Some small changes that make a mixing layer very quiet

Numerical solutions of the adjoint of the perturbed compressible flow equations are used to seek (and find) small inflow perturbations that bring a directly simulated randomly excited subsonic two-dimensional mixing layer flow into a 'nearby' but much quieter 'state'. This is discussed in the context of common wave-packet or instability wave models of free-shear-flow noise, which are known to qualitatively predict aspects of subsonic jet noise. Emprical eigenfunctions are used to demonstrate that a more regular wave-packet character underlies the pertubed flow, which otherwise appears superficially unchanged. This shows for the first time that the sensitivity of wave-packet type noise sources to subtle details of their space-time structure can, at least in principle, be exploited to effect large (>10dB) reductions in the noise from a nonlinearly active free shear flow.