The response of an axisymmetric jet placed at various positions in a standing wave

We explore experimentally whether axisymmetry breaking can be exploited for the open-loop control of a prototypical hydrodynamic oscillator, namely a low-density inertial jet exhibiting global self-excited axisymmetric oscillations. We find that when forced transversely or axially at a low amplitude, the jet always transitions first from a period-1 limit cycle to $\mathbb {T}^2$ quasiperiodicity via a Neimark–Sacker bifurcation. However, we find that the subsequent transition, from $\mathbb {T}^2$ quasiperiodicity to $1:1$ lock-in, depends on the spatial symmetry of the applied perturbations: axial forcing induces a saddle-node bifurcation at small detuning but an inverse Neimark–Sacker bifurcation at large detuning, whereas transverse forcing always induces an inverse Neimark–Sacker bifurcation irrespective of the detuning. Crucially, we find that only transverse forcing can enable both asynchronous and synchronous quenching of the natural mode to occur without resonant or non-resonant amplification of the forced mode, resulting in substantially lower values of the overall response amplitude across all detuning values. From this, we conclude that breaking the jet axisymmetry via transverse forcing is a more effective control strategy than preserving the jet axisymmetry via axial forcing. Finally, we show that the observed synchronization phenomena can be modelled qualitatively with just two forced coupled Van der Pol oscillators. The success of such a simple low-dimensional model in capturing the complex synchronization dynamics of a multi-modal hydrodynamic system opens up new opportunities for axisymmetry breaking to be exploited for the open-loop control of other globally unstable flows.

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Self-excited instability regimes of a confined turbulent jet flame at elevated pressure

Buschhagen

Gejji

Scalo

et al. 2022

Physics of Fluids

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The dynamic response of a confined, premixed turbulent jet flame is investigated at high thermal power densities ([Formula: see text] MW/m2/bar) and turbulence levels ([Formula: see text] 5 [Formula: see text]). As equivalence ratio and inlet jet velocity are varied at these conditions, multiple instability modes coexist: a low-amplitude instability ([Formula: see text] 9%) with longitudinal-mode fluctuations (1L and 2L) and two high-amplitude ([Formula: see text] 20%), high-frequency, transverse instability modes. While the axial modes are ubiquitous across the operational envelope, the transverse mode selection is sensitive to the equivalence ratio and reactant jet velocity. A linear stability analysis (LSA) of the confined base flow is performed to explore the flame perturbation in response to the density and temperature gradients, and the shear-layer instabilities in the flow. The high-frequency combustion instabilities are associated with a combined azimuthal hydrodynamic mode of the reactant jet, (1) at the combustion chamber near field and, (2) downstream in the fully developed region of the combustor. An excellent agreement was observed between the convectively unstable modes identified by the temporal LSA and the self-excited combustion instabilities in the experiment.

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The response of an axisymmetric jet placed at various positions in a standing wave

Abstract: Abstract

Cited by 3 publications

References 41 publications

Tailoring the gain and phase of the flame transfer function through targeted convective-acoustic interference

Tailoring the gain and phase of the flame transfer function through targeted convective-acoustic interference

Asynchronous and synchronous quenching of a globally unstable jet via axisymmetry breaking

Self-excited instability regimes of a confined turbulent jet flame at elevated pressure

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