Low-frequency sound radiated by a nonlinearly modulated wavepacket of helical modes on a subsonic circular jet

A possible fundamental physical mechanism by which instability modes generate sound waves in subsonic jets is presented in the present paper. It involves a wavepacket of a pair of helical instability modes with nearly the same frequencies but opposite azimuthal wavenumbers. As the wavepacket undergoes simultaneous spatial-temporal development in a circular jet, the mutual interaction between the helical modes generates a strong three-dimensional, slowly modulating 'mean-flow distortion'. It is demonstrated that this 'mean field' radiates sound waves to the far field. The emitted sound is of very low frequency, with characteristic time and length scales being comparable with those of the envelope of the wavepacket, which acts as a non-compact Source. A matched-asymptotic-expansion approach is used to determine, in a self-consistent manner, the acoustic field in terms of the envelope of the wavepacket and a transfer factor characterizing the refraction effect of the background base flow. For realistic jet spreading rates, the nonlinear development of the wavepacket is found to be influenced Simultaneously by non-parallelism and non-equilibrium effects, and so a composite modulation equation Including both effects is constructed in order to describe the entire growth-attenuation-decay cycle. Parametric studies pertaining to relevant experimental conditions indicate that the acoustic field is characterized by a single-lobed directivity pattern beamed at an angle about 45 degrees-60 degrees to the jet axis and a broadband spectrum centred at a Strouhal number St approximate to 0.07-0.2. As the nonlinear effect increases, the radiation becomes more efficient and the noise spectrum broadens, but the gross features of the acoustic field remain robust, and are broadly in agreement with experimental observations.