Accession Number : AD0670576

Title :   INTERACTION BETWEEN AIR FLOW AND AIRBORNE SOUND IN A DUCT. VOLUME III. SUMMRY REPORT.

Descriptive Note : Final rept. Feb 65-Jan 66,

Corporate Author : GOETTINGEN UNIV (WEST GERMANY) PHYSIKALISCHES INSTITUT

Personal Author(s) : Mechel,F. P. ; Mertens,P. A. ; Schilz,W. M.

Report Date : NOV 1967

Pagination or Media Count : 46

Abstract : The report briefly summarizes experiments during 1963-64 on the interaction between subsonic airflow and airborne sound in a duct. Also summarized are the results of the following four experiments conducted in 1965. The first experiment investigates the effect of the wall impedance on the stability and phase velocity of boundary layer waves. Measurements revealed a step in the dispersion curve of the flow velocity; this agrees with the dispersion curve of the sound velocity for a transition from capactive to inductive wall impedance. The second experiment investigates a flow boundary layer modified by sinusoidal suction (as opposed to stationary suction) through a porous plate imbedded in the wall of a duct. The alternating motions present in the suction elements excite boundary layer disturbances which may lead to a quicker transition into turbulence as compared to stationary suction. The third experiment was performed to improve the stability of a flow boundary layer by the application of an appropriate wall impedance. It was concluded that the real part of the impedance must be below 0.2 rho c. The final and most detailed experiment investigates the reflection coefficient of cross-sectional area changes and apertures in a tube with sound superimposed on an air flow. Smooth cross-sectional transitions show no acoustical or flow losses, whereas abrupt area changes cause flow separation, turbulence, and flow losses, all of which are associated with acoustic losses. These losses are described as an ohmic resistance which is added to the impedance of the transition. (Author)

Descriptors :   (*AERODYNAMIC NOISE, *DUCTS), (*SOUND TRANSMISSION, POROUS MATERIALS), BOUNDARY LAYER, ACOUSTIC IMPEDANCE, TURBULENCE, WALLS, FLOW SEPARATION, REFLECTION, STABILITY, AIRBORNE, BOUNDARY LAYER TRANSITION, PROPAGATION, ORIFICES, ABSORPTION, WEST GERMANY

Subject Categories : Aerodynamics
      Acoustics
      Fluid Mechanics

Distribution Statement : APPROVED FOR PUBLIC RELEASE