Transient evolution of the global mode in turbulent swirling jets: experiments and modal stability analysis

TL;DR

This study combines experiments and modal stability analysis to investigate the transient development of the global mode in turbulent swirling jets, revealing the link to internal stagnation points and validating frequency predictions.

Contribution

It introduces a combined experimental and stability analysis approach to study transient global mode evolution in turbulent jets, challenging previous nonlinear mode hypotheses.

Findings

Global mode onset linked to internal stagnation point formation

Linear stability analysis accurately predicts global mode frequency during transient

No evidence found for nonlinear global mode as previously suggested

Abstract

Modal linear stability analysis has proven very successful in the analysis of coherent structures of turbulent flows. Formally, it describes the evolution of a disturbance in the limit of infinite time. In this work we apply modal linear stability analysis to a turbulent swirling jet undergoing a control parameter transient. The flow undergoes a transition from a non-vortex breakdown state to a state with a strong recirculation bubble and the associated global mode. High-speed Particle Image Velocimetry (PIV) measurements are the basis for a local linear stability analysis of the temporarily evolving base flow. This analysis reveals that the onset of the global mode is strongly linked to the formation of the internal stagnation point. Several transition scenarios are discussed and the ability of a frequency selection criterion to predict the wavemaker location, frequency and growth rate of the global mode are evaluated. We find excellent agreement between the linear global mode frequency and the experimental results throughout the entire transient. The corresponding growth rate qualitatively conforms to experimental observations. We find no indication for a nonlinear global mode as proposed by previous studies.