Presented at the NASA/AFOSR/ARO Workshop on Physics of Forced Unsteady Separation April 17-19, 1990 NASA Ames Research Center, Moffett Field, CA 9403

Ahmed, S.

Carr, L.W.

Chandrasekhara, M.S.

Calhoun, Institutional Archive of the Naval Postgraduate School

Calhoun: The NPS Institutional ArchiveFaculty and Researcher Publications Faculty and Researcher Publications Collection1990-04Comparison of pitch rate history effects ondynamic stallChandrasekhara, M.S.NASA/AFOSR/ARO Workshop on Physics of Forced Unsteady Separation April 17-19,1990 NASA Ames Research Center, Moffett Field, CA 94035http://hdl.handle.net/10945/50208N94- 34968COMPARISON OF PITCH RATE HISTORY EFFECTS ONDYNAMIC STALLM.S.ChandrasekharaNavy-NASA Joint Institute of AeronauticsDepartment of Aeronautics and AstronauticsNaval Postgraduate School, Monterey, CAL.W.CarrAeroflightdynamics DirectorateU.S.Army ARTA and Fluid Dynamics Research BranchNASA Ames Research Center, Moffett Field, CAandS.Ahmed *MCAT Institute, San Jose, CAPresented at the NASA/AFOSR/ARO Workshop on Physics of Forced Unsteady SeparationApril 17-19, 1990NASA Ames Research Center, Moffett Field, CA 940351. INTRODUCTIONDynamic stall of an airfoil is a classic case of forced unsteady separated flow. Flowseparation is brought about by large incidences introduced by the large amplitude unsteadypitching motion of an airfoil. One of the parameters that affects the dynamic stall process isthe history of the unsteady motion, (McCroskey 1). In addition, the problem is complicatedby the effects of compressibility that rapidly appear over the airfoil even at low Machnumbers at moderately high angles of attack. Consequently, it is of interest to know theeffects of pitch rate history on the dynamic stall process. This abstract compares the resultsof a flow visualization study of the problem with two different pitch rate histories, namely,oscillating airfoil motion and a linear change in the angle of attack due to a transientpitching motion.2. DESCRIPTION OF THE RESEARCHStroboscopic schlieren studies were conducted while a 3 in. chord, NACA 0012 airfoilwas executing unsteady motion. Two separate motion histories were considered. The firstwas a sinusoidal variation of the angle of attack and the second was a rapid ramp motionof the airfoil. Two independent drives were designed to produce the necessary pitch ratehistories an_t are described in Carr and Chandrasekhara 2 and Chandrasekhara and Carr 3respectively. A large body of data enveloping a Math number M = 0.2 - 0.45 was collected.Since the pitch rate continuously changes for an oscillating airfoil, the angles of attack at* On Leave from National Aeronautical Laboratory, Bangalore, India23which the pitch rates match were obtained by comparing them with those available for theramp type motionexperiment. The angle of attack was varied from 0 - 600 in the rampmotion. The corresponding variation for the oscillatory motion wasa,= C_o+ _msin(_ot) = 10 ° + 10 °sin(wt)Data was also obtained at other values of the amplitude of oscillation (2 o and 50).However, to achieve a proper comparison, only the case of 10 degree amplitude that resultsin a total angle of attack range of 0 - 20 o will be used.3. RESULTS AND DISCUSSIONFig. 1 shows the schlieren pictures at M = 0.2 at an instantaneous angle of attack ofapproximately 17 ° for the two pitch rate histories at a non-dimensional pitch rate definedas _+ ac = 0.025. As can be seen from the figure, the flow over the airfoil in ramp= t-v-g_motion has already reached deep stall conditions, whereas that over the oscillating airfoilshows a clearly defined dynamic stall vortex at 60% chord location, indicating that theairfoil is still producing dynamic lift. At a higher (_+ value of 0.03, the two flows are nearlyidentical even at an angle of attack of _ 15 °.Similar results were obtained at M = 0.25, 0.3 and 0.35. In all cases, at low pitchrates, deep stall occured over the airfoil in ramp type motion at the angles of attackfor which the flow over the oscillating airfoil was dominated by a strong, tightly wounddynamic stall vortex which was still located over the upper surface. This result was true,despite the fact that at lower angles of attack, the two flows appeared nearly identical.In addition, in instances where the dynamic stall vortex could still be identified for thetransient pitching case, it was significantly diffused, indicating that it was in a disorganisedstate as opposed to the oscillating case, where it was well organised. This trend persistedin the Mach number range that extended into the compressible regime, namely beyondM = 0.3. A table of the results for the different conditions is included to summarise theresults discussed.It is somewhat surprising to note the trends obtained in this comparison. An expla-nation of this effect could be offered for this as follows: A sinusoidal motion produces pitchrates that increase from 0 to 0.035 during the pitch-up phase for k = 0.1 and an amplitudeof 10 degrees. Its maximum occurs at the mean angle of attack. Beyond this, the pitchrate decreases, but at the angle at which the comparions were made (17.07 °) in Fig. 1 , thepitch rate is still significant (0.025). For the ramp motion, the pitch rate reaches a constantvalue by _ _ 6 °. Chandrasekhara and Carr 4 have shown that stall can be delayed to higherangles of attack by increasing the pitch rate. It appears from the pitch rate variation withangle of attack that an oscillating motion can produce higher amounts of vorticity whichwill cause the dynamic stall vortex to be more organised and coherent. This leads to theconclusion that motion with continuously changing acceleration can support larger flowgradients and thus is more desirable.244. CONCLUSIONSThe study shows that pitch rate history is a very important parameter in the analysisof dynamic stall. Pitch rate history plays a dominant role by controlling the strength andbehavior of the dynamic stall vortex. Vorticity created by repetitive motion appears tohave the energy to sustain higher pressure gradients in the flow.5. REFERENCES1. McCroskey, W.J., "The Phenomenon of Dynamic Stall", NASA TM 81264, March 1981.2. Carr, L.W. and Chandrasekhara, M.S., "Design and Development of a CompressibleDynamic Stall Facility", AIAA Paper No. 89-0647, Jan. 1989.3. Chandrasekhara, M.S. and Carr, L.W., "Design and Development of a Facility forCompressible Dynamic Stall Studies of a Rapidly Pitching Airfoil", Proc. 13 th ICIASF,Goettinegen, W.Germany, September 1989.4. Chandrasekhara, M.S. and Carr, L.W., "Flow Visualization Studies of the Mach NumberEffects on the Dynamic Stall of an Oscillating Airfoil", AIAA Paper No. 89-0023, Jan.1989.Table 1. Comparison of Pitch Rate History Effects throughFlow VisualizationM = 0.2, k -- 0.1No. Ramp Type Motion Oscillatory Motion a+1. a = 170 a = 17.07 ° 0.025Nearly deep stall Tightly wound vortexTransverse scales large at _60% chord= 150 c_ = 15.230 0.03Flow nearly identical in both cases.M -- 0.2, k : 0.0751. a = 130 e_ = 13.820 0.025Very nearly identical flow in both cases25M -- 0.25_ k --- 0.1°..a = 180Deep stall, trailingvortex, large transverseflow scalesa = 170Vortex present, butdisorganised at 55%chordIndications of flow breakdowna = 150c_ = 1S.t °Vortex at 75% chordand well organiseda = 17.070Well organised vortexat 50% chord0.020.025a= 15.230 0.03Flow very nearly similar in the two casesM -- 0.25_ k "- 0.0751. a = 16.50 a = 16.5 ° 0.02Deep stall. Shear layer Well organised at vortexvortex at mid-chord, _ 60%large transverse scalesa = 130 0.025Beginnings of a vortex,a= 13.50Imprint of a vortex26M -- 0.3, k -- 0.1...o_ = 180Vortex well above thesurface, near deep stalllarge transverse disturbanceDisorganised flow= 170Vortex at 65% chordflow getting disorganised,large vortexo = 150Vortex at 15% chordOther features of flow nearly alikeo = 18.1 oVortex near 90e_ chordtransverse disturbancegetting largero = 17.10Vortex at _55-60% chordWell organised flowo = 15.23 ovortex at 15% chord0.020.0250.03M -- 0.3, k -- 0.075.2.= 16.50 a = 16.50 0.02Total flow breakdown organised vortex at 55% chordo = 130 o_ = 130 0.025Flow nearly identical in the two casesM -- 0.35, k = 0.1o.= 17 0Large vortex, but notorganised= 150Vortex at 30% chordOtherwise nearly identical flowa = 17.07.10Organised large vortexat the same locationo_ = 150votrex at 25_ chord0.0250.0327Ramp motion Oscillatory motionoL+ = 0.025(x= 17 ° k = 0.10, o_= 17.07 °oL+ = 0.03oL= 15° k = 0.10, oL= 15.23 °oL+ = 0.025OL= 13 ° k = 0.075, o_= 13.82 °Figure 1. Comparison of Pitch Rate History Effects(M = 0.20)28Ol--Wc-OOEoc-CimZ_OnoCwmmiimOOOo,IVlVlO,,_j,mlOO-I-OOIIdocLE:tjimt')CCZI--r._oIIOOVlVlOIIE_Cm't:EOI,-(--c'_..CIE.OOc"r,.)_2,9O_-0q_C_.-0o_mm8OII÷o0O_mmUlCPII3000n_CLL_0C0wmNmmor)0m U_c =5Vlimm0VlO"_.. c4oOooO..QOLO"OCOOVlVlO"::tOBOVlVl,.l,..acLWI'--mm"OL_OE¢,#'3c-"(J3LOmmO..cLmmm(-tO(:!.)LO...F:OOb"OcOcO-"OO"O(1)L31WOmaoOOOEc--Oc-OOO.o_'_XolO>.,"OOim(I) Oc-"OOQ.OCOemOEc-c-c-.O"O (D_m"_ Oe--.._ c-3280.0C0MPARISION OF TIME HISTORY+M = 0.20, ot = 0.025........... RAMP MOTIONOSCILLATORY MOTION60.040.020.00.0, , ,-- -*,*- ''" "-" • ''.- -- ....... • .......... r ........; ,°.,,".':: .,;' !.................... :..:'. ..... ;...........................................................................e•° i0.00 0.02 0.04 0.06 0.08 O.tOtime,s ecRun No. 00129BSLOPES 0 - i0 Deg:0 - 30 Deg:0 - 57 Deg:1510.681309.371285.5033VARIATION OF PITCH RATE WITH ANGLE OF ATTACK0.05OSCILLATORY MOTION, RAMP MOTION+C.)O. 04 .........................'.............: .........: ........._ ....................'.........................i,.........i ..i:::!:::::::::::0m 0.020.01 ...............................:......................................................:0.00 '0.0 5.0 10.0 15.0 20.0Angle of attack,34

Comparison of pitch rate history effects on dynamic stall

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Comparison of pitch rate history effects on dynamic stall

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