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This paper explores the screech closure mechanism for different axisymmetricmodes in shock-containing jets. While many of the discontinuities in tonalfrequency exhibited by screeching jets can be associated with a change in theazimuthal mode, there has to date been no explanation for the existence ofmultiple axisymmetric modes at different frequencies. This paper provides justsuch an explanation. As shown in previous works, specific wavenumbers arisefrom the interaction of waves in the flow with the shocks. This provides newpaths for driving upstream-travelling waves that can potentially close theresonance loop. Predictions using locally parallel and spatially periodiclinear stability analyses and the wavenumber spectrum of the shock-cellstructure suggest that the A1 mode resonance is closed by a wave generated whenthe Kelvin-Helmholtz mode interacts with the leading wavenumber of theshock-cell structure. The A2 mode is closed by a wave that arises due tointeraction between the Kelvin-Helmholtz wave and a secondary wavenumber peak,which arises from the spatial variation of the shock-cell wavelength. Thepredictions are shown to closely match experimental data, and possiblejustifications for the dominance of each mode are provided based on the growthrates of the absolute instability.

Closure mechanism of the A1 and A2 modes in jet screech