1、-UsingZLraand E. B. Plentovicti .-=_;_:;_,.(NASA-T_-GG36) CHARACTERIZATIONCAVITY FLOW FIELDS USING PRESSUREDATA OBTAINED IN THE LANGLEYO.3-METER TRANSONIC CRYOGENICTUNNEL (NASA) 5 pOF N93-22876UnclasHI/D2 01537047m.Provided by IHSNot for ResaleNo reproduction or networking permitted without license
2、from IHS-,-,-|- _ ! R ll. n I B uuu iJilliL7=i .i!Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-NASA Technical Memorandum 4436Characterization of Cavity FlowFields Using Pressure DataObtained in the Langley 0.3-MeterTransonic Cryogenic TunnelM. B.
3、Tracy and E. B. PlentovichLangley Research CenterHampton, VirginiaNational Aeronautics andSpace AdministrationOffice of ManagementScientific and TechnicalInformation Program1993Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Provided by IHSNot for Re
4、saleNo reproduction or networking permitted without license from IHS-,-,-AbstractStatic and fluctuating pressure distributions were obtained along thefloor of a rectangular-box cavity in an experiment performed in theLangley 0.3-Meter Transonic Cryogenic Tunnel. The cavity studiedwas 11.25 in. long
5、and 2.50 in. wide with a variable height to obtainlength-to-height ratios of _._, 6.7, 12.67, and 20.0. The data presentedherein were obtained for yaw angles of 0 and 15 over a Mach numberrange from 0.2 to 0.9 at a Reynolds number of 30 106 per footwith a boundary-layer thickness of approximately 0.
6、5 in. The resultsindicated that open and transitional-open cavity flow supports tonegeneration at subsonic and transonic speeds at Mach numbers of 0.6and above. Further, pressure fluctuations associated with acoustic tonegeneration can be sustained when static pressure distributions indicatethat tra
7、nsitional-closed and closed flow fields exist in the cavity. Cavitiesthat support tone generation at 0 yaw also supported tone generation at15 yaw when the flow became transitional-closed. For the latter cases,a reduction in tone amplitude was observed. Both static and fluctuatingpressure data must
8、be considered when defining cavity flow fields, andthe flow models need to be refined to accommodate steady and unsteadyflOWS.IntroductionCavities in aerodynamic surfaces can generateboth steady and unsteady disturbances in otherwiseuniform flow fields. Changes in static pressure distri-butions insi
9、de the cavity can result in large pressuregradients, and the unsteady flow can generate self-sustaining oscillations which, in turn, generate acous-tic tones that radiate from the cavity. Both steadyand unsteady flows can present difficulties for storeseparation from an internal weapons bay. The for
10、-mer can cause large nose-up pitching moments, andthe latter can cause structural vibration of the store.To ensure safe separation, the various flow fields thatdevelop about a cavity must be characterized. Theexperimental and computational results from stud-ies of the mean flow field within a cavity
11、 have beenreported in references 1-14. Cavity acoustic resultshave been reported in references 1, 2, 6, and 15-27.The purpose of this study is to characterize thecavity flow fields observed at subsonic and transonicspeeds by using the complimentary static and fluctu-ating pressure data measured alon
12、g the length of thecavity. Previous publications (refs. 28 and 29) sep-arately analyzed the static and fluctuating pressuredata obtained in this experiment for the effects ofReynolds number and yaw angle, and they found nosignificant effect due to Reynolds number (separatefrom boundary-layer thickne
13、ss) in either the static orthe fluctuating pressure data. The static results in-dicated that the various types of flow fields occurredfor length-to-height ratios (1/h) that were differentfrom those observed at supersonic speeds. Specifi-cally, the cavity with l/h = 6.7, which would supportopen flow
14、at supersonic speeds, showed transitional-open flow at a free-stream Mach number (Mc) of 0.6and tended toward open flow as the Mach numberwas increased to 0.9. The cavity with l/h = 12.67,which would support transitional flow at supersonicspeeds, showed closed flow at M_c = 0.6 and tendedtoward tran
15、sitional-closed as the Mach number wasincreased to 0.9. The acoustic results, based on fluc-tuating pressures measured at a single location onthe forward floor of the cavity, agreed with super-sonic observations in most cases. A notable exceptionwas the cavity with I/h = 12.67 in which tones de-velo
16、ped when the Mach number was reduced to 0.6.The tone amplitude and bandwidth were observedto change from transonic through subsonic with de-creasing Mach number for all cases for which tonesoccurred.The intention of this study is to extend the anal-ysis to more thoroughly characterize the various fl
17、owfields and the transitions between them at subsonicand transonic speeds. Some previously publisheddata (acoustic spectra and static pressure distribu-tions) are presented to demonstrate both the cases forwhich static and fluctuating pressure data were con-sistent and the cases for which they were
18、not. Addi-tional unpublished acoustic data measured along thecavity floor are used to generate mode shape plots.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SymbolsBLCCpDfmhkLN2lMoomN2PfPsPt_o0P_q_R_SPLU_27O_56*and Abbreviationsboundary-layer cont
19、rolpressure coefficient, q_diameter, ftfrequency of acoustic mode, Hzcavity height, in.empirical ratio of shear layer andfree-stream velocities, a functionof Mocliquid nitrogencavity length, in.free-stream Mach numberacoustic mode numbernitrogenmeasured fluctuating pressure,psimeasured surface stati
20、c pressure,psifree-stream total pressure, psifree-stream static pressure, psifree-stream dynamic pressure psifree-stream unit Reynolds num-ber, per ftsound pressure level, dBfree-stream total temperature, Kfree-stream velocity, fpsdistance in streamwise direction,in.distance in spamvise direction, i
21、n.distance normal to tunnel side-wall, in.empirical phase between instabil-ities in shear layer and pressurewaves, a function of I/hratio of specific heat of test gasat constant pressure to that atconstant volumeboundary-layer thickness, in.boundary-layer displacementthickness, in.boundary-layer mom
22、entumthickness, in. yaw angle, degBackgroundAt supersonic speeds, four types of mean cavityflow were defined in references 13 and 14, and thesefour types (open, closed, transitional-closed, andtransitional-open) will be briefly discussed. Thefirst flow type generally occurs when the cavity isdeep, a
23、s found in bomb bays, and is termed opencavity flow. This flow type generally occurs forl/h _ 13. Fig-ure 2 provides a sketch of the flow field and typicalpressure distributions for closed cavity fow. In thisflow type, the flow separates at the forward face ofthe cavity, reattaches at some point alo
24、ng the cavityfloor, and then separates again before reaching therear cavity face (fig. 2(a). This creates two distinctseparation regions, one downstream of the forwardface and one upstream of the rear face. This flow pro-duces an adverse static pressure gradient (fig. 2(b)that can cause the separati
25、ng store to experiencelarge nose-up pitching moments. However, acoustictones are not present for shallow cavities where theflow is closed.The third and fourth mean cavity flow types(transitional-closed and transitional-open) occur forcavities with values of l/h that fall between thosefor closed cavi
26、ty flow and open cavity flow, (i.e.,values of I/h between 10 and 13). Transitional-closed cavity flow in the past has been referred toas transitional cavity flow (ref. 4); however, theimpingement shock and the exit shock that normallyoccur for closed cavity flow coincide and produceProvided by IHSNo
27、t for ResaleNo reproduction or networking permitted without license from IHS-,-,-a singleshockasshownin figure3(a). Similartothe resultfor closedcavityflow,largelongitudinalpressuregradientsoccur(fig.3(b)in thecavitythatcancontributetolargenose-uppitchingmoments.With a smallreductionin I/h from a va
28、lue cor-responding to transitional-closed cavity flow, theimpingement-exit shock wave abruptly changes to aseries of expansion and compression wavelets indi-cating that the shear layer no longer impinges onthe cavity floor. This flow field is referred to astransitional-open cavity flow. For this flo
29、w field, asindicated in figure 3(c), longitudinal pressure gradi-ents in the cavity are not as large as those shown fortransitional-closed cavity flow (fig. 3(d), and conse-quently the problem of store nose-up pitching mo-ment is not as severe as that for closed cavity flow.The acoustic fields corre
30、sponding to the transitionalflow fields have not been determined.The mechanism that produces acoustic tonesfor open cavity flow fields is understood to bcareinforcement between instabilities in the shear layerthat bridges the cavity and pressure waves that aregenerated in the cavity when the shear l
31、ayer impingeson the aft wall. Acoustic tones occur at discrete fre-quencies that correspond to characteristic pressurepatterns (standing waves or modes) in the cavity. Al-though no satisfactory method exists to predict toneamplitude, the frequencies at which they occur canbe predicted by a semiempir
32、ical equation determinedby Rossiter in reference 1 and modified by Heller,Holmes, and Covert in reference 15. The modifiedRossiter equation is given asl m-a- = (1)+_The determination of transitional-closed andtransitional-open cavity flows, as well as of open andclosed cavity flows, has been made by
33、 observation ofthe static pressure distribution in the cavity. Fig-ures l(b), 2(b), 3(b), and 3(d) provide typical staticpressure distributions for each flow type that can beused as a guideline for determining the type of cavityflow.Cavity flow regimes are generally defined in termsof the length-to-
34、height ratio of the cavity. However,there are other parameters that can affect the exactvalue of l/h where the flow transitions from closedto open. Some of these parameters include Machnumber (ref. 1), the ratio of cavity width to cavityheight (ref. 4), and the ratio of boundary-layer heightto cavit
35、y height (ref. 3). Cavity parameters and free-stream conditions should match when making datacomparisons.Experimental MethodsWind Tunnel DescriptionThe tests were conducted in the 13- by 13-in.adaptive-wall test section of the Langley 0.3-MeterTransonic Cryogenic Tunnel (0.3-m TCT). A sketchof the t
36、unnel is presented in figure 4. The 0.3-m TCTis a fan-driven, cryogenic pressure tunnel that usesgaseous nitrogen as a test medium. It is capable ofoperating at stagnation temperatures from approx-imately 80 K to 327 K and at stagnation pressuresfrom 1.2 to 6.0 arm. The fan speed is variable so that
37、the empty test section Mach number can be variedcontinuously from about 0.20 to 0.95. This combi-nation of test conditions provides a test envelope ofReynolds numbers up to about 100 106 per foot.Additional details of the tunnel and its range of op-eration may be found in references 30 and 31.Figure
38、 5 presents a sketch showing details of theflow region in the adaptive-wall test section, andfigure 6 presents a photograph of the test section.All four walls are solid. The sidewalls are rigid,whereas the top and bottom walls are flexible andmovable. The flexible top and bottom walls arecomputer co
39、ntrolled, with feedback provided on thewall positions and pressure distributions to achievealignment with model streamlines. This producesflow in the vicinity of the model that approachcs theflow which would be obtained for free-air conditions.Specific information on the adaptive-wall test sectionan
40、d a brief description of the strategy used to contourthe walls can be found in reference 32.Model DescriptionA rectangular cavity model was mounted on aturntable installed in the sidewall of the 0.3-m TCTto produce an angle of attack of 0 . Figure 7 showsthe cavity with pressure instrumentation prio
41、r toinstallation in the tunnel. The cavity was 2.50 in.wide by 11.25 in. long and the height was variedto obtain l/h ratios of 4.4 (h - 2.56 in.), 6.7 (h =1.68 in.), 12.67 (h = 0.89 in.), and 20.0 (h = 0.56 in.).The turntable could be rotated with respect to theflow to position the cavity with a yaw
42、 angle of 0 and15 .The model was instrumented with 18 static pres-sure orifices and 19 flush-mounted dynamic pressuretransducers. Sixteen of the dynamic pressure trans-ducers were mounted along the centerline (13 on thecavity floor and 3 on the tunnel sidewall adjacent toProvided by IHSNot for Resal
43、eNo reproduction or networking permitted without license from IHS-,-,-thecavity),1eachontheforeandaft wallsat half-depth and an additionalsealedtransduceron thecavityfloor. The instrumentationlayoutis shownin figure8. TableI providesthe measuredposi-tionsof thestaticpressureorificesandtableII pro-vi
44、desthemeasuredpositionsofthedynamicpressuretransducers.Test ConditionsThe model was tested in the 0.3-m TCT at Machnumbers from 0.2 to 0.9, unit Reynolds mnnbersranging from 2.0 x 106 to 100 106 per ft (free-stream total temperatures ranging from 105 K to320 K), and yaw angles of 0 and 15 (tile limi
45、tsof the range of the turntable). The data presentedin this report were obtaincd with a Reynolds num-ber of 30 106 per ft and a nominal free-stream to-tal temperature of 112 K. The boundary-layer thick-ness was approximately equal to 0.5 in. Details ofthe boundary-layer measurements and calculations
46、are given in reference 28, and table III summarizesthese calculated boundary-layer parameters. Theboundary-layer measurements were not made at thenominal test conditions indicated in table IV becauseof tunnel time constraints. The flexible test sectionwalls were set to a “streamlined“ shape for each
47、 testcondition.InstrumentationSurface static pressures. Because of the largechanges in dynamic pressure over the operating rangein the 0.3-m TCT (a factor of about 75), a high-precision capacitive-type transducer is used for pres-sure measurements. The electrical outputs from thetransducers are conn
48、ected to individual signal condi-tioners. The signal conditioners are set on autorangeto keep the transducer signal within voltage limits forthe data acquisition system for all pressure ranges.The transducers have a maximum range from -100to 100 lb/in 2 and have an accuracy of 0.25 percentfrom 25 pe
49、rcent of negative full scale to 100 percentof positive full scale. Additional details of the 0.3-mTCT pressure instrumentation system can be foundin reference 30.For the experimental data reported herein, eachorifice was sampled 40 times over a 1-sec period; thesedata were then averaged to produce the mean valuefor each data point.Fluctuating pressures.
