle of attack was increased into stall. The boundary-layer features can generally be determined to within±2% chord.
Data Reduction and Uncertainty
The time-dependent lift voltages were digitallyfiltered and the lift coefficient (C,) was calculated andcorrected for wind-tunnel interference effects. Detailson the digital filtering process are discussed below.The wind-tunnel correction procedures were carried outusing methods similar to those given by Selig et al.15
and Giguere and Selig.18 The root-mean-square of thefluctuating lift coefficient (C,rw) was also computedfrom the time series data.
Strouhal numbers were calculated from the powerspectra of the both the lift and hot-film signals. Thepeak frequency of each power spectrum wasdetermined as the midpoint of the -3 dB bandwidth.This frequency and tunnel conditions were then used tocalculate the Strouhal number. The uncertainty in thepeak frequency was estimated to be plus or minus onehalf of the -3 dB bandwidth. Therefore, the uncertaintyin the Strouhal number is higher for broader low-frequency peaks. The typical relative uncertainty in theStrouhal number was ±3 %.
The experimental uncertainty was computed forthe reduced quantities following the method presentedin Coleman and Steele.19 These calculations wereincluded in the data reduction routines so that thevariation in the uncertainty with angle of attack could
be easily ascertained. The calculated uncertainties onlyinclude bias errors based upon 20:1 odds. Theuncertainty in the angle of attack was ±0.15°. Therelative uncertainty in the freestream velocity was±0.90% at 50 ft/sec. The uncertainty in the mean liftcoefficient is shown in Fig. 4, along with data fromother facilities. The error bars are ±2 to 3 % of themean C/. Agreement amongst the data is very goodwithin the linear range and diverges after stall whichmay be due to large-scale unsteadiness which occursfor the E387 airfoil.
1.50n
1.25-
1.00-
(J 0.75-
0.50-
0.25-
0.00-
E387 ;
Present Data :Ref. 17 :Ref. 20 :
0.0 5.0 20.0 25.010.0 15.0a(Deg.)
Fig. 4 Comparison of present data with data fromother facilities for the E387 airfoil.
Lift-Balance Frequency Response
The lift balance used in this study was designedfor unsteady lift measurements where the frequencyresponse was a primary concern. Fortunately, the flowfrequencies investigated were very small, less than 10Hz. Several attempts were made to model and quantifythe dynamics of the lift-balance mechanism and someof these results are documented here.
A simple second order system model of the liftbalance was used to estimate the natural frequency ofthe system as well as the magnitude and phaseresponse. Since the strain-gauge load cell deflectedlinearly over its operating range it was modeled as aspring with a constant of 2100 Ibf/in. The smallcompression spring was of negligible stiffness relativeto the load cell. The mass of the system was 3.2 Ibm.The resulting natural frequency was approximately 80Hz. A second order system model suggests that thereshould be no magnitude attenuation and no phase
199American Institute of Aeronautics and Astronautics
difference for frequencies much less than the naturalfrequency, depending on the amount of damping withinthe system.
The characteristics of the airfoil models alsoplayed a role in the overall system dynamics. Asshown in Fig. 3, the airfoils vary in thickness and therewas a large variation in the structural compositions.The result of these two effects was a large range in thenatural bending and torsional frequencies. Thesefrequencies were estimated for some of the modelsassuming that the models were simply supportedbeams. The fundamental bending resonance was high,on the order of 100 Hz for the LRN(1)-1007 model.However, the fundamental torsional resonance wasmuch lower, about 45 Hz. These are crude estimates,given the complexity of the geometry and compositecomposition, however, agreement with experiment isfairly good, as shown below.
Detailed investigations were conducted toevaluate the estimates of the natural frequencies givenabove. An accelerometer was used to analyzevibrations with the flow on. The spectra shown in Fig.5 compare the frequency content of the wake hot-film,lift-balance and accelerometer (located on the modellower surface at the near end in Fig. 2) signals. Theairfoil in Fig. 5 is the previously studied LRN(1)-1007and the low-frequency peaks (at « 4 Hz and « 8Hz) inthe hot-film spectra correspond to the fundamental andfirst harmonic of the low-frequency oscillation. Thelift-balance spectrum also contains these peaks, and inaddition, the broad peaks centered at approximately 38Hz and 85 Hz. These frequency peaks correspond tothe fundamental torsional resonance of the model andthe lift-balance resonance, respectively. This issupported by other evidence as well. For example,when the freestream velocity was reduced by a factor oftwo, the low-frequency oscillation peaks occurred at alower frequency, consistent with previous data.3'7However, the 38 and 85 Hz peaks remained at thesefrequencies which further suggests they are structural inorigin. The lift spectra of other airfoils exhibiting low-frequency oscillations also contained a similar 85 Hzpeak. The 38 Hz peak was not the same, but usually itoccurred between 30 and 50 Hz. This further supportsthe conclusion that the 85 Hz peak is related to the liftbalance and the 38 Hz peak is related to the modelstructure. In another test, a slight tapping on thesurface of the airfoil model produced a frequency ofabout 40 Hz from the accelerometer. This again wouldindicate that the estimated torsional resonance of 45 Hzis fairly accurate.
It was also important to determine the magnitudeand phase response of the system at the lowfrequencies. A rig was designed to drive the model at a
10 n
-10-
3- "20 "0)I -30 Ho.S -40 -
-50 -
-60-
-70
Accelerometer
0 10 20 30 40 50 60 70 80 90 100Frequency (Hz)
Fig. 5 Comparison of spectra from wake hot-film,lift-balance and accelerometer signals for theLRN(1)-1007 airfoil at a = 15°.
'•5°lE374
1.25-
1.00-
0.75-
0.50-
0.25
0.00
D&:o
rO.24
-0.20
-0.16
-0.12 4
-0.08
-0.04
0.005.0 10.0 15.0 20.0 25.0
a (Deg.)
A GI Increasing aT C, Decreasing aA Clrms Increasing a, 20 Hz Low-Pass Filterv C,rm Decreasing a, 20 Hz Low-Pass FilterO c[rm All cc, Unfiltered
Fig. 6 Effect of filtering lift data for the E374 airfoil.
known input frequency. The lift-balance signal (theoutput) and the input signal were measuredsimultaneously for comparison in the time domain.This procedure was repeated for several frequencies.The results showed no magnitude attenuation orphase difference up to a frequency of 3.5 Hz.Unfortunately, this procedure could not be carried outat higher frequencies due to equipment limitations.
200American Institute of Aeronautics and Astronautics
The 80 Hz frequency was also present in the lift-balance data, suggesting it is structural in origin (the liftbalance) and not flow related. Based upon this and thevalidity of the second order system model, the time-dependent lift data should suffer negligible magnitudeattenuation or phase discrepancy for frequencies lessthan 10 Hz.
The lift signal was digitally low-pass filtered witha 20 Hz cut-off to remove the unwanted contribution ofmodel and lift-balance natural frequencies. The 8th
order Butterworth filter was designed using MATLABand implemented using a zero-phase delay, forward-reverse filtering algorithm. This means that the datawere actually filtered twice (in opposite directions intime) to ensure that there was no phase delay, whichresulted in a 16th order filter. Filtering the data had theeffect of lowering the Clrms, however, this effect wasrelatively small, as shown in Fig. 6. The figureillustrates that most of the energy of the lift fluctuationswas contained within the low frequencies. Low-passfiltering the Clma data with a 20 Hz cut-off eliminatedthe model and lift-balance resonances and ensured thatthe data from each airfoil are very comparable.
Results
Time-Dependent Lift Data
A summary of the time-dependent lift data isshown in Table 1, which classifies the airfoils tested bystall type. The stall type was determined frominterpretation of the lift curves and flow visualizationdata. These data show that the airfoils having thin-airfoil stall and a combination of thin-airfoil andtrailing-edge stall have the highest Clrms levels at
maximum lift, indicating large-scale unsteady flow. Ofthese, the Clrms values for the combination stall type arenearly twice that for the pure thin-airfoil stall cases.The variation of Clfna with angle of attack is presentedbelow for representative airfoils in each category andthe frequency content of the fluctuating lift is addressedin the following section.
The mean and fluctuating lift coefficients (C, andC,rms) for the Ultra-Sport airfoil are shown in Fig. 7a asa function of angle of attack. The variation in C/ at stallwas typical for the trailing-edge stall type. The plotshows how the Clrms gradually increased as a increasedbeyond maximum lift. The relatively high levels ofClrms (~ 0.04) occurred for very high angles of attack (a> 18°) and can be attributed to broad-band unsteadiness(from 0 to 20 Hz), with bluff-body shedding beginningat a = 22°.
This behavior contrasted with the E472 which hada leading-edge stall type. The lift data, Fig. 7b, showthe abrupt loss of lift at stall which is a trademark of theleading-edge stall type. The Clrms level also changedabruptly with the loss of lift, increasing from less than0.01 to almost 0.04. It is also interesting to note thatthe Clrms remained high until the lift was recovered asthe angle of attack was decreased beyond stall. Thishysteresis in the mean lift is fairly well known21 and thepresent data suggest that the fluctuating lift exhibitssimilar behavior.
Data for the combination trailing-edge andleading-edge stall type are shown in Fig. 7c, for theclassic CLARK-Y airfoil. This combination stall typewas determined from interpretation of the lift and flowvisualization data. The mean and fluctuating lift trendscontained elements of each stall type. The mean liftgradually decreased beyond stall (characteristic of
201American Institute of Aeronautics and Astronautics
trailing-edge stall) until a = 19°, where an abrupt dropoccurred (characteristic of leading-edge stall). Thefluctuating lift gradually increased to this point as well,then increased sharply and there was some hysteresis.
The lift data for the NACA 64A010 shown in Fig.7d illustrates typical trends for airfoils having the thin-airfoil stall type. There was a distinctive change inslope associated with the formation of a separationbubble at a = 4° in Fig. 7d and a gentle stall, followedby gradual lift recovery with increasing incidence. TheCtrms variation was quite different from the previouscases. The Clrms increased substantially (to 0.06) asmaximum lift was attained, then decreased to the usualvalue of 0.04. These same trends in both mean andfluctuating lift were also observed in the NACA 0009and MA409 airfoil data. The frequency spectra of thehighest C,rms values contained distinct low-frequencypeaks.
The highest levels of unsteadiness were observedfor airfoils with combination thin-airfoil and trailing-edge stall types. Data for these airfoils are shown inFig. 7e for the representative LRN(1)-1007 airfoil(henceforward, "LRN"). The Clrms peaks for theseairfoils were extremely high and occurred almostexactly at the value of maximum lift, as shown for theLRN. However, the peaks in each case quickly dropoff and the Clrms values were about 0.04 for deep stall,which was similar to the other cases discussed above.These features are also shown in Fig. 6 for the E374airfoil. As mentioned in the Introduction, the unsteadyflow which accompanies the stall of the LRN has beenwell documented and it is obvious that the high valuesof the fluctuating lift coefficient resulted from theunsteady flow.
Frequency Content of the Fluctuating Lift
type. For the pure thin-airfoil stall airfoils, the low-frequency unsteadiness was much more discernible inthe lift spectra versus the wake hot film. For example,Fig. 8 compares hot-film and lift-balance spectra for theNACA 64AGIO airfoil at a = 10°, which corresponds tomaximum lift. The low-frequency peak atapproximately 4 Hz was discernible in both spectra, butthe peak was much sharper in the lift spectrum.Unfortunately, the lift spectrum also contained theunwanted contributions from the model and balanceresonances. Detailed frequency data were acquired forthis and the other two thin-airfoil stall airfoils and thefrequencies were converted to Strouhal numbers andplotted as a function of angle of attack (Fig. 9). Thevalues of the Strouhal number were very low and theyshowed a generally increasing trend with angle ofattack, as with the LRN airfoil.9 The slopes shown onthe plot were determined from simple linear regression.
0 -
-5 -
-10 -
-15 -
-20-
-25 -
-30-
-35 -
-40 -
-45 -
-50
A - Fundamental model torsional resonanceB - First harmonic of torsional resonanceC - Lift-balance resonance
Hot Film
0 10 20 30 40 50 60 70 80 90 100Frequency (Hz)
Fig. 8 Wake hot-film and lift spectra for the NACA64A010 airfoil at a = 10°. Spectra offset by 10 dB.
A key objective of this study was to identifylarge-scale, or low-frequency, oscillations associatedwith the unsteady flowfield of stalled airfoils. Thespectra of the fluctuating lift for the trailing-edge,leading-edge and combination trailing-edge/leadingedge stall types were distributed evenly over the low-frequencies (< 20 Hz). At higher frequencies therewere some amplitude peaks associated with the modeland balance natural frequencies as discussed above.Also for high angles of attack (> 20°) there were peaksat the bluff-body shedding frequencies which werediscernible in both the lift and hot-film spectra.
The airfoils exhibiting distinct low-frequencyunsteadiness at stall were of the thin-airfoil andcombination thin-airfoil/trailing-edge stall categories.As indicated by the Clrms data presented above, theunsteadiness was more pronounced for the latter stall
It has already been noted above that the large-scale unsteadiness was most intense with thecombination thin-airfoil/trailing-edge stall airfoils. Liftand hot-film power spectra for the LRN airfoil at a =15° were already presented in Fig. 5. Note the intensityof the fundamental and its first harmonic whichindicates the level of periodicity associated with thelow-frequency oscillation. The variation in St with afor the three combination stall type airfoils is shown inFig. 10. The results for the LRN airfoil were consistentwith those presented by Bragg et al.9 The data showedthe same general trends as in Fig. 9, except that theStrouhal number showed a stronger dependence uponthe angle of attack.
Flow Visualization Results
The surface-oil-flow visualization method provedto be very valuable in defining the stall types of each ofthe airfoils since the boundary-layer characteristicswere determined as the angle of attack was increasedinto stall. The boundary-layer features were very two-dimensional in character. This is in a time-averagedsense, since the oil is allowed to flow for 20 to 30minutes. The two-dimensionality was characteristic ofall airfoils tested, except for the E387, even for anglesof attack above maximum lift. Figure 11 shows aboundary-layer state plot for the E374 airfoil. Therewas a separation bubble on the upper surface whosereattachment moved aft on the upper surface withincreasing angle of attack, which is characteristic of thethin-airfoil stall type. There was also substantialturbulent boundary-layer separation which movedslightly forward on the upper surface with increasingangle of attack, which is characteristic of trailing-edgestall. Therefore, the E374 was classified as a having acombination of the two stall types. This flowfieldbehavior is also very similar to that of the LRN airfoilreported by Bragg et al.9 and this comparison is furtherdiscussed below.
Discussion
Airfoil Stall Type and Flowfield Unsteadiness
The data presented here show a distinctrelationship between stall type and low-frequency/large-scale unsteady flow. Trailing-edgestall types exhibited minimal lift fluctuations until theangle of attack was increased well above Cima.Leading-edge stall types developed force fluctuationsimmediately past stall that accompanied the abrupt lossof lift. Thin-airfoil stall types exhibited largerfluctuations in lift which increased with angle of attack
0.030-1- -°- -
0.025-
0.020-
0.015-1
c/30.010-
0.005-
LRN(1)-1007, Lift, ASt/Aa = 0.00576LRN(1)-1007, Hot Film, ASt/Aa = 0.00569E374, Lift, ASt/Aa = 0.00414E374, Hot Film, ASt/Aa = 0.00422E387, Lift, ASt/Aa = 0.00567E387, Hot Film, ASt/Aa = 0.00468 .
11.0 12.0 13.0 15.0 16.0 17.014.0a (Deg.)
Fig. 10 Strouhal number variation with angle ofattack for combination thin-airfoil/trailing-edgestall airfoils. Frequency source noted in legend.
Fig. 11 E374 airfoil boundary-layer state versusangle of attack as determined from surface-oil-flowvisualization.
into stall and began to decrease beyond Clm(U. Themost severe force fluctuations at stall were found onairfoils having a combination of thin-airfoil andtrailing-edge stall. The fluctuations associated with thelatter two stall types contained distinct low-frequencycomponents. The data suggested that the low-frequency oscillation studied for the LRN airfoil alsooccurred on other airfoils with similar stallingcharacteristics.
204American Institute of Aeronautics and Astronautics
Having formulated these conclusions based uponthe present data, some comments can be made onprevious investigations of two-dimensional airfoil stall.Beginning with the trailing-edge stall type, Jones1 notedthat "violent fluctuations" existed, but not until aftermaximum lift was obtained. In fact, judging from hisobservations he is likely referring to the bluff-bodyshedding regime. In this study, bluff-body sheddingdid not occur for trailing-edge stall airfoils until a >20°. Essentially, he reports only "minor" fluctuationsnear maximum lift. This is consistent withMcCullough and Gault4 who do not report anyunsteady characteristics for trailing-edge stall.
For airfoils having the leading-edge stall type,McCullough and Gault4 note two different cases ofunsteady flow. The stall of the NACA 63r012 wasfound to be so violent that, "the runnel speed wasreduced immediately after the occurrence of the stall."This observation is very consistent with the sharpincrease in C,rms just after C/max was attained for theE472 airfoil in Fig. 7b. Gault5 described the unsteadystalling behavior of an NACA 63-009 airfoil asinvolving a cyclic change between stalled and unstalledconditions at Qmax and that the flow was part of acirculatory motion above the airfoil surface. AlthoughMcCullough and Gault4 classify this airfoil as having aleading-edge stall type, they describe the post-stall flowas more similar to thin-airfoil stall than leading-edgestall. Thus, the flowfield may contain weak low-frequency unsteadiness similar to the thin-airfoil stallairfoils in the present data. This is not unlikely, since areduction in Reynolds number may cause the stall typeof the NACA 63-009 to change to the thin-airfoil stalltype.
The rise of lift fluctuations C,max is a commoncharacteristic of airfoils exhibiting the thin-airfoil stall.Even in the first investigations, Jones1 noted the"violent force fluctuations" which were quasi-periodicnear Clmw!, but quickly became irregular at lower orhigher incidences. These observations are consistentwith the very small angle of attack range for which adefinite frequency was discernible in the present data(see Fig. 9). In their study of thin-airfoil stall on theNACA 64A006 airfoil, McCullough and Gault6
reported "large [and] relatively low-frequencyfluctuations of the velocity associated with theseparated boundary layer" at Clmax. In fact, they evenestimated a frequency of the oscillation whichcorresponds to St « 0.09. This is much larger thanthose presented here, but also much smaller than thebenchmark value of 0.20. Further, the authors notedthat the (mean) lift data in the vicinity of stall showedconsiderable scatter which they attributed to thebuffeting of the model. Again, this would suggest an
increase in the fluctuating lift not unlike that shown inFig. 7d. McCullough and Gault4 also studied the stallof a 4.23% thick double-wedge airfoil which had asharp leading-edge. A separation bubble wasimmediately formed whence the angle of attack waschanged. Again, the authors reported the presence of acirculatory motion associated with the airfoil flowfield,indicating low-frequency unsteadiness. Few anomaliesarise when previous data are reviewed in light of thepresent data.
Comments on the Magnitude of the Fluctuating Lift
In a review article on normal force fluctuations onairfoils, Mabey2 analyzes data from various sourcesfrom airfoils having different stall types. He proposes alinear relationship between fluctuating normal forceand the extent of trailing-edge, boundary-layerseparation, based on data for two-dimensional airfoilsat Reynolds numbers of 1.5-1.8 x 106 and Machnumbers from 0.5 to 0.9. He refers to the normalizedseparation length as the distance from the leading edgeto the separation point per unit chord (xsep /c). Forairfoils with trailing-edge stall, this length decreases asthe angle of attack is increased. Mabey proposes that,as the separation length decreases the magnitude of theforce fluctuations increase linearly. An analysis of thepresent data for the trailing-edge stall type shows that alinear relationship may be too simplistic. Themagnitudes of the Clrms were higher and more stronglydependent upon the separation length in Mabey's data,than for the present data.
The present data are plotted in this way for thetrailing-edge stall Ultra-Sport and the combinationleading-edge/trailing-edge stall CLARK-Y along withMabey's linear prediction (Fig. 12). The experimentalCirms values fall well below this line. It is possible thatthe differences are attributable to the low-Reynoldsnumber at which the present data were acquired. Forexample, Fig. 12 shows data for the NACA 63-018section with aspect ratio 4, at Re = 500,000. There ismuch better agreement with the present data at Re =300,000. Mabey notes that in even in the two-dimensional case for the NACA 63-018 (at Re =500,000), the maximum normal force rms is 0.05,which compares very well with the present data.Therefore, the applicability of the Mabey's linearprediction may be limited to higher Reynolds andMach numbers.
There are some caveats within these comparisonsof fluctuating force data. Mabey provides noinformation about the bandwidth of the fluctuatingnormal force data. At the same time he comments onthe effects of force-balance frequency response, but
205American Institute of Aeronautics and Astronautics
0.08-iUltra-Sport, Re = 300,000CLARK-Y, Re = 300,000NACA 63-018, AR = 4, Re = 500,000, Ref. 2
0 Q^. - - - - Mabey Linear Prediction, Ref. 2
0.06-
0.05-
;0.04-
0.03-
0.02-
0.01-
0.00-
Fig. 12 Comparison of the variation in fluctuatinglift versus distance from airfoil leading edge toboundary-layer separation point (xsm /c).
does not quantify this in terms of fluctuating normalforce. He also makes approximations about the extentof separation, which may not be exactly correct. Inspite of these difficulties, the order of magnitude of theC,rms data are comparable.
The Thin-airfoil Stall Type
The thin-airfoil stall type is of special interest inthis study because of the large separation bubble whichforms on the upper surface as the angle of attack isincreased. The low-frequency unsteadiness associatedwith this type of stall was briefly discussed above, andis now presented in more detail as it relates to theleading-edge laminar separation bubble. The firstevidence of a bubble on the NACA 64AGIO airfoil isthe reduction of the lift curve slope at a « 4° in Fig. 7d.It is likely that there was already a small bubble thathas broken down into a long bubble, as described byTani,22 and this would also result in an attendantincrease in drag. The boundary-layer state plot, derivedfrom the flow visualization (Fig. 13) shows howrapidly this bubble grows as the angle of attack isincreased. Also, note that the trailing-edge separationis minimal, thus resulting in the thin-airfoil designation.The bubble reattachment location in the oil-flow patternat higher angles of attack was somewhat ambiguous(±5 % chord) and likely resulted from this region beingvery unsteady. This is not unexpected as Mabey23
suggests that this region is the most unsteady part of thebubble, in terms of fluctuating pressure. The
frequency data in Fig. 9 for these airfoils indicates thatthe unsteadiness occurs at very low frequencies.Mabey2 shows fluctuating lift spectra for stallingairfoils which have frequency peaks that convert toStrouhal numbers of 0.011 (at a = 8.5°) and 0.013 (at a= 10.0°) which would fit nicely on the plot in Fig. 9.Unsteadiness in large separation bubbles is notuncommon and low-frequency disturbances have beendocumented for bubbles associated with a backward-facing step or blunt flat plate.24'25 The low-frequencyunsteadiness is of practical importance as Mabey^indicates that buffet at these low frequencies are morelikely than higher frequencies to excite aircraftstructural modes.
Fig. 13 NACA 64A010 airfoil boundary-layer stateversus angle of attack as determined from surface-oil-flow visualization.
The Thin-airfoil and Trailing-edge Stall Combination
The fluctuating lift data presented here show analarming trend for the combination thin-airfoil andtrailing-edge stall cases. The maximum Clrmsassociated with these airfoils is nearly double themaxima of the thin-airfoil stall types. Also, the low-frequency unsteadiness is very pronounced in both thelift and in the wake (see Fig. 10). From the dataacquired in this study, it is apparent that this low-frequency oscillation studied in detail for the LRNairfoil occurs in much the same form for the otherairfoils. A comparison of the time signals is shown inFig. 14. All three time series show the extremelyintense and low-frequency lift oscillation associated
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Fig. 14 Wake hot-film voltage and lift coefficienttime series for the combination thin-airfoil/trailing-edge stall airfoils at angles of attack near maximummean lift.
with the unsteadiness. The total fluctuation in C, isabout 37% of the mean for the LRN, about 35% of themean for the E374 and about 30% of the mean for theE387. While the value of 37% for the LRN is lowerthan the 50% reported by Zaman et al.,3 it is still verylarge. The wake hot-film voltage time series for bothairfoils shows the periodic decrease in velocity as theairfoil stalls and the wake increases in size, causing thehot-film to be engulfed in low-speed fluid. As the flowreattaches and the wake decreases in size, the velocityvoltage is higher and more uniform." The time seriesdata also show that the oscillation frequency is higherfor the LRN(1)-1007 than for the E374 and E387.This is evident in the frequency data given in Fig. 10.
As previously discussed with thin-airfoil stall, thefrequency is considered extremely low and may berelated to the leading-edge bubble present on theseairfoils (e.g., see Fig. 11) in the same way as for thethin-airfoil stall. The LDV flowfield measurements ofBroeren and Bragg10 performed on the LRN uppersurface showed that the oscillation was related to thequasi-periodic growth and bursting of the leading-edgebubble. The "bursting of the bubble" occurred whenthe bubble reattachment merged with the trailing-edge
separation. Not surprisingly, Mabey2 describes asimilar scenario of a downstream moving bubblereattachment merging with an upstream movingtrailing-edge separation on a supercritical airfoil. Thismay explain why the lift fluctuations are more severefor the combination stall type case. That is, for purethin-airfoil stall, the amount of trailing-edge separationis very small and the force fluctuations are smaller inmagnitude. This suggests that the trailing-edgeseparation may amplify, or enhance, the unsteadiness inthe separation bubble. However, more detailedmeasurements would be required to validate thistheory.
A "superposition" type of analysis may be appliedto summarize the entire data set. That is, Mabey2
proposes that the movement of the trailing-edgeseparation contributes to some of the force fluctuationsand that there may be separation bubble excitationwhich also contributes to the force fluctuations. Giventhe present data, it would seem that Mabey is correct inthat airfoils with trailing-edge separations have low liftfluctuations at stall (Clirm < 0.04) and airfoils with largeseparation bubbles, like the thin-airfoil stall category,have moderate to high lift fluctuations at stall ( 0.04 <QmB < 0.08). For airfoils having both large leading-edge bubbles and trailing-edge separations the liftfluctuations are indeed very high (C,rms > 0.08). This isa very simple analysis of a complex phenomenon.However, it may well provide the framework fordetermining the relative contributions of leading-edgeand trailing-edge separations to force fluctuations onstalled airfoils.
Summary and Conclusions
Time-dependent lift measurements have beencarried out on 12 airfoils with different stallingcharacteristics to determine the influence of stall typeon low-frequency unsteadiness at stall. In addition tothe lift data, wake hot-film data, frequency data andflow visualization data were acquired at a Reynoldsnumber of 300,000. The lift force fluctuations weredetermined from the root-mean-square value of thefluctuating lift after low-pass filtering the data with a20 Hz cut-off to remove unwanted contributions frommodel structural and lift-balance resonances.
The data presented in this paper show a distinctrelationship between stall type and low-frequency/large-scale unsteady flow. Trailing-edgestall airfoils experience the least lift fluctuation at stall(Cirms < 0.04). For leading-edge stall airfoils the liftfluctuations increase sharply to Clrms « 0.04 with theabrupt loss of lift associated with this stall type. Forthin-airfoil stall types, the fluctuating lift begins to
207American Institute of Aeronautics and Astronautics
increase substantially in magnitude before the stall andhas peak values of Clrms < 0.08, nearly double that ofthe previous two stall types. For these airfoils, distinctlow-frequency oscillations, occurring at Strouhalnumbers less than 0.02, are present in the fluctuatinglift spectra. This behavior is likely related tounsteadiness in the large laminar separation bubbleassociated with this stall type. A combination of thin-airfoil and trailing-edge stall types results in C;im!magnitudes which are nearly double that for pure thin-airfoil stall types. The energy is also contained withinlow-frequency oscillations detectable in the lift andwake hot-film spectra. The data suggest that the low-frequency oscillation studied for the LKN(1)-1007airfoil also occurs on other airfoils with similar stallingcharacteristics. It appears that the unsteadinessassociated with the laminar separation bubble isamplified by the trailing-edge separation, resulting inthe large lift fluctuations. However, more research isnecessary to completely validate this conclusion.Finally, the low-frequencies associated with the lattertwo cases are of practical importance since buffet atthese frequencies may be likely to excite aircraft wingstructural modes.
Acknowledgments
This work was funded through a NASA GraduateStudent Researchers Program Fellowship. The authorswish to acknowledge K.B.M.Q. Zaman of the NASALewis Research Center for his contributions to thisresearch.
References
1. Jones, B.M. "An Experimental Study of the Stallingof Wings," Aeronautical Research Council Reportsand Memoranda (ARCR&M), No. 1588, Dec. 1933.
2. Mabey, D.G., "Review of Normal ForceFluctuations on Aerofoils with Separated Flow,"Progress in Aerospace Sciences,Vo\. 29, 1992,pp. 43-80.
3. Zaman, K.B.M.Q., McKinzie, D.J., and Rumsey,C.L., "A Natural Low-Frequency Oscillation OverAirfoils Near Stalling Conditions," Journal of FluidMechanics, Vol. 202, 1989, pp. 403-442.
4. McCullough, G.B. and Gault, D.E., "Examples ofThree Representative Types of Airfoil-Section Stall atLow-Speed," NACA TN 2502, Sept. 1951.
5. Gault, D.E., "Boundary-Layer and StallingCharacteristics of the NACA 63-009 Airfoil Section,"NACA TN 1894, June 1949.
6. McCullough, G.B., and Gault, D.E., "Boundary-Layer and Stalling Characteristics of the NACA64A006 Airfoil Section," NACA TN 1923, Aug. 1949.
7. Bragg, M.B., Heinrich, D.C., and Khodadoust, A.,"Low-Frequency Flow Oscillation over Airfoils nearStall," AIAA Journal, Vol. 31, No. 7, July 1993, pp.1341-1343.
8. Bragg, M.B., Heinrich, D.C., and Zaman, K.B.M.Q.,"Flow Oscillation Over Airfoils Near Stall," ICASPaper 94-4.5.2, 19th Congress of the InternationalCouncil of the Aeronautical Sciences ConferenceProceedings. Vol. 2, Sept. 1994, pp. 1639-1648.
10. Broeren, A.P., and Bragg, M.B., "Phase-AveragedLDV Flowfield Measurements About an Airfoil inUnsteady Stall," AIAA Paper 96-2494-CP, Proceedingsof the 14th AIAA Applied Aerodynamics Meeting. NewOrleans, June 1996, pp. 921-931.
11. Broeren, A.P., "Phase-Averaged FlowfieldMeasurements About an Airfoil in Unsteady Stall,"M.S. Thesis, University of Illinois at Urbana-Champaign, 1996.
12. Farren, W.S. "The Reaction on a Wing WhoseAngle of Incidence is Changing Rapidly— Wind-Tunnel Experiments with a Short-Period RecordingBalance," Aeronautical Research Council Reports andMemoranda (ARCR&M), No. 1648, Jan. 1935.
13. Reda, B.C., "Observations of Dynamic StallPhenomena Using Liquid Crystal Coatings," AIAAJournal, Vol. 29, No. 2, Feb. 1991, pp. 308-310.
14. Bragg, M.B., Khodadoust, A., and Spring, S. A."Measurements in a Leading-Edge Separation Bubbledue to a Simulated Airfoil Ice Accretion," AIAAJournal, Vol. 30, No. 6, June 1992, pp. 1462-1467.
15. Selig, M.S., Guglielmo, J.J., Broeren, A.P., andGiguere, P., Summary of Low-Speed Airfoil Data—Volume 1, SoarTech, Virginia Beach, VA, 1995.
208American Institute of Aeronautics and Astronautics
16. Selig, M.S., Lyon, C.A., Giguere, P., Ninham,C.P., and Guglielmo, J.J., Summary of Low-SpeedAirfoil Data—Volume 2, SoarTech, Virginia Beach,VA, 1996.
17. Lyon, C.A., Broeren, A.P., Giguere, P.,Gopalarathnam, A., and Selig, M.S., Summary of Low-Speed Airfoil Data—Volume 3, SoarTech, VirginiaBeach, VA, 1997.
18. Giguere, P., and Selig, M.S., "Freestream VelocityCorrections for Two-Dimensional Testing with SplitterPlates," AHA Journal, Vol. 35, No. 7, July 1997, pp.1195-1200.
19. Coleman, H.W., and Steele, W.G.,Experimentation and Uncertainty AnalysisforEngineers, John Wiley and Sons, New York, 1989.
20. McGee, R.J., Walker, B.S., and Millard, B.F.,"Experimental Results for the Eppler 387 Airfoil atLow-Reynolds Numbers in the Langley Low-Turbulence Pressure Tunnel," NASA TM 4062, Oct.1988.
21. Selig, M.S., Guglielmo, J.J., Broeren, A.P., andGiguere, P., "Experiments on Airfoils at Low-ReynoldsNumbers," AIAA Paper 96-0062, Jan. 1996.
22. Tani, I., "Low-Speed Flows Involving SeparationBubbles," Progress in Aeronautical Sciences, Vol. 5,1964, pp. 70-103.
23. Mabey, D.G., "Analysis and Correlation of Data onPressure Fluctuations in Separated Flow," Journal ofAircraft, Vol. 9, No. 9, Sept. 1972, pp. 642-645.
24. Driver, D.M., Seegmiller, H.L., and Marvin, J.G.,"Time-Dependent Behavior of a Reattaching ShearLayer," AIAA Journal, Vol. 25, No. 7, July 1987, pp.914-919.
25. Kiya, M., and Sasaki, K., "Structure of a TurbulentSeparation Bubble," Journal of Fluid Mechanics, Vol.137, 1983, pp. 83-113.
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