Visualization of Noise from Cold Supersonic Jets

The acoustic field of small, cold supersonic jets has been studied by shadowgraph visualization. Preliminary experiments showed the importance of choosing an optimum film position for the visualization of sound waves. The interpretation of shadowgraphs is discussed. The apparent radiation field from the supersonic region of the flow is of relatively high frequency. In this field the order of dominance of the sound sources in the present case is (i) radiation from the nozzle, (ii) radiation from shock‐turbulence interaction in the flow, and (iii) Mach‐wave radiation. When the jet flow is deflected, the jet‐deflector interaction becomes the principal noise source. The significance of the nozzle radiation is believed to result from the small scale of the jet used and demonstrates the necessity for care when extrapolating model scale acoustic data. The comparative in significance of Mach‐wave radiation is probably due to the lower exhaust velocity of the cold jets. It is suggested that Mach reflection of spherical waves may be an important factor in the generation of “Mach waves.”