The leading edge flow field of an airfoil executing a. sinusoidal
oscillatory motion and experiencing dynamic stall under
compressibility conditions has been studied using a two component
LDV system. Phase averaged mean velocity measurements
and some flow quantities derived from it are presented and discussed.
The results indicate extremely large accelerations of the
flow are present around the leading edge with mean velocity values
60% higher than and instantaneous velocities as large as 1. 75
times the free stream velocity. The velocity profiles at certain
locations over the airfoil resemble that of a wake

Chandrasekhara, M.S.

Van Dyken, R.D.

Calhoun, Institutional Archive of the Naval Postgraduate School

Calhoun: The NPS Institutional ArchiveFaculty and Researcher Publications Faculty and Researcher Publications Collection1991-09Oscillating Airfoil Velocity Field During LargeAmplitude Dynamic StallChandrasekhara, M.S.Symposium on Turbulent Shear Flows, 8th, Munich, Federal Republic of Germany,Sept. 9-11, 1991, Proceedings. Vol. 2 (A92-40051 16-34). University Park, PA,Pennsylvania State University, 1991: Oscillating airfoil velocity field during largeamplitude dynamic stallhttp://hdl.handle.net/10945/50358OSCILLATING AIRFOIL VELOCITY FIELD DURING LARGE AMPLITUDE DYNAMIC STALL M.S.Chandras('kharat .! and R.D.Van Dyken+ t Adjunct Research Prof('ssor and Assistant Director :j: Graduate Student Navy-NASA .Joint Institute of Aeronautics and Fluid Mechanics Laboratory. NASA Ames Research Center Departm('ut of Aeronautics and Astronautics Naval Postgraduate School, Monterey, CA 93943 1. SUMMARY The leading edge flow field of an airfoil executing a. sinu-soidal oscillatory motion and experiencing dynamic stall under compressibility conditions has been studied using a two compo-nent LDV system. Phase averaged mean velocity measurements and some flow quantities derived from it are presented and dis-cussed. The results indicate extremely large accelerations of the flow are present around the leading edge with mean velocity va)-ues 60% higher than and instantaneous velocities as large as 1. 75 times the free stream velocity. The velocity profiles at certain locations over the airfoil resemble that of a wake. 2. INTRODUCTION The problem of dynamic stall of an airfoil is a unique case of forced unsteady separated flows wherein the leading edge flow separates, but the airfoil does not lose lift. It is a complex prob-lem governed by the extremely large flow accelerations that are present around the airfoil leading edge that could r('sult in lo-cally supersonic flow even at very low free stream Mach numbers of 0.2, flow transition to turbulence, movement of the transi-tion point due to the unsteady motion, formation of shock(s) and their interaction with the local boundary layer, the eventual large scale separation at the leading edge with large amounts of vorticity being added to the flow, dependence of all of the above phenomena on the parameters of the unsteady motion such as amplitude, mean angle of attack, degree of unsteadiness and so on. These complexities have defied all attempts to compute the problem with any degree of success at the resolution needed. The understanding of the associat('d flow physics is crucia.l hefOl'e allY progress can be made in controlling the dynamic stall flow. The work to be described pertains to the measurement of flow fi('ld in the leading edge region of an airfoil oscillating at a large amplitude when compressibility effects just set iu. 3. EXPERIMENT DESCRIPTION The study is a part of the dynamic stall research project underway at the Navy-NASA Joint Institute of A('ronautics \)('-tween the Naval Postgraduate School and NASA AllIes R('search Celltpr(ARC). The experiments were conducted in the Com-pressible Dynamic Stall Facility (CDSF) in the Fluid Mechanics Laborat.ory(FML) at ARC. Ref. 1 provides a full description of the facility and its capabilities. It is a unique unsteady flow facility equipped with a drive system located a.t the t.op of the test section of an indl'aft wind tunnel connert('d to an ('vacuation compressor. The drive oscillates an airfoil mounted between two optical glass windows sinusoidally. Encoders mounted on the drive prm'ide the airfoil position information fOntinuouslv. Two component LDV data was obtained for M = 0.:1 at a reduced fre-quency of 0.0.5. Velocities were mappped in a region ('nveioping -.16, :::; '''Ie :::; 0.167 and 0.083 :::; Ylc ::s: O.Hi,. The colllplete details of the LDV signal data validation procedures as well as the ensemble averaging method followed could be found in Chan-drasekhara and Ahmed. j Mailing Address: M.S. 2/;0-1. NASA Am£'.< R,s£arch Cen/(I'. Mo]Jell Fifld. CA .9.IOS5 t Presently: Research Srienlist. Naval Weapons Cente,', China Lake. CA 4_ RESULTS 4.1. Phase Distribution of Velocity Fig. 1 shows the distribution of the velocity wit h th(' phase angle of oscillation,4>, at xlc = 0.067. It should he noted til(' drive system causes the airfoil to first to go t.hrough a downward motion to an angle of attack, 0 = 0 d('g. at 4'> = 90°. ;wd then it pitches the airfoil up to an angle of attack of 20 (lPg. at 4> = 270° through the static stall angle of 12,4 deg. and back to the mea.n angle of attack of 10 deg. The velocity drops to its lowest valne at 4> = 90° (at 0 = ( 0 ) and inCl'pas('s rapidly. For ex. at y/c = 0.167, the velocity is apPl'OxinlatPiy <,qnal to t.he free stream velocity [Too at 4> = 900 and rearil(,s I.!il·" at 4> = 2160 , where 0 = 1.5.90 • This angle corresponds to the dynamic stall angle as determined by the schlieren stmlies of Chandrasekhara and Carr. From this point, the velocity drops rapidly to 1.25Uoo ' However, in the fully separated flow. the velocities are still large, 0 (Uoo ). One of the most interesting results seen in this graph is the dramatic variation in the behavior seen in the ':i/c locations closer to the surface of the airfoil. For y I c = O.OR:~ (showll by the symbol \i'),the velocity decreases to 0.9U,:;oo at. 4> = 90°. ami begins to increase as expected during the upstroke of til(' air-foil. However, at 4> = 1.5.5°, at a = 5.5°, the v('lority drops suddenly to O.4Uoo over 155° :::; 4> ::; :202°, rorn'spoll(lillg 10 5.50 ::; a 13.,°. Such a drop can be attributed to the prl'Sl'llCe of a separation bubble that the LDV probe volume encounters as the airfoil pitches up. Once the buhble bursts, the velocit~' increases qllickly as th(' ollt('r fluid gushps into t h!' void ('I'('al<,d by the bursting hubble. Eventually at this loratioll. th!' airfoil blocks off the beams and thus no data can be obtained until a phase angle of ~ 330°, when they are unblocked again. The val-ues seen for the phase angles in between are an artifact of t hl' data processing program. (Also, the copy of the profile seen he-low is also due to software requiring a pseudo-recta,ngular grid with equal number of points in each column of the data file.) At the higher locations, the phase angle range over which this dramatic drop occurs decreases as can be expected due to I he shape of the bubble. Fig. 1 also shows t.hat the buhble burst.s at 4> = 202°, at aU ylc locations as could be expected. Dnrillg this process, the velocity increases. In this figure, exn'pling al ylc = 0.167, all other measurement. points are within the separatioll bubble and the phase variation seen at y Ic = 0.1 G, is typical of all points in the flow. 4.2. Vplocity Dist.ribut.ion at. xlc = 0.OG7 The velocit.y profiles at various phase angles from [(j:2 d(,-grees to 219 degr('es are shown in Fig. 2. At. 4> = 1(;2°. til<' fluid is seen t.o accelerate rloser to th(' surfar('. Also, slpep challges can be seen in t.he velocity in til(' core of the bubbl('. Ev('nt nally as thp bubble is cleared ( at a phase angle of 20:2 degrl'('s and b(,Y011C1). the velocity profile becomes wake-like. It appears that the fluid nearer the surfac(' acrelE'rates bot.h above and t hl' Ill'-low the bubble ami the low velocit.v fluid that was in the huhhle has not yet mixed with t.he fluid silrroullding it ('ven though the bu bble has hurst. 4.3. Vf'locity Cont.ours at <I> = 17·lo.a = R.95° The vl'lority contonrs at all statiolls for a phas(' angl(' of J ,.J degr('('s and an angle of attack of 8.95 degrees are shown in Fig. :1. Thl' rangE' of the velocit.ies l'ncoullt.E'r('d herE' is frolll 0.95[.'.;v t.o I A!i[1"",. The int.eresting feature seen here is that. pock('ts of fluid at the highest velocity could be found 10 - 1.5% a bove the airfoil upper stIl'facc, and not dose to I he surraC'(' as tlH'orised by the moving wall effect. This indicates that the cfl"ects of the sllrface accd(,l'ation al'l:' not just confined to the airfoil boundary lawr. whirh ill Ihis case is estimated to be about 0.15 nllll. Ali-ol'lt('r inleresting result is that at. the leading edge, the \'docity is I.:IS(T". duc 1.0 Ihc suction peak. It decreases for a short distance illllll('dia.I ... I,v following it, but increases again as the streamlines ,lI'P aCf{'Il'ralt,d around the bubble. and ill til .. olll.N flow. It, is worth nol illl-\ hNe that I.h(' largest llIP'J.II vdocitiPs lll('asured were about I.G I.illl(,s th(' free stream value with the instantaneous val-ucs r('aching about 1.75Uoo at some locations in the flow. 5. CONCLUSIONS The dramatic changes in the leading edge flow fi!?ld of an oscillating a.irfoil undergoing large amplitude dynamic stall has bcclI capl.llrpd using a lloll-intrusivp measurelll('nt t('chniqlle. Of partricular int('rcst are the very large time averaged mean veloc-itips (~ I.G U",) at locations far away from the airfoil surfan', tht' formation and bursting of the sepa.ration bubble. and the 2.0 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2 ~ 1.1 :-- 1.0 """ 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0 6. + x '0 "J .~ 0.0 90.0 0.117 0.100 0.083 0.083 :~ : ... ~ •.....••.. i'.~~~: •••••. 180.0 270.0 360.0 Pltase AngiE', 1> Fig. I. Disiribntion of ,\bsolntp Velocity with Phas!? }\nglp al. xl" = 0.(1),. (Origin Shirt"d by O.lli.", at Each .rIc). . u --:.., 0.20 0.15 0.10 0.05 0.00 -0.05 -0.10 -0.20 CONTOUR 1 2 3 4 II 6 7 8 9 10 11 12 13 VALUE 0.900 0.9110 1.000 1.0110 1.100 1.1110 1.200 1.2110 1.300 1.350 1.400 1..50 1.500 • NO CONTOUR LINES FOUND -0.15 -0.10 -0.05 rcslllting wa.kE'-lik!? velocity distributions. These results indicate the ext.relll!?ly complex nature of the flow being studi('d and have provided the first. ever documentation of the velocity field in these !lows. 6. REFERENCES 1. CAHH, L.W. and ClIANDRASEl\llAltA, M.S., "Design and Development of a Compressible Dynamic Stall Fa.cility", to a.ppear in J. ;l irccraft. 2. CJIANDRASEKHARA, M.S. and CARR. L.W. "Flow Visualiza.tion Studies of the Mach Number Effecls 011 the Dy-Ilamic Stall of Oscillating Airfoils", J. Ail'emft, Vol. 27, No. G, June 1990, pp .. 516-522. 3. CHANDRASEKJIARA, M.S., and AHMED,S., "J,,,,ser V('lorimetry Measurements of Oscilla.ting Airfoil Dynamic Sta.lI Fidd". illAA Pa.per No. 91-1799, to be presented at the AlA A nnd Fluid Dynamics, Plasma Dyna.mics and Lasers Conf0l'ellce. 0.20 0.15 .... , ..... · .......... 1 •• -.:::.. 0.10 .... · .. · .. : .. · .... · .. ; .... $:PIiMfn$~W~t(Jf4 >. 0.05 ................... : .......... : .......... : .......... : ......... . 0.00 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 Absolule Velocity, J~ Fig. 2. Vdorit.y Profiles at x./c = 0.OG7; 0,1>= W2°, 6.¢ = 3° for Successive Profiles. 0.00 0.05 0.10 0.15 0.20 x/r Fig. 3. V('locilY Cont.o1lrs at M 0.:1. k 0.05, Cl' = 10° - 10° sin·wl. 

Oscillating Airfoil Velocity Field During Large Amplitude Dynamic Stall

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Oscillating Airfoil Velocity Field During Large Amplitude Dynamic Stall

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