1、NASA TECHNICALMEMORANDUMCMOsCMCONASA TM X-3292F H1CO EXMEASUREMENTS OF FARFIELDSOUND GENERATION FROMA FLOW-EXCITED CAVITYPatricia J, W. Block and Hanno HellerLangley Research CenterHampton, Va. 23665NATIONAL AERONAUTICS AND SPACE ADMINISTRATION WASHINGTON, D. C. DECEMBER 1975Provided by IHSNot for R
2、esaleNo reproduction or networking permitted without license from IHS-,-,-1. Report No.NASA TM X-32922. Government Accession No.4. Title and SubtitleMEASUREMENTS OF FARFIELD SOUNDGENERATION FROM ACAVITY7. Author(s)Patricia J. W. Block andFLOW-EXCITEDHanno Heller9. Performing Organization Name and Ad
3、dressNASA Langley Research CenterHampton, Va. 2366512. Sponsoring Agency Name and AddressNational Aeronautics andWashington, D.C. 20546Space Administration3. Recipients Catalog No.5. Report DateDecember 19756. Performing Organization Code8. Performing Organization Report No.L-1037610. Work Unit No.5
4、05-03-12-0311. Contract or Grant No.13. Type of Report and Period CoveredTechnical Memorandum14. Sponsoring Agency Code15. Supplementary NotesHanno Heller: Bolt Beranek and Newman, Inc., Boston, Massachusetts.16. AbstractResults of 1/3-octave-band spectral measurements of internal pressures andthe e
5、xternal acoustic field of a tangentially blown rectangular cavity are com-pared. Proposed mechanisms for sound generation are reviewed. Spectra anddirectivity plots of cavity noise are presented. Directivity plots show a slightlymodified monopole pattern. Frequencies of cavity response are calculate
6、d usingexisting predictions and are compared with those obtained experimentally. Theeffect of modifying the upstream boundary layer on the noise is investigatedand its effectiveness was found to be a function of cavity geometry and flowvelocity.17. Key Words (Suggested by Author(s)Cavity noiseAerody
7、namic soundDirectivity19. Security dassif. (of this report) 20.Unclassified18. Distribution StatementUnclassified - Unlimited/Subject Category 71Security Classif. (of this page) 21 . No. of Pages 22. Price“Unclassified 45 $3.75For sale by the National Technical Information Service, Springfield, Virg
8、inia 22161Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-MEASUREMENTS OF FARFIELD SOUND GENERATIONFROM A FLOW-EXCITED CAVITYPatricia J. W. Block and Hanno Heller*Langley Research CenterSUMMARYResults of 1/3-octave-band spectral measurements of inter
9、nal pressures and the externalacoustic field of a tangentially blown rectangular cavity are compared. Proposed mechanismsfor sound generation are reviewed. Spectra and directivity plots of cavity noise are presented.Directivity plots show a slightly modified monopole pattern. Frequencies of cavity r
10、esponseare calculated using existing predictions and are compared with those obtained experimentally.The effect of modifying the upstream boundary layer on the noise is investigated and itseffectiveness was found to be a function of cavity geometry and flow velocity.INTRODUCTIONAirframe noise, which
11、 is the noise produced by airflow over an aircraft, has beenrecently recognized as the noise floor for aircraft noise reduction (refs. 1 and 2). Subse-quently, much research effort has been aimed at identifying the noise sources on the airframeand determining their relative contribution to the overa
12、ll airframe aerodynamic noise.In 1974, Healy (ref. 3) presented a technique for estimating the level of the overall air-frame noise and a nondimensionalized airframe-noise spectrum for an aircraft in the cruise con-figuration. Gibson (ref. 4) extended the applicability of this technique to larger ai
13、rcraft inthe cruise configuration and further investigated the noise produced by the individual com-ponents present in the landing configuration such as the main and front wheel wells, landinggear, and flaps. He reports a substantial increase in perceived noise level when the aircraftchanges from th
14、e cruise to the landing configuration and concludes that the main contributorsto the noise increase were the landing-gear/wheel-well assembly and the wing/flap assembly.A state-of-the-art review of airframe noise by Hardin (ref. 5) revealed a number ofpapers concerning the production of sound by man
15、y types of aerodynamic surfaces in a flowincluding cavities. A rectangular cavity is the simplest model which can be used to describethe wheel well on an airframe. However, the majority of the papers concerning cavities wereBolt Beranek and Newman, Inc., Boston, Massachusetts.Provided by IHSNot for
16、ResaleNo reproduction or networking permitted without license from IHS-,-,-aimed only at understanding or reducing the large-scale internal cavity pressure oscillationsthat occur in such an arrangement. The magnitude of this internal pressure field was foundto be a function of fluid velocity, cavity
17、 dimensions, and the type of boundary layer pre-ceeding the cavity and the spectrum was shown to take on -a periodic or random characterdepending on these same parameters. Although sound radiation was observed there was nocomparison of the internal pressure field with the external sound field nor di
18、rectivity measure-ments in these studies, in fact no quantitative measurement of the external sound field wasmade.It appears reasonable to assume that the internal pressure field, which has been measuredextensively (refs. 6 to 17), is related to the external sound field (ref. 8). If this relationshi
19、pis strong, the sound-producing mechanisms can be understood in terms of the models alreadyput forth in these references as the cause of the large pressure fluctuations inside the cavity.Further, the application of internal pressure reducing designs could apply to noise reduction.The intent of this
20、paper is to investigate the effect of the change in velocity, cavitylength, and boundary-layer thickness on the farfield sound generated by a flow-excited cavity.Predictions schemes will be reviewed and presented in light of the data obtained in thisexperiment. The present work contains preliminary
21、sound-field measurements at eight farfieldpositions with respect to a simple rectangular cavity blown tangentially by a jet. It reportsthe first systematic attempt to measure the amplitude, spectra, and directivity of farfieldradiated cavity noise. The external sound field is compared with internal
22、pressure field whichwas measured at one point within the cavity.SYMBOLSc speed of soundD cavity depth, cmf frequency, Hzfm modal frequency, Hz, where m = 1, 2, . . .ky ratio of average vortex convection velocity to the free-stream velocity .L streamwise length of the cavity, cmM Mach numberm mode nu
23、mber2Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-p acoustic pressure inside cavity, N/m (Pa)P0 peak pressure at open end of cavity, N/m (Pa)are the polar and azimuthal angles, respectively. The air-flow was provided through a 7.6-cm-square nozzle
24、 mounted to a flat plate in which thecavity was placed. The plate was long enough so that the trailing-edge noise from the platewas not appreciable. Test velocities were 43 and 86 m/sec. The cavity was placed approx-imately 2.5 cm from the edge of the square nozzle. The trailing edge of the cavity (
25、seefig. l(t) was at least 15 cm upstream of the expected transition region of the jet for allconfigurations. The cavity configurations that were tested are listed in table II. The entiresequence of tests was repeated with a roughness element located on the lower lip of theProvided by IHSNot for Resa
26、leNo reproduction or networking permitted without license from IHS-,-,-nozzle as shown in figure l(a). Eight farfield microphones (sensors 2 to 9) shown in figure 1were placed approximately 0.8 m from the leading edge of the cavity to measure radiatedsound produced by the flow-excited cavity. This d
27、istance placed the microphones in the far-field geometrically and acoustically for frequencies above about 900 Hz. Sensors 2, 6, 7,and 9 were located beyond the edge of the flat plate but in the same plane. Sensor .1 waslocated in the center of the rearward facing wall of the cavity to measure the d
28、ynamic pres-sure at this point inside the cavity.The instrumentation used to obtain the data consisted of nine pressure sensors and theirpower supplies, amplifiers for signal conditioning, a real-time 1/3-octave analyzer, and an x-yplotter. Sensor 1 was a 1/4-inch piezoelectric dynamic pressure sens
29、or with flat frequencyresponse from 10 Hz to 20 kHz and a linear amplitude response (1 percent) to 180 dB (re2 X 10“ N/irr). Sensors 2 to 9 were 1/2-inch free-field condenser microphones withflat frequency response to 40 kHz. For each test condition (table II) on-line data wereobtained for each sens
30、or in the form of 1/3-octave-band spectra. All pressure levels (hydro-dynamic and acoustic) are relative to 2 X 10“ N/m .BACKGROUNDIn order to understand better the .data obtained in this experiment it is beneficial toreview the mechanisms that have been proposed as the cause of both the random and
31、thelarge-scale pressure fluctuations inside the cavity. There are four basic types of mechanismsor flow patterns. These source mechanisms can be classified as primary hydrodynamic orprimary acoustic. These mechanisms can be described with the help of figure 2 where thecavity is viewed from the side
32、and the flow is moving from left to right. The first sourcetype, which is purely hydrodynamic, is called the captive-vortex model and is shown in fig-ure 2(a). Captive vortices (cellular flow) have been observed for certain values of the cavitylength-to-depth ratio (refs. 8 and 12). The oscillatory
33、motion of this captive-vortex systemwithin the cavity has been considered as a cause of large-scale1 periodic pressure fluctuations(refs. 7, 8, and 12). When the cavity dimensions did not permit cellular flow, the large-scalesteady pressure oscillations diminished and the internal pressures were ran
34、dom. These flowpatterns were observed in deeper cavities where the length-to-depth ratio was less than about 2.The second type of source mechanism arises from the shear-layer oscillation which isshown in figure 2(b). Shear-layer oscillation (similar to that occurring with edge tones andvortex format
35、ion) is caused by the interaction of the shear layer shed by the leading edge ofthe cavity with the downstream edge of the cavity. This shear layer is folded into the cavityto form a vortex. As the vortex grows and travels down the cavity length the shear layermoves upward. As the vortex exits from
36、the cavity as a whole, it causes the shear layer tobend into the cavity. After the vortex is shed, the downward position of the shear layerProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-caused by the mass exit forces the fluid to enter the cavity. A
37、t this point another vortexbegins to form. Smoke tunnel photographs (ref. 9) and shadowgraphs (ref. 13) show this flowsequence clearly. The explanation and analysis originally given by Nyborg (ref. 6) for thisjet/edge interaction was extended by Spee (ref. 14) for a cavity, obtaining a relationship
38、for. .the frequencies of this type of shear-layer oscillation:2vrf Ltan - = - (m - 1, 2, . . .) (1)The normal displacement of the shear layer over the mouth of the cavity is similar to themechanism that excites a Helmholtz resonator. The resonance of the volume of air withinthe cavity is also though
39、t to enhance the sound radiation for deep cavities when excited inthis manner in the resonance region of the cavity (ref. 9).Another mechanism causing shear-layer oscillation is that proposed by Heller and Bliss(ref. 17) as the result of pressure measurements and a water-table visualization study. C
40、onsidera shallow cavity under an external high-speed flow. It appears that at the cavity trailing edgethere occurs a periodic mass intake and mass efflux, caused by an oscillatory motion of theshear layer at the trailing edge. The mass intake creates a region of overpressure in thetrailing-edge regi
41、on which sends out an internal pressure front, traveling upstream within thecavity towards the leading edge. There it reflects and travels downstream back towards thetrailing edge. This cavity internal pressure wave forces the shear layer “up,“ since the pressure-wave front divides a region of overp
42、ressure and underpressure, to which the shear layer neces-sarily has to adjust. Hence, the shear layer will bulge out into the free stream when abovea region of overpressure and cave in when above a region of underpressure. Thus, the shearlayer will assume some wavelike motion which is a result of t
43、he cavity internal pressure-wavemotion, leading to a periodic up-and-down motion at the trailing edge, which in turn createsthe overpressure necessary to sustain the process. Obviously there is a self-sustaining feedbackmechanism involved. At subsonic flow speeds, which are of interest in the contex
44、t of thispaper, the shear layer will tend to roll up, forming discrete vortices. Hence, the appearanceof discrete vortices is considered in this study as a byproduct of the shear-layer motion. Thefrequencies of this type motion are given by equation (2).The third mechanism is a hydrodynamic/acoustic
45、 feedback model suggested by Rossiter(ref. 13) and shown in figure 2(c). An interaction of the intense sound emanating from thetrailing edge with the shear layer will maintain a periodic shedding of vortices at the leadingedge. As these vortices travel downstream and impinge on or pass the trailing
46、edge soundis emitted. This sound travels upstream to interact with the shear layer at the leading edge,and the process is repeated. Rossiter obtained the following empirical prediction formulafor the frequency of the periodic pressure fluctuations:Provided by IHSNot for ResaleNo reproduction or netw
47、orking permitted without license from IHS-,-,-_ V m - 0.25 / i - /-f = r -T- (m = 1, 2, . . .) (2)L - + MkvIn reference 16, Heller, Holmes, and Covert modified Rossiters formula in order to iimprove the frequency prediction for higher Mach numbers. However, they point out thatfor Mach numbers below
48、0.5 his formula underpredicts the resonant frequency.The above three models are examples of hydrodynamic resonance. The captive vortexmodel is dependent on all dimensions whereas the oscillating-shear-layer and feedback modelshave preferred frequency predictions based on length only. All three model
49、s have velocitydependence. It is conceivable that under the proper conditions (dimensions and velocity) any,all, or none of the mechanisms for oscillatory response may exist.The fourth type of source mechanism, which is purely acoustic, is called an acousticresonator and is shown in figure 2(d). Broadband sound generated in the shear layer excitesthe acoustic modes of the ca
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