[PDF] Interaction Between Air Flow And Airborne Sound In A Duct eBook

Author : Fridolin P. Mechel
Publisher :
Page : 64 pages
File Size : 45,25 MB
Release : 1965
Category : Air ducts
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Several studies of the interaction between air flow and airborne sound in a duct were made. Three projects were investigated: (1) The excitation of boundary layer distortions in a laminar boundary layer by simulated oscillatory flexural waves was investigated. Results show boundary layer waves are excited whenever the phase velocity of the flexure wave is in the instability range of the phase velocity of boundary layer waves. (2) The propagation of a pressure pulse wave front and the acoustic impedance of porous absorbers are examined in ducts with air flow. Measurements prove the sound energy of the wave front is directed towards the walls and the absorber impedance becomes nonlinear when the absorber is penetrated by the flow. (3) the effect on the acoustic radiation impedance of an orifice with flow discharge restricted by fences and diaphragms was investigated. The acoustic resistance was shown to increase at low frequencies due to the restrictions.

Author : F. P. MECHEL
Publisher :
Page : 18 pages
File Size : 15,6 MB
Release : 1962
Category :
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The interaction of a surface bending wave with the flow boundary layer is of practical interest. Several types of surfaces conducting bending waves suitable for boundary layer interaction are tested. For the investigation of the influence of the flow velocity profile in a ducton the sound attenuation the sound pressure profiles in a wide duct with flow were measured. The acoustic impedance of an absorber with flow along its surface was measured. The sound radiation from an orifice with flow discharge depends on the flow velocity. The reflection coefficient of the orifice increases with flow velocity. It depends strongly on the turbulence level of the jet behind the orifice. (Author).

Author : F. P. Mechel
Publisher :
Page : 46 pages
File Size : 32,58 MB
Release : 1967
Category :
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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).

Author : P. A. Mertens
Publisher :
Page : 34 pages
File Size : 45,82 MB
Release : 1967
Category :
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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. To accomplish this, the wave fronts were photographed with a schlieren optics in a flow duct with a large flow boundary layer thickness. Results of these experiments agree with the theories of geometrical acoustics and can be incorporated into the design of ducts having air flow and airborne sound. Also investigated were properties of porous absorbers in the presence of an air flow. Results show 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. (Author).

Author : F. P. Mechel
Publisher :
Page : 55 pages
File Size : 10,55 MB
Release : 1967
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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).

Author : F. Mechel
Publisher :
Page : 51 pages
File Size : 31,26 MB
Release : 1963
Category : Acoustics
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This report, Volume 4, presents the results (1961-1962) of several studies of the interaction between air flow and airborne sound in a duct. Three projects are discussed: (1) Part A: The effects of various sound fields on the excitation of the boundary layer of a flat plate are discussed relative to the stagnation point and the boundary layer itself. (2) Part B: The change in the cross-sectional distribution of the sound pressure in an absorbent duct with air flow is examined with respect to shape of the velocity profile of the flow. (3) Part C: The dependence of the acoustic radiation impedance of a tube orifice during flow conditions is investigated as a function of the flow velocity and turbulence level. Results show that as the flow increases the radiation impedance decreases.

Author : F. Mechel
Publisher :
Page : 62 pages
File Size : 39,21 MB
Release : 1963
Category : Acoustics
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This report, Volume 4, presents the results (1961-1962) of several studies of the interaction between air flow and airborne sound in a duct. Three projects are discussed: (1) Part A: The effects of various sound fields on the excitation of the boundary layer of a flat plate are discussed relative to the stagnation point and the boundary layer itself. (2) Part B: The change in the cross-sectional distribution of the sound pressure in an absorbent duct with air flow is examined with respect to shape of the velocity profile of the flow. (3) Part C: The dependence of the acoustic radiation impedance of a tube orifice during flow conditions is investigated as a function of the flow velocity and turbulence level. Results show that as the flow increases the radiation impedance decreases.