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Cross-variable amplitude-frequency coupling during intermittency in a turbulent thermoacoustic system
Published online by Cambridge University Press: 21 February 2025
Abstract
We investigate flame–acoustic interactions in a turbulent combustor during the state of intermittency before the onset of thermoacoustic instability using complex networks. Experiments are performed in a turbulent bluff-body stabilised dump combustor where the inlet airflow rate is varied quasi-statically and continuously. We construct a natural visibility graph from the local heat release rate fluctuations ($\dot {q}'$) at each location. Comparing the average degree during epochs of high- and low-amplitude acoustic pressure oscillations (
$p'$) during the state of intermittency, we detect frequency modulation in
$\dot {q}'$. Through this approach, we discover unique spatial patterns of cross-variable coupling between the frequency of
$\dot {q}'$ and the amplitude of
$p'$. The frequency of
$\dot {q}'$ increases in regions of flame anchoring owing to high-frequency excitation of the flow and flame during epochs of high-amplitude
$p'$ dynamics. However, the frequency of
$\dot {q}'$ decreases in regions associated with flame-front distortions by large coherent vortices. In experiments with continuously varying airflow rates, the spatial pattern of frequency modulation varies with an increase in the average amplitude of
$p'$ owing to an increase in the epochs of periodic
$p'$ dynamics and the size of vortices forming in the flow. Dynamic shifts in the location of flame anchoring induce low-frequency fluctuations in
$\dot {q}'$ during very-high-amplitude intermittent
$p'$ dynamics. Our approach using conditional natural visibility graphs thus reveals the spatial pattern of amplitude-frequency coupling between the co-evolving flame and the acoustic field dynamics in turbulent reacting flows.
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- © The Author(s), 2025. Published by Cambridge University Press