1、. .- t AN EXPERIMENTAL INV,ESTIGATION 7 A OF THE AIRFLOW OVER A CAVITY TWITH ANTIRESONANCE DEVICES by Donald A, Bzlell Ames Research Center I ofett Field, Cal$ 94035 I NATIONAL AERONAUTICS AND SPACE ADMINISTRATION WASHINGTON, D. C. MARCH 1971 - - E Provided by IHSNot for ResaleNo reproduction or net
2、working permitted without license from IHS-,-,- TECH LIBRARY KAFB, NM 19. Security Clanif. (of this report) Unclassified I111111 11111 IIIII 11l1111111111111111111111111 21. NO. of Pages 22. Price 20. Security Classif. (of this page) 73 2. Government Accession No. I 1. Report No. NASA TN D-6205 4. T
3、itle and Subtitle AN EXFERIMENTAL INVESTIGATION OF THE AIRFLOW OVER A CAVITY WITH ANTIRESONANCE DEVICES 7. Author(s) Donald ABuell Uwd 9. Performing Organization Name and Address $3.00 NASA Ames Research Center Moffett Field, Calif., 94035 12. Sponsoring Agency Name and Address National Aeronautics
4、and Space Administration Washington, D.C. 20546 15. Supplementary Notes - 0133126 3. Recipients Catalog No. 5. Report Date March 1971 6. Perfbrming Organization Code 8. Performing Organization Report No. A-3691 10. Work Unit No. 188-48-01 -01 -00-2 1 11. Contract or Grant No. 13. Type of Report and
5、Period Covered Technical Note 14. Sponsoring Agency Code 16. Abstract Airflow over deep cavities was investigated in a wind tunnel and in flight at high subsonic speeds at equivalent or achal altitudes of 7 to 12 km. The cavities were large, with opening dimensions on the order of 1 m. Pressures wer
6、e measured in and near the cavities with and without externally mounted devices intended to suppress resonance in the cavities. Some of the devices reduced the pressure fluctuations from as much as 50 to 2 or 3 times the amplitude occurring in a normal attached boundary layer. The pressure data were
7、 analyzed for spectral content, coherence, phase, shear-layer thickness, and shear-layer location. 17. Key Words (Suggested by Author(s) Cavity flow Antiresonance device Resonance Spoiler 18. Distribution Statement Unclassified - Unlimited Provided by IHSNot for ResaleNo reproduction or networking p
8、ermitted without license from IHS-,-,-Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-TABLE OF CONTENTS Page NOTATION v SUMMARY . 1 INTRODUCTION . 1 APPARATUS . 2 Wind Tunnel and Cavities . 2 Antiresonance Devices 3 Airplane . 3 Instrumentation . 3 T
9、ESTS . 4 Wind Tunnel . 4 Airplane . 5 CORRECTIONS 5 SPECTRAL ANALYSIS 5 RESULTS AND DISCUSSION 6 Root-Mean-Square Pressure Fluctuations . 6 Static Pressures 8 Frequency and Phase at Resonance 9 Pressure Spectra . 10 Cross-Spectral Characteristics . 11 Velocity Profiles . 12 Light Scattering . 13 CON
10、CLUSIONS 13 REFERENCES 17 TABLE 18 FIGURES . 19 APPENDIX - ACOUSTICAL NORMAL MODES OF THE WIND-TUNNEL CAVITY . . 15 . ll1 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Provided by IHSNot for ResaleNo reproduction or networking permitted without lic
11、ense from IHS-,-,-CP f G H h L 1 M m mV n P, 4 U V XYY YZ Y 6 NOTATION F - P, pressure coefficient, frequency of oscillation, Hz 9 spectral density of pressure, (N/m2 ) /Hz height of wraparound spoiler (fig. 5 and table l), cm height of inlet to cambered-plate diffuser (fig. 5 and table l), cm maxim
12、um streamwise dimension of cavity opening, m span of antiresonance device (fig. 3 and table l), m Mach number index of acoustical normal mode approximately tangent to cavity walls in plane parallel to opening index of vortex shedding mode index of acoustical normal mode along radii of cavity in plan
13、e parallel to opening index of acoustical normal mode perpendicular to cavity opening free-stream static pressure, N/m2 mean value of measured pressure, N/m2 rms of measured pressure fluctuation about mean value, N/m2 1 free-stream dynamic pressure, 2 pV2, N/m2 velocity computed from rake measuremen
14、ts, m/s free-stream velocity, m/s coordinates for pressure measuring locations (fig. 1 l), m coherence bet ween pressure fluctuations, GaGb square of modulus of cross spectrum between pressures a and b boundary-layer thickness on wind-tunnel wall with no cavity, 0.13 m V Provided by IHSNot for Resal
15、eNo reproduction or networking permitted without license from IHS-,-,-e mean phase angle between pressure fluctuations (positive when pressures at the first mentioned location lead), deg p free-stream density, kg/m3 vi Provided by IHSNot for ResaleNo reproduction or networking permitted without lice
16、nse from IHS-,-,-r AN EXPERIMENTAL INVESTIGATION OF THE AIRFLOW OVER A CAVITY WITH ANTIRESONANCE DEVICES Donald A. Buell Ames Research Center SUMMARY Airflow over deep cavities was investigated in a wind tunnel and in flight at high subsonic speeds at equivalent or actual altitudes of 7 to 12 km. Th
17、e cavities were large, with opening dimensions on the order of 1 m. Pressures were measured in and near the cavities with and without externally mounted devices intended to suppress resonance in the cavities. Some of the devices reduced the pressure fluctuations from as much as 50 to 2 or 3 times th
18、e amplitude occurring in a normal attached boundary layer. The pressure data were analyzed for spectral content, coherence, phase, shear-layer thickness, and shear-layer location. INTRODUCTION The investigation reported herein was initiated in support of plans to install an infrared telescope in an
19、airplane in order to carry it above the infrared-absorbing troposphere. Since windows pass only a small portion of the infrared spectrum, it is desirable to have the telescope enclosure open to the airstream: thus the goal of the investigation was to examine the airflow over and in a cavity represen
20、tative of a telescope enclosure, and to develop devices that would minimize pressure and light disturbances within the enclosure. The same goal with respect to pressure disturbances usually applies to any type of cavity on an airplane: therefore the configurations of the cavities used in the investi
21、gation were kept simple and as general as possible so that the results of the investigation would have wide applicability. The tests were limited to deep cavities (depth greater than width) and to high subsonic speeds. The airflow over cavities has been studied extensively in the past, especially at
22、 low speeds. The mechanism that produces “organ piping” has been of particular interest. Blokhintsev (ref. 1) showed that airflow over a deep cavity causes vortices to form at the mouth and to be shed at a given reduced frequency or multiples thereof. Dunham (ref. 2) photographed the formation of th
23、e vortices and noted that the fundamental reduced frequency is obtained when one vortex at a time is in the cavity opening. The first overtone occurs when two vortices are present, etc. These vortices move downstream and strike the rear of the cavity opening, exciting one of the natural acoustic mod
24、es of the cavity if the shedding frequency coincides with the acoustic frequency. Rossiter (ref. 3) observed flow at both subsonic and supersonic speeds over a variety of cavity shapes and proposed an empirical equation for the frequency of vortex shedding. The response of the acoustic modes depends
25、 on the impedance of the air in the cavity opening as well as on the impedance of the cavity walls. Harrington (ref. 4) noted an interaction between Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-shedding frequency and cavity response frequency whic
26、h he ascribed to vorticity effects on the impedance at the cavity opening. Plumblee, Gibson, and Lassiter (ref. 5) examined the effect of the moving airstream on the impedance at the opening and set forth a method for computing the acoustical response characteristics of a cavity. Response frequencie
27、s computed according to the method of Plumblee and excitation frequencies computed with the aid of Rossiters equation were further checked by East (ref. 6) in low-speed experiments. Unfortunately, neither method of computation establishes the mode to be expected, and some of the factors in the equat
28、ions, such as the velocity of the shed vortices, are not precisely defined for the general case. Nevertheless, the resonance phenomenon seems to be reasonably well understood, at least in a practical sense. Many investigations have also been directed toward suppressing resonance. For example, Rossit
29、er used small spoilers upstream of the cavity to reduce periodic pressure fluctuations. Certain adaptations of flow deflectors in the upstream position may accomplish the same purpose, although the primary function of deflectors is usually to reduce steady-state air velocity in the cavity (e.g., ref
30、. 7). Many other schemes have been proposed and tested, but none lent themselves as readily to the present application as those mentioned. It appears that very few investigators have concerned themselves with the goal of the present investigation - simultaneous suppression of resonance and minimizat
31、ion of the associated random pressure fluctuations. The present investigation consisted basically of measuring pressures in cavities with and without various antiresonance devices. To obtain full-scale Reynolds numbers the cavities were large, with an opening greater than 1 meter. Moreover, the rela
32、tionship between the tunnel boundary-layer thickness and the size of the opening was representative of what would be expected on an airplane. A few pressure measurements were also made in a Boeing KC-1 35 airplane in which an open telescope enclosure had been installed. The latter measurements were
33、made with the cooperation of MIT Lincoln Laboratories and the U.S. Air Force, which operated the airplane. It might be mentioned that air injection was tried in the cavities installed in the wind tunnel and found unsatisfactory as a resonance suppressor, but the crudeness of the injection system ren
34、dered the results inconclusive. Another goal of the wind-tunnel tests was the minimization of light disturbances, and optical measurements were made for this purpose; however, problems due to vibration of the optical system were not satisfactorily resolved, and the results can only be considered qua
35、litative. APPARATUS Wind Tunnel and Cavities The majority of the tests were performed in the Ames 6- -y 6-Foot Supersonic Winc Tunnel. The test section of this facility is square in cross section and has slots in the floor and ceiling to permit transonic testing. The cavities were formed by removing
36、 a window in the side of the wind-tunnel test section and attaching large welded steel tubing of circular cross section as illustrated in figure 1 : the wall thickness of the tubing was 6.4 mm. The “shallow” cavity was formed by installing the wind-tunnel window at the end of the first section of tu
37、bing, at which point the tubing was supported from the 2 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-r tunnel framework. The deeper cavity was formed by moving the window to the end of two additional sections of tubing cantilevered from the fist
38、section. Figure 2 is a photograph of the deep cavity structure. Antiresonance Devices The devices employed to suppress resonance were modifications of two basic configurations. One configuration was the high-aspect ratio “diffuser” illustrated in figure 3: the device was constructed so as to allow a
39、ir to pass beneath a ramp, through the diverging channel formed by the ramp and the wall, through a porous surface, and over the cavity opening. It was intended that this diffusion process would cover the opening with a thick blanket of low-energy air. The porous surface consisted of an expanded-met
40、al gridwork normally covered with perforated steel sheet to provide the desired porosity. The height of the diffuser was approximately equal to the thickness of the boundary layer on the wind-tunnel wall. Table 1 gives the geometric details of this and the other antiresonance devices. A porous spoil
41、er (figs. 3 and 4) was derived from ,the diffuser configuration by cutting away the ramp between ribs, and a second variation was obtained by substituting a perforated steel sheet for the 45” ramp and omitting the covering on the expanded-metal framework. The second basic configuration was a long di
42、ffuser wrapped around the cavity opening as shown in figures 5 and 6. It consisted of a cambered plate with a streamlined cross section mounted on a porous support. A “wraparound” spoiler (fig. 7) was created by removing the plate. The various heights and porosities tested are given in table 1. Airp
43、lane The airplane in which measurements were made was a Boeing KC-135 operated by the U.S. Air Force under the project name “Press.” The airplane contained a telescope in a cavity that was open to the airstream. The airplane and the rectangular opening to the cavity are shown in figure 8. The telesc
44、ope was mounted parallel to the axis of the airplane, and objects to the side of the airplane were viewed by means of a dynamically stabilized flat mirror. Figure 9 indicates the location of the larger objects in the cavity: the mirror, gimbals, torque motors, and the front end of the telescope tube
45、. The cavity was protected by a permanently mounted 45” diffuser of the same general shape and height as the 45” diffuser tested in the wind tunnel. This configuration was, in fact, originally developed for the airplane from work described in reference 7. Although the diffuser was no longer than the
46、 actual cutout in the side of the fuselage, a sliding door partially covered the cutout at the top leaving the diffuser extended, in effect, about 30 cm beyond the upper side of the opening. Instrumentation The primary instrumentation for the tests was a number of differential-pressure transducers e
47、ach 3.2 mm in diameter. The transducers were basically 1.2 mm circular diaphragms connected to semiconductor strain gages of approximately 2500 a and installed so that the diaphragms were 3 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-I I I I I I
48、I I I II I i I 1 i I I I I1 ll11l1ll11l IIIIII I I I I 111 I I Ill11 II I II 111 I II 111 III1111 Ill 1111 I II I II II I 11III111111 1111 11111 1111 1111III flush with the mounting surface. The transducers were connected to individual direct-current power supplies and amplifiers; the signals were recorded on a frequency-modulated magnetic-tape recorder. The frequency response of a complete system was limited primarily by the speed of the recorder. The response was flat from 0 to 2500 Hz in the wind-tunnel system and was flat from 0.5 to 10,000 Hz in the airplane. The connection from the
