Numerical study of the effects of a secondary flow on screech noise generated by supersonic jets

The effects of a secondary flow (often referred to as coflow) on the noise components generated by turbulent jets have been investigated in several experimental studies of the literature. For a supersonic jet, in particular, surrounding the jet by a secondary flow can suppress Mach wave radiation by rendering all turbulent motions intrinsically subsonic. For a shock-containing supersonic jet, it was also found to significantly affect the screech tones emerging at specific frequencies due to the establishment of aeroacoustic feedback loops. It has been reported that the secondary flow leads to a reduction in amplitude of the screech tones and can also cause a mode switching, i.e. a shift in the
dominant screech mode, or a change in the tone frequencies.

The effects of a secondary flow on screech tones can be explained by the modifications of the components involved in the screech feedback loops, namely the downstream-propagating jet instability-waves/coherent-structures, the shock-cell structure, the upstream-propagating acoustic waves resulting from the instability-wave /shock-cell interactions and the receptivity at the nozzle lip.
In most studies, the changes in the screech tone properties are attributed to the fact that a secondary flow can stretch the axial development of the shock cells, altering the spacing and strength of the shocks, and can modify the jet spreading rate and turbulence levels. A secondary flow is also likely to disrupt the feedback loops by diverting the propagation path or damping the upstream-propagating acoustic waves, leading to reduced or suppressed screech.

More information on the link below.