INTERACTION BETWEEN AIR FLOW AND AIRBORNE SOUND IN A DUCT: VOLUME II. ACOUSTIC EXCITATION OF BOUNDARY LAYER AND RADIATION IMPEDANCE OF ORIFICES.
Final rept. Feb 64-Jan 65,
GOETTINGEN UNIV (WEST GERMANY) PHYSIKALISCHES INSTITUT
Pagination or Media Count:
The report presents the results of three experimental studies on the interaction between air flow and airborne sound in a duct. The first experiment concerns the excitation and suppression of boundary-layer distortions created by flexural wave-like wall oscillations. Mathematical approximations show and experimental results confirm that the stability of the boundary layer is shifted to higher Reynolds numbers by the flexural wave. The second experiment relates the boundary-layer produced by static suction stabilization to the sound field influencing it. Results show that the particle velocity of sound penetrating a porous section of the wall leads to an increase of boundary-layer distortions and that this effect could be suppressed when the flow velocity of the static suction exceeded a threshold value. The final experiment shows the influence of a superimposed air flow on the acoustic impedance of orifices terminating a tube and of constriction, enlargements, and nozzles within a tube. Results show that if the frequency is scaled according to the Mach number, then the ratio between the reflection coefficient gamma pm at Mach number M and the reflection coefficient at zero flow velocity gamma po is independent from the frequency. Author
- Fluid Mechanics