Theory of the acoustic feedback of subsonic free jets which impinge on obstacles, and comparison with experiments
Abstract
When a circular subsonic free jet impinges on an obstacle, blown flaps (STOL) or the ground (VTOL) for example, the sound pressure level is increased by about 10 dB, for certain aerodynamic and geometric parameters. The feedback mechanism, which causes this effect, reinforces the natural ordered structure of the jet turbulence - periodic ring vortices travel downstream. A pressure wave which originates from an obstacle inside the jet, spreading upstream in the jet, causes this reinforcement of the axisymmetric vortex mode. This pressure wave is explained by wave fronts travelling obliquely to the jet axis and totally reflected at the jet boundary layer.
- Publication:
-
Pract. Probl. on Longitudinal and Cross-blown Free Jets (ESA-TT-206)
- Pub Date:
- November 1975
- Bibcode:
- 1975pplc.nasa...84N
- Keywords:
-
- Feedback;
- Jet Impingement;
- Sound Generators;
- Subsonic Flow;
- Externally Blown Flaps;
- Free Jets;
- Ground Effect (Aerodynamics);
- Jet Flow;
- Short Takeoff Aircraft;
- Turbulent Jets;
- Vertical Takeoff Aircraft;
- Acoustics