INTERACTION BETWEEN AIR FLOW AND AIRBORNE SOUND IN A DUCT.
GOETTINGEN UNIV (WEST GERMANY) PHYSIKALISCHES INSTITUT
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The sound propagation in moving air with locally varying flow velocity is investigated by means of shock waves from a spark discharge. For this purpose the resulting wave front is photographed with a schlieren optics in a flow duct with great flow boundary layer thickness. The results reveal that by this method the known theories of geometrical acoustics can fairly be represented and can experimentally be completed. From the measuring results statements can be made about the favourable design of absorbing flow ducts. The properties of porous absorbers are determined in the presence of an air flow. There a penetration of the absorber by the air flow yields a change of the flow resistance compared with still air. The acoustic flow resistance and the differential static flow resistance are experimentally found to have the same value. Moreover it is proved that turbulent motions along the surface of the absorber do not affect the flow resistance appreciably.
- Fluid Mechanics