On the effect of spatial source coherence on the radiation of jet noise

The effect of spatial source coherence on jet noise has been investigated theoretically for a normalized cross-spectral density of the source quantity based on a convected Gaussian form and for different special source intensity distributions, Lighthill's approach being used. As a coherence parameter the ratio of two length scales has been introduced which characterize the spatial extension of the coherence and source volumes. It is found that for angles near to the jet axis the radiation increases with increasing source coherence, but can decrease for larger angles. The radiated sound power of a frequency component at low Mach numbers has a maximum for a certain value of the coherence parameter which depends on the Strouhal number. For convection Mach numbers approaching unity, the noise radiation is always increased by increasing source coherence. The present results have to be modified when effects such as mean flow-turbulence interaction or refraction are included, but these are neglected here.