Contains research objectives and summary of research on three research projects.U. S. Navy - Office of Naval Research (Contract N00014-76-C-0090

Ingard, K. Uno

Patrick, William P.

Singhal, Vijay K.

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X. PHYSICAL ACOUSTICSAcademic Research StaffProf. Uno IngardGraduate StudentsVijay K. SinghalGeorge P. SucciU. S. Navy - Office of Naval Research (Contract N00014-76-C-0090)Uno IngardOur research in acoustics is focused on noise generation by turbulent flow, andinteraction of sound with flow and structures, with particular emphasis on acousticallyinduced flow instabilities and structural instabilities. Specifically, we are investigatinpanalytically and experimentally interaction of jet noise with acoustical modes in ducts,and acoustically induced instabilities of control valves.1. EFFECT OF FLOW ON THE ACOUSTIC RESONANCES OF ANOPEN-ENDED DUCTUno Ingard, Vijay K. SinghalWe have analyzed the effect of flow on the acoustic resonances of an open-ended,hard-walled duct. The flow produces acoustic losses both in the interior of the ductand at the ends. Unless the duct is very long, typically 100 times the diameter, thelosses at the ends dominate. At flow Mach numbers in excess of 0. 4 the losses areso large that axial duct resonances are almost completely suppressed. The plane waveGreen's function for the duct with flow is expressed in terms of the (experimentallydetermined) pressure reflection coefficients at the ends of the duct, and the flow depen-dence of the complex eigenfrequencies of the duct is obtained. Some observationsconcerning the noise produced by the flow in the duct have been reported.12. FLOW EXCITATION AND NONLINEAR COUPLING OF ACOUSTICMODES IN A SIDE BRANCH CAVITY IN A DUCTUno Ingard, Vijay K. SinghalFor the first time we have observed nonlinear coupling of acoustic modes in sidebranch cavities in ducts excited acoustically by flow. As part of a broader study offlow noise in ducts, we have investigated the excitation by flow of the acoustic modesof a side branch cavity in a duct. Under certain conditions, we found evidence ofcoupling between the different cavity modes and the axial modes of the main duct.A paper on this work is being prepared for publication in the Journal of the AcousticalSociety of America.PR No. 117(X. PHYSICAL ACOUSTICS)3. A NEW APPROACH TO ACOUSTIC FILTERING WITH LINED DUCTSUno Ingard, William P. PatrickIn a different area, we have devised a new method of acoustic filtering at lowfrequencies. We consider a lined duct divided into two parallel branches by means of arigid partition. The phase speed of the fundamental mode in each branch depends onthe liner configuration and can differ markedly from the free-space phase speed. Ifthe liners in the two branches are not the same, the corresponding phase speeds willbe different and a relative phase lag between the waves in the two branches results.This, in turn, leads to interference between these wave components (and their reflec-tions) both at the exit and the entrance of the parallel branch pair. We have demon-strated both analytically and experimentally that this interference can be exploited forthe purpose of obtaining a low-frequency attenuation which is substantially larger thanthe attenuation of the unpartitioned duct. A paper on this subject was presented at theAcoustical Society of America Meeting, in San Francisco, November 1975.2 We are nowpreparing this paper for publication in the Journal of the Acoustical Society of America.References1. U. Ingard and V. K. Singhal, "The Effect of Flow on the Acoustic Resonances of anOpen-Ended Duct," J. Acoust. Soc. Am. 58, 788-793 (1975).2. U. Ingard and W. P. Patrick, "A New Approach to Acoustic Filtering with LinedDucts," J. Acoust. Soc. Am. 58, Supplement No. 1, p. S6 (1975).PR No. 117

Physical Acoustics

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Physical Acoustics

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