It was demonstrated that the addition of a secondary flow to a supersonic jet can reduce Mach wave emission when the convective velocity of the jet eddies are also subsonic values, provided that the secondary flow eddies are also subsonic with respect to the ambient (Papamoschou 1997; Papamoschou, Debiasi 1999).
Direct Computation of Mach wave radiation in an axisymmetric supersonic jet, AIAA Journal 35(1): 1574-1580.
Mach wave elimination from supersonic jets, AIAA Journal 35(10): 1604-1611.