Lagrangian analysis of intermittent sound sources in the flow-field of a bluff-body stabilized combustor

Abstract

Experiments were performed in a partially premixed bluff-body stabilized combustor in the regimes of combustion noise, intermittency, and thermoacoustic instability. Simultaneous measurements of unsteady pressure fluctuations and flow-field using time-resolved two-component particle image velocimetry reveal dominant dynamics at 141.9 Hz which is responsible for thermoacoustic instability. In the intermittent regime that presages thermoacoustic instability, there are two distinct frequencies: a low-frequency component at 30.7 Hz dominant in the velocity spectra (hydrodynamic mode) and a higher frequency component at 176.4 Hz dominant in the pressure spectra (acoustic mode). Examining the phase relationship between the two modes in the intermittent regime using a variant of the Dynamic Mode Decomposition (DMD) confirms that the appearance of bursts of periodic pressure oscillations coincide with the time instants when the hydrodynamic and the acoustic modes are phase synchronized. To identify the flow structure dynamics observed only during sound production, we compute ridges in the fields of backward-time finite time Lyapunov exponents. The roll up of shear layers from the dump plane and the leading edge of the bluff body and subsequent impingement on combustor walls are identified as the dominant features of the flow during thermoacoustic instability as well as during the bursting stage of intermittency. We show convincingly that these identified dynamics correspond to the acoustic mode using DMD filtered flow fields comprising only of the acoustic mode.

Time–frequency localisation of intermittent dynamics in a bistable turbulent swirl flame

“…Spectral POD methods outlined by Sieber et al (2016) and Towne, Schmidt & Colonius (2018) provide additional ranking of the energetic modes according to Time-frequency localisation of intermittent dynamics 882 A30-3 their dynamic relevance but lack built-in mechanisms for separating closely packed frequency components and/or localising discontinuities in the temporal behaviours of the extracted modes. A variant of DMD that extracts time coefficients based on dynamic modes (Alenius 2014;Premchand et al 2019) is capable of capturing temporal disruptions in coherent structures with well-defined frequencies but is not suitable for handling the non-periodic dynamics that governs the flame-shape switching.…”

“…A variant of DMD that extracts time coefficients based on dynamic modes (Alenius 2014; Premchand et al. 2019) is capable of capturing temporal disruptions in coherent structures with well-defined frequencies but is not suitable for handling the non-periodic dynamics that governs the flame-shape switching.…”

The generation of screech tones by shock leakage

“…Again, following the approach of Premchand et al. (2019), in this work the particle advection is performed backwards in time, such that the maxima of the FTLE contours represent attracting rather than repelling LCS, as indicated in figure 5.

Figure 5.Schematic showing fluid parcel advection in forward time; the red line represents the forward time FTLE, and the blue line the backward time FTLE.

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“…In addition to facilitating an objective definition of the saddle points, LCS also enable a more intuitive comparison between the structures observed in the flow visualizations of § 3 and the velocity data of § 4. In this work the identification of LCS is performed by tracking contours of maxima in the field of finite-time Lyapunov exponents (FTLEs), following the methodology of Premchand et al (2019).…”

“…Again, following the approach of Premchand et al (2019), in this work the particle advection is performed backwards in time, such that the maxima of the FTLE contours represent attracting rather than repelling LCS, as indicated in figure 5.…”