INVESTIGATIONS OF
AERODYNAMICALLY GENERATED SOUND by
H. S. RIBNER
U. S. A. F. Contract No. AF 49(638)-249 FINAL TECHNICAL REPORT
JANUARY, 1962 UTIA REPORT NO. 81
INVESTIGATIONS OF
AERODYNAMICALLY GENERATED SOUND
by H. S. RIBNER
U. S. A. F. Contract No. AF 49(638)-249 Final Technical Report
•
JANUARY. 1962 UTIA REPORT NO. 81
FOREWORD
The research reported herein was carried out under US AF Contract No. AF 49( 638)-249, monitored by the Mechanies Division of the Air Force Office of Scientific Research. The contract period was the four calendar years 1958 through 1961. This is the final Technical Report required for compliance with the terms of the Contract (Item 2b( 2) of Supplemental Agreement 3( 60-770».
A number of people participated in the various investi-gations: their names appear in the References. The work was initiated by Dr. G. N. Patterson and the major effort in supervision was divided between Prof. B. Etkin - who was responsible for the Aeolian tones research - and Dr. H. S. Ribner. The last mentioned, as author of this document, has recorded a group activity, and the contributions of all participants are sincerely appreciated. Thanks are also tendered to Dr. J. H. T. Wade for direction of the work during academie 1960-61 while the author was on leave of absence in England.
The continued interest and financial support of the Air Force Office of Scientific Research are gratefully acknowledged.
SUMMARY
A four-year experimental and theoretical program has been carried out on aerodynamically generated sound. Thirteen investigatiol").s were completed. They dealt with (1) Aeolian tones: the sound emitted by a rod in all airstrearn, and the associated forces; (2) boundary layer noise-rigid wall: the direct sound generated by the turbulent boundary layer of a sp inning thick-walled cylinder; (3) boundary layer noise - fle~ible wall: the sound generated by panel vibration excited by turbulent duct flqW (simu-lation of boundary layer flow); (4) aerodynamic noise theory, particularly jet noise.
( 1) I. INTRODUCTION
This is the final or summary report on research on Aero-dynamically Generated Sound under the support of Contract AF 49( 638)-249. The four- year program has been aimed at developing a better understanding of the mechanisms of flow noise and related phenomena via theory and
experiment. The principal areas have been Aeolian tones, boundary layer noise. and jet noise. Twelve individual reports and notes have been pub-lished and two Ph. D. -theses will be published (Refs. 1 to 14). The follow-ing paragraphs will serve to introduce these papers and show how they relate-to one another and to the subject areas. The major results are summarized in a final section preceding the bibliography.
11. SURVEY PAPER (1958)
Reference 1 consists of material assembled for presentation at the "Jet Engines and Noise" session of the ICAS meeting in Madrid,
Spain, September, 1958. IIl. AEOLIAN TONES
A rod or wire in an airstream emits a fairly definite tone. The phenomenon is relevant to - among other things - the excitation of vibration of a slender missile on its launching pad. The investigation at UTIA antedated the subject Contract and produced an interim report (Ref. 15). This early work showed the need for measurement of pressures on the rod, of the correlation of these pressures along the rod, and of the inte-grated forces on the rod. These matters were investigated in detail in References 2 to 4.
An
overall summary of the completed project, superseding Reference 1, is in print (Ref. 5).IV. BOUNDARY-LAYER NOISE
The aerodynamic noise radiated by a turbulent boundary layer flowing over a rigid surface (one that cannot add to the sound by vibrating) is known to be relatively weak at subsonic speeds; thus it is easily masked by other noises, particularly in a wind tunnel.
An
experi-mental arrangement that avoids this masking utilizes the turbulent bound-ary layer on a thick-walled rotating cylinder. A comprehensive investiga-tion with such a cylinder has been carried out as a Ph. D. program (Ref. 6).( 2)
The turbulent boundary layer flowing over the relatively flexible skin of an aircraft in flight excites appreciabIe skin vibration,pro-moting strong noise in the interior. This behaviour has been simulated in the laboratory by means of a long turbulent-flow duet: a thin metal panel was flush mounted in one wall, and the sound radiated therefrom was investi-gated. The duet facility is described in Reference 7 and the final results of some four years' investigation with it in Reference 8.
V. JET NOISE
Acoustic energy flow in a moving stream (e. g., a jet) is
examined theoretically in Reference 9, The results are applied in Reference 10 to explore the question of enhancement of the generated sound by motion of the eddies.
In Reference. 11 Lighthill' s theory is applied to two regions of "similar" profiles in a jet. The analysis yields the nóise power emitted by a "slice" of jet as a funetion of the distance x of the slice from the nozzle. This leads to a suggestion for jet mufflers.
An acoustic principle of broad applicability is put forward in Reference 12. The so- called acoustic near field in a general acoustical situation is represented as a distribution of "sources" of fluid dilatation; the acoustic far field can be computed from this near field "source" pattern.
A theory of flow noise - ,emphasizing jet noise - essentially based on this principle (although antedating it) is put forward in Reference 13.
VI. SHOCK- VORTEX INTERACTION
The material of Reference 14 was presented at the 1957 Heat Transfer and Fluid Mechanics Institute, Pasadena. The present content is that of the Stanford University Press preprint with minor corrections and the addition of a list of symbols. Publication in this form, motivated by the limited circulation of the preprint, was supported by the present Contract, although the work preceded the Contract.
The analysis applies directly to a specific shock-tube experi-ment. In addition, it has some relevance to the problem of sound augmen-tation when jet turbulence in an overchocked jet passes through the shock pattern.
( 3)
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VII. RESUME OF MAJOR RESULTS
The experimental and theoretical investigations summarized herein have contributed to our understanding of Aerodynamically Generated Sound in a number of respects. Some of the more important results are:
1. ..A.eolian Toiles - A plausible theory of the generation of almost pure tones by a rod in a wind has been developed and the principa~ para-meters identified and measured. In particular, the connection between the rms. fluctuating lift and the correlation length of the shedding vortices has been recognized, and light has been shed on both by measurements. Substi-tution of the measured parameters in the theoretical equation yields sound pressures in acceptable agreement with experiment.
2. Boundary Layer Noise: Rigid Wall - The rotating cylinder experiment has yielded both near field and far field rreasurements of sound pressures and spectra. In the near field the nondimensional spectra for a variety of speeds collapse onto a single smooth curve. In the far field the measurements go beyond those of other investigators in yielding acoustic power and efficiency.
3. Boundary Layer Noise: Flexible Wall - A turbulent flow channel facility has been constructed. The sound generated by turbulent excitation of a panel flush-mounted in a wall has been intensively studied as a function of flow speed, panel thickness and turbulence scale (oe duct depth) . The data include careful measurements of panel damping, power output and spectra, and corresponding statistical measurements (spatial correlations) of the exciting turbulence pressures on the duct wall. These results are relevant to the prediction of boundary layer noise levels in aircraft.
4. Jet Noise - A compatible alternative to the quadrupole theory of jet noise has been developed; it appears to be conceptually simpIer. In this view turbulent fluctuations of momentum flux generate unsteady press-ures within the flow and nearby. In response to the presspress-ures the fluid elements contract or expand. These dilatations act essentially* like simple sources to radiate sound to the far field. By contrast, Lighthill' sapproach proceeds directly from the momentum flux (quadrupoles) to the far field, bypassing the dilations in the flow.
The strength distribution of noise sources along a jet has been derived via both theoretical models with identical results. Among other things, the effects of eddy convection have been treated analytically, demonstrating downstream enhancement; moreover, the apparent paradox of infinite emission at the Mach angle is resolved in the process.
*
A small part of the dilatation corresponds to the sound pressure and is not source-like.(4) -REF-ERENCES
Publications under the subject Contract are listed below in the order referred to in the text (Nos. 1 to 14).
*Reference 15 is a publication outside the Contract. loEtkin, B 0 , Ribner, H. S. 2. McGregor, D. M. 3. Prendergast, V. 4. el Baroudi, M. Y. 5. Keefe, R. T. 6. Wilson, L. N. 7. Maestrello, L.
"Canadian Research in Aerodynamic Noise", UTIA Review No. 13, July 1958, AFOSR TN 58-860, AD 203 662
"An
Experimental Investigation of theOscillating Pressures on a Circular
Cylinder in a Fluid Stream", UTIA TN No. 14 (June, 1957), AFOSR TN 58-339, AD 154 343. Also note by Etkin, B., and McGregor, D. M. in Physics of Fluids, Vol. 1, No. 2, p. 162, 1958.
"Measurement of Two-Point Correlations of the Surface Pressure on a Circular Cylinder", UTIA TN No. 23, July, 1958, AFOSR TN 58-861, AD 203 674
"Measurement of Two-Point Correlations of Velocity Near a Circular Cylinder Shedding a Karman Vortex Sheet", UTIA TN 31, Jan., 1960, AFOSR TN 60-835
"An
Investigation of the Fluctuating ForcesActing on a Stationary Cylinder in a Subsonic Stream and of the Associated Sound Field", UTIA Rep. No. 76, AFOSR 2147 (In print)
"An
Experimentéi.l Investigation of the NoiseGenerated by the Turbulent Flow Around a Rotating Cylinder", UTIA Rep. No. 57, April 1959, AFOSR. TN 59-487, AD 215780; also
(abridged) J. Acoust. Soc. Amer. az.,.10, pp. 1203-7. "UTIA Air Duct Facility for Investigation of Vibration Noise Induced by Turbulent Flow Past a Panel (Boundary-Layer Noise) ", UTIA TN No. 20, April 1958, AFOSR TN 58- 358, AD 154 263
8. Ludwig, G. R. 9. Ribner, H. S. 10. Ribner, H. S. 11. Ribner, H. S. 12. Ribner, H. S. 13. Ribner, H. S. 14. Ribner, H. S. *15. Korbaeher, G. K. Etkin, B. . Keefe, R. T. ( 5)
"Vibration Noise Indueed by Turbulent Flow Past a Panel (Boundary Layer Noise) ", (rough draft of UTIA Report; AFOSR TN number to be applied for af ter revision and printing)
"Note on Aeoustie Energy Flow in a Moving Medium", UTIA TN No. 21, April 1958, AFOSR TN 58-360, AD 154 265
"Energy Flux from an Aeoustie Souree
Contained in a Moving Fluid Element and lts Relation to Jet Noise", Jour. Aeous. Soe. Amer., Vol. 32, No. 9, Sept. 1960, pp. 1159-60 (L)
"On the Strength Distribution of Noise Sourees Along a Jet", UTIA Rep. No. 51, April 1958, AFOSR TN 58-359, AD 154 264. Also note in Jour. Acous. Soc. Amer., 1958, Sept. 1, p. 876
"'Incompressible' Acoustic Near Field as a Spatial Distribution of Sources Generating the Far Field", Jour. Acous. Soc. Amer., Vol. 33, No. 6, p. 850, June 1961 (abstract from Program 61st Meeting Aeous. Soe. Amer., Philadelphia, May 10-13, 1961)
"A Theory of the Sound From Jets and Other Flows in Terms of Simple Sources", UTIA Rep. No. 67, July, 1960, AFOSR TN 60-950 "The Sound Generated by Interaction of a Single Vortex with a Shock Wave", UTIA Rep. No. 61, June, 1959, AFOSR TN 59-1166
"Acoustic Radiation from a Stationary Cylinder in a Fluid Stream", UTIA Rep. No. 39, May, 1956. Also Jour. Acous. Soc. Amer., Vol. 29, No. 1, Jan. 1957
