Continuation of Near-Acoustic Fields of Jets to the Far Field: Part II Experimental Validation and Noise Source Characteristics

This is the second of a two-part paper. This part reports the results of a systematic experimental study of the near acoustic field of a high-speed jet. The aim is to shed light on the noise source characteristics of the jet. A microphone array consisting of twenty-one semicircular rings of microphones is built for the experiment. The microphone array extends from one diameter from the nozzle exit to a distance of thirty-one diameters downstream. The microphones lie on a conical surface of ten degrees half-apex angle surrounding the jet. Two-point space-time pressure correlations are measured. There are two principal objectives in this study. The first objective is to validate the near field to far field continuation method, developed in the Part I paper, with experimental data. For this purpose, two series of validation tests are performed. First is a comparison of far field noise spectra between those continued from near field measurements on the conical surface and those measured directly in the far field. Another test series consists of comparisons of far field directivities at selected Strouhal numbers, again, between those continued from the near field measurements to the far field and those measured directly in the far field. Favorable agreements are found. These tests not only validate the continuation method developed in the Part I paper but also support the recognition that the two-point space-time pressure correlation function on the conical surface is the equivalent source of jet noise. This realization offers a way to investigate the characteristics of jet noise sources by analyzing those of the equivalent noise sources. The second objective of this experimental study is to investigate the noise source location, strength, size, convection speed, degree of randomness and other characteristics along the length of the jet. The noise source can also be decomposed into azimuthal modes. It will be shown that the axisymmetric noise source mode is the most dominant. Contributions to the radiated noise from modes higher than the second mode are negligible. Other modal characteristics such as propagation speed, location of maximum intensity and others will be reported. Finally, strong evidence will be presented to show the wave-like nature of the source of high-speed jet noise.