On the Connection Between Near and Far Pressure Fields of a Turbulent Jet

The paper presents the development of a simple stochastic model for the noise source of a round turbulent jet. The model is defined on a surface at the rotational/irrotational boundary of the jet and comprises a linear pressure event with random origin and random helical mode. The probability density function for the event’s axial origin is derived from the statistics of coherent structures in shear layers. The distribution for the helical mode is mostly empirical and driven by the need to match the polar directivity of sound emission in the far field. The generic form of the event is a self-similar wavepacket whose shape parameters are determined by least-squares matching of the far-field sound pressure level at a wide range of frequencies and polar angles. Initial results indicate that the model reproduces, in a qualitative sense, key statistics of the jet acoustic field, including the nearfield space-time correlation, the broadening of the far-field spectral density with increasing polar angle from the jet axis, and the coherence between the near and far fields. For the latter, the analysis indicates that the rapid decline in the coherence with increasing polar angle is primarily due to the randomness of the event’s axial origin.