Closure mechanism of the A1 and A2 modes in jet screech

TL;DR

This study explains the closure mechanisms of A1 and A2 jet screech modes by analyzing wave interactions with shock structures, matching predictions with experimental data and revealing how different modes are sustained.

Contribution

It provides a novel explanation for the existence of multiple axisymmetric modes at different frequencies in jet screech, linking wave interactions to mode closure.

Findings

A1 mode is closed by a wave from Kelvin-Helmholtz interaction with the shock-cell leading wavenumber.

A2 mode closure involves interaction with a secondary shock-cell wavenumber.

Predictions closely match experimental observations and growth rate analyses support mode dominance.

Abstract

This paper explores the screech closure mechanism for different axisymmetric modes in shock-containing jets. While many of the discontinuities in tonal frequency exhibited by screeching jets can be associated with a change in the azimuthal mode, there has to date been no explanation for the existence of multiple axisymmetric modes at different frequencies. This paper provides just such an explanation. As shown in previous works, specific wavenumbers arise from the interaction of waves in the flow with the shocks. This provides new paths for driving upstream-travelling waves that can potentially close the resonance loop. Predictions using locally parallel and spatially periodic linear stability analyses and the wavenumber spectrum of the shock-cell structure suggest that the A1 mode resonance is closed by a wave generated when the Kelvin-Helmholtz mode interacts with the leading wavenumber of the shock-cell structure. The A2 mode is closed by a wave that arises due to interaction between the Kelvin-Helmholtz wave and a secondary wavenumber peak, which arises from the spatial variation of the shock-cell wavelength. The predictions are shown to closely match experimental data, and possible justifications for the dominance of each mode are provided based on the growth rates of the absolute instability.