Wavepacket coupling in screeching twin-jets

An analysis of the symmetry-locking mechanism in screeching twin-jet systems dominated by axisymmetric modes is performed in this work. The different waves supported by the flow in a range of jet conditions are obtained by means of a twin-jet vortex sheet model, which considers the shear layer as an infinitesimal region. Analysis of the bands of existence of upstream waves in the flow suggest that the jet separation greatly affects the ability of the flow to support anti-symmetric screech modes, while symmetric modes remain relatively unaffected by this parameter. Comparison with acoustic data shows that most tones lie in the frequency bands of existence of guided jet modes, supporting the hypothesis that resonance is closed by these waves. The dominant symmetry for each condition is obtained by means of a symmetry-imposed spectral proper orthogonal decomposition of schlieren data, which provides both mode shapes and energies of the most amplified coherent structures in the flow. Overall, it is shown that symmetric modes are more energetic for very low spacings, and no clear dominance is found for large spacings.