Global response to forcing in a subsonic jet: Instability wavepackets and acoustic radiation

The linear flow response of a turbulent subsonic jet to low-level harmonic forcing is investigated in a fully non-parallel framework. The analysis is carried out for the configuration studied by Sandberg et al., and the predictions are validated against their DNS data. The present formalism relies on a singular mode analysis of the linear global resolvent operator for a forced compressible flow. We obtain the optimal distribution of volume forces inside the jet pipe that gives rise to the maximal flow response. Two definitions of optimality are explored, one based on the integrated energy of the instability wavepacket in the near field, the other based on the radiated acoustic power. Both definitions give similar results for the flow response, overall in good qualitative agreement with the nonlinear DNS results. The approach opens a new perspective on the instability response of jets and the associated noise generation.