1、NASA Technical Paper 3669Cavity Unsteady-Pressure Measurements atSubsonic and Transonic SpeedsMaureen B. Tracy and E. B. PlentovichLangley Research Center Hampton, VirginiaNational Aeronautics and Space AdministrationLangley Research Center Hampton, Virginia 23681-2199December 1997Provided by IHSNot
2、 for ResaleNo reproduction or networking permitted without license from IHS-,-,-Available electronically at the following URL address: http:/techreports.larc.nasa.gov/ltrs/ltrs.htmlPrinted copies available from the following:NASA Center for AeroSpace Information800 Elkridge Landing RoadLinthicum Hei
3、ghts, MD 21090-2934(301) 621-0390National Technical Information Service (NTIS)5285 Port Royal RoadSpringfield, VA 22161-2171(703) 487-4650Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-ContentsSummary 1Introduction . 1Symbols 1Background 2Cavity How
4、 Field Types for Supersonic Speeds . 2Subsonic and Transonic Flow Field Types 3Experimental Methods . 4Model Description . 4Wind Tunnel and Test Conditions 4Instrumentation and Measurements . 5Data Reduction . 5Results and Discussion . 6Presentation of Data . 6Effect of Transducer Location 7Effect o
5、f Cavity IIh and Correlation With Open, Transitional, and Closed Flows 7Flow Field Analysis Using Cross-Channel Analysis Between Transducers . 8Effect of Cavity Width . 8Effect of Cavity Depth . 8Effect of Mach Number . 8Concluding Remarks 9References 9Figures 11iiiProvided by IHSNot for ResaleNo re
6、production or networking permitted without license from IHS-,-,-Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SummaryAn experimental investigation was conducted in theLangley 8-Foot Transonic Pressure Tunnel to determinethe flow characteristics of
7、rectangular cavities with vary-ing relative dimensions at subsonic and transonic speeds.Cavities were tested with width-to-depth ratios w/h of 1,4, 8, and 16 for length-to-depth ratios llh of 1 through17.5. The maximum cavity dimensions were 42.0 in. inlength, 9.6 in. in width, and 2.4 in. in depth.
8、 The bound-ary layer approaching the cavity was turbulent and hadan approximate thickness of 0.5 in. Unsteady- and meanstatic-pressure measurements were made at free-streamMach numbers M* from 0.20 to 0.95 at a unit Reynoldsnumber per foot R* of approximately 3 106.Unsteady-pressure results are pres
9、ented in this paper,which is a companion paper to one previously publishedon static-pressure results (NASA TP-3358).Unsteady-pressure results indicate that, as Uhincreases, cavity flows changed from resonant to nonres-onant with resonant amplitudes decreasing gradually.Resonant spectra are obtained
10、largely in cavities withmean static-pressure distributions characteristic of openand transitional flows. Resonance does occur for closedflow in some cases. Other results indicate that increasingcavity width or decreasing cavity depth while holding IIhfLxed has the effect of increasing resonant ampli
11、tudes andsometimes inducing resonance. The effects due tochanges in width are more pronounced. Decreasing M*has the effect of broadening the resonances. The effectsof varying length and M* on the resonant frequenciesare consistent with the Rossiter equation. The values ofthe resonant frequencies dis
12、play a slight sensitivity tochanges in width w and depth h for low values of llh.IntroductionWith renewed interest in internal carriage of storesand the need to safely separate stores over the entireflight envelope of the aircraft, knowing the cavity flowenvironments for all operational speeds is im
13、portant.Many investigations, both primarily experimental (refs. 1through 29) and primarily computational (refs. 30through 40), have been conducted to study the flow fieldsin rectangular cavities. These studies largely concen-trated on mean static-pressure distributions and/orunsteady-pressure spectr
14、a in cavities. They were con-ducted at speeds ranging from subsonic through hyper-sonic, with the largest amount of effort concentrated onsupersonic speeds since military aircraft generally oper-ate supersonically. Radiated acoustic pressure (refs. 24,25, 27, and 28) and store separation characteris
15、tics(refs. 18 through 23) have also been obtained in somestudies.Carrying weapons internally has aerodynamicadvantages in flight. Cavities (open weapons bays) inaerodynamic surfaces, however, can generate both steadyand unsteady flow disturbances. Changes in mean static-pressure distributions inside
16、 the cavity can result in largepressure gradients, and the unsteady flow disturbancescan generate self-sustaining oscillations which, in turn,generate acoustic tones that radiate from the cavity. Boththe steady and the unsteady flows can present difficultiesfor store separation from an internal weap
17、ons bay. Thesteady flows can generate large nose-up pitchingmoments, and the unsteady flows can induce structuralvibration. To ensure safe carriage and separation for sub-sonic and transonic speeds, the flow fields that developin cavities must be thoroughly characterized. The experi-mental study des
18、cribed herein was designed to accom-plish this by obtaining mean static-pressure distributionsand unsteady-pressure spectra in cavities with varyingrelative dimensions. The primary parameter of interestwas the cavity length-to-depth ratio llh because the cav-ity flow field is known to depend on Uh (
19、ref. 6). Twoother parameters, free-stream Mach number M* and theratio of cavity width-to-depth wlh, are included in thisstudy as they can affect the values of l/h at which flowtypes change (refs. 6 and 2, respectively). (Other parame-ters that affect the values of Uh at which flow typeschange, not e
20、xamined in this study, include the ratio ofboundary-layer height to cavity depth (ref. 1) and thelocation of stores in the cavity (ref. 21).) Static-pressuredata from this study have been used to define flow fieldtypes and to determine parameter sensitivities (ref. 5).Unsteady-pressure results are p
21、resented in this paper andare used to identify parameter combinations that supportcavity resonance and the effects of parameter changes.This report is a companion paper to the previously pub-lished paper on smile-pressure results, NASA TP-3358(ref. 5). An electronic “Supplement to NASA TP-3669“conta
22、ining the spectral data presented graphically in thisreport in ASCII format is available on request as aCD-ROM. A request form is included at the back of thisreport.SymbolsAI, A2, A3ACaooC Pff,FPLconstants from ramped sinusoidal functionalternating currentfree-stream acoustic wave speed, fpsp-p*pres
23、sure coefficient,q*frequency, Hzreduced frequencyfrequency of lengthwise acoustic mode, Hzfluctuating-pressure level, normalized withrespect to q*, dBProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-h cavity depth, in.k empirical ratio of shear layer
24、and free-stream velocities, function of M*, 0.57(ref. 6)1 cavity length, measured in streamwisedirection, in.Mach numberfree-stream Mach numberlongitudinal mode numberoverall sound pressure level, dBmeasured surface static pressure, psimeasured unsteady pressure, psiroot-mean-square pressure,f(p) 2,
25、 psifree-stream static pressure, psifree-stream total pressure, psifree-stream dynamic pressure, psifree-stream unit Reynolds number per footsound pressure level, normalized withrespect to audible sound, 2.9 x 10-9 psi, dBTt, free-stream total temperature, FU velocity, fpsU* free-stream velocity, fp
26、sw cavity width, in.x distance in strearnwise direction, positivedownstream, in. (see fig. 4(b)y distance in spanwise direction, positive leftfacing upstream, in. (see fig. 4(b)z distance normal to flat plate, positive down,in. (see fig. 4(b)tx empirical constant related to phase betweeninstabilitie
27、s in shear layer and upstreamtraveling pressure waves, function of llh,0.25 (ref. 6)y ratio of specific heat of test gas at constantpressure to that at constant volume, 1.4 forair8 boundary-layer thickness, measured at cen-ter of leading edge of cavity, in.BackgroundFlow field types for transonic sp
28、eeds have been iden-tified based on a detailed evaluation of static-pressuremeasurements (ref. 5) and referenced to established flowfield types for cavities in supersonic flows (refs. 2, 20,and 21). The flow field types identified for supersonicMM*mOASPLPpIPrmsP*P t,*q*R*SPL2speeds are used for refe
29、rence because off-surface flowvisualization and extensive computational studies areavailable for validation. Figure 1 presents schematicsof the supersonic flow field types and the associatedcharacteristic mean static-pressure distributions. Theseflow field types-open, closed, transitional-open, andt
30、ransitional-closed-are briefly discussed in the follow-ing paragraphs. (It is important to note that the use of theword “transitional“ in relation to cavity flows does notrefer to transition from laminar to turbulent flow.) Fol-lowing this discussion, a description of the flow fieldtypes identified
31、for transonic flows is presented. As withsupersonic cavity flow types, the subsonic and transonicflow types are defined by their characteristic mean static-pressure distributions and the values of llh identified asbounding the various flow types are approximate, limitedby the increments used in vary
32、ing l/h.Cavity Flow Field Types for Supersonic SpeedsThe first cavity flow field type identified for super-sonic speeds generally occurs when the cavity is “deep“(a small value of I/h), as is typical of bomber aircraftbays, and is termed “open cavity flow“ (fig. 1(a). Openflow occurs in cavities wit
33、h values of l/h less than orequal to 10. For this regime, the flow essentially bridgesthe cavity and a shear layer is formed over the cavity.This flow produces a nearly uniform static-pressure dis-tribution along the floor of the cavity which is desirablefor safe store separation. However, when open
34、 cavityflow occurs, a cavity resonance can be sustained. Themechanism that produces this resonance is understood tobe the reinforcement between instabilities in the shearlayer and upstream-traveling pressure waves generated atthe aft wall by the time-varying impingement of the shearlayer. These osci
35、llations can generate high intensityacoustic tones that can induce vibrations in the surround-ing structure, including the separating store, and lead tostructural fatigue (refs. 29 and 40). The frequencies atwhich these tones occur can be predicted with a semi-empirical equation known as the modifie
36、d Rossiter equa-tion (ref. 8)1 m - x(llh) .f_U-_M_,_I + (y- 1)/2M2 + 1/k(M)1Here fm is the frequency of a given lengthwise acousticmode; l, the cavity length; U*, the free-stream velocity;m, the longitudinal mode number; M*, the free-streamMach number; and y, the ratio of specific heat of the testga
37、s at constant pressure to that at constant volume (takento be equal to 1.4 for perfect gases). Two empirical con-stants are in this equation: ix, which depends on llh and isrelated to the phase between the instabilities in the shearlayer and the upstream traveling pressure waves, and k,Provided by I
38、HSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-which depends on M.o and gives the relative speed ofthe instabilities in the shear layer to free stream. Thevalues for the coefficients tx and k are taken fromreference 6 as 0.25 and 0.57, respectively. (The value o
39、ftx was obtained for an IIh value of 4.0 and k was an aver-age of values obtained for a range of M from 0.40 to1.20.) The modification (ref. 8) of the Rossiter equation(ref. 6) equates the cavity sound speed to the stagnationsound speed to accommodate high-speed flows.The second type of cavity flow
40、identified for super-sonic speeds occurs for cavities that are “shallow“ (largevalues of Uh), as is typical of missile bays on fighter air-craft, and is termed “closed cavity flow“ (fig. l(b).Closed flow occurs for cavities with values of Uh greaterthan or equal to 13. In this regime, the flow separ
41、ates atthe forward face of the cavity, reattaches at some pointalong the cavity floor, and separates again before reach-ing the rear cavity face. This flow produces a meanstatic-pressure distribution with low pressure in the for-ward region, a plateau in the attached region, and highpressure in the
42、aft region. Impingement and exit shocksare observed. The adverse static-pressure gradient pro-duced by closed cavity flow can cause the separatingstore to experience large pitching moments that turn thestore nose in_to the cavity. Acoustic tones generally donot occur for closed cavity flow at supers
43、onic speeds.The third and fourth cavity flow field types definedfor supersonic speeds are termed “transitional(transitional-open and transitional-closed)“ and are flowfields that occur for cavities that have values of llh thatfall between closed and open cavity flow, that is,values of llh between ap
44、proximately I0 and 13.Transitional-closed cavity flow (fig. l(c) occurs whenIIh is decreased from a value corresponding to closedcavity flow. The change in flow field type is signaled bythe collapse of the impingement and exit shocks into asingle shock and the disappearance of the plateau in themean
45、 static-pressure distribution. The shock signifiesthat although it does not remain attached, the flow hasimpinged on the floor. Similar to closed cavity flow,large static-pressure gradients occur along the cavityfloor and can contribute to large nose-up pitchingmoments. With a very small reduction i
46、n l/h from thevalue corresponding to the transitional-closed cavityflow, the impingement-exit shock wave abruptly changesto a series of compression wavelets; this indicates thatalthough the shear layer no longer impinges on the cavityfloor, it does turn into the cavity. This type of flow isreferred
47、to as “transitional-open cavity flow“ (fig. l(d).For this type of flow field, longitudinal pressure gradi-ents in the cavity are not as large as for transitional-closed cavity flow, and consequently, the problem ofstore nose-up pitching moment is not as severe as closedcavity flows. The acoustic fie
48、lds for the transitional-closed and transitional-open flow fields have not beendetermined.Subsonic and Transonic Flow Field TypesFigure 2 (reproduced from ref. 5) gives the charac-teristic static-pressure distributions for the various flowfield types defined for subsonic and transonic speeds. Aswith
49、 supersonic flow, open and closed cavity flow occur.For the range of I/h between those for open and closedflow, a gradual change occurs from open to closed flowand thus a single transitional type flow is defined (ratherthan transitional-open and transitional-closed as forsupersonic flow). In this regime, the flow turns into thecavity and may or may not impinge on the cavity
