Global instability of low-density jets

Coenen, Wilfried; Lesshafft, Lutz; Garnaud, Xavier; Sevilla, A.

doi:10.1017/jfm.2017.203

Cited by 42 publications

(68 citation statements)

References 32 publications

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“…This picture ( Fig. 2d) resembles the spectrum of the slowly developing jet of Coenen et al [7], reproduced in our Fig. 1.…”

Section: A Ginzburg-landau Model Problemsupporting

confidence: 88%

“…However, the shape of the arc branch in case 8 displays some differences with respect to all other cases, and similar mode patterns have been reported from jet calculations on long domains by Coenen et al [7], where no artificial damping was applied. In the vicinity of the least stable mode (circled in Fig.…”

Section: Possible Remedies Tested For a Non-parallel Jetsupporting

confidence: 81%

“…Note that those affected original eigenvalues are not merely altered by the feedback, but they rather disappear abruptly from the spectrum as they fall below the new branches. The spectra obtained with C = 10 −6 and 10 −2 resemble those of pure helium jets, as shown in figure 6 of [7], and those of cylinder wakes obtained by Marquet et al [20, their figure 16]. The least stable original eigenvalue lies just above the feedback branches, apparently unaffected.…”

Section: A Ginzburg-landau Model Problemsupporting

confidence: 63%

“…This paper investigates the effect of spurious pressure feedback, due to domain truncation, on the eigenmode spectrum of incompressible open flow problems. The investigation is motivated by observations made in recent linear instability studies of jet flows [12,7], where a prominent family of eigenmodes (black symbols in Fig. 1), referred to as the 'arc branch' from here on, was found to present features that suggest a resonance between the inflow and outflow boundaries.…”

Section: Introductionmentioning

confidence: 99%

“…Arc branch modes are found to be robust with respect to those details [3], but strongly dependent on the numerical domain length. Coenen et al [7] show that arc branch eigenfunctions of a jet are characterised by an integer number of wavelengths between the inflow and outflow boundaries, and they suggest an analogy with acoustic modes in a pipe of finite length. This analogy implies that arc branch modes are the result of unwanted resonance between the numerical boundaries, potentially leading to a spurious instability of the numerical system.…”

Section: Introductionmentioning

confidence: 99%

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Artificial eigenmodes in truncated flow domains

Lesshafft

2017

Theor. Comput. Fluid Dyn.

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Whenever linear eigenmodes of open flows are computed on a numerical domain that is truncated in the streamwise direction, artificial boundary conditions may give rise to spurious pressure signals that are capable of providing unwanted perturbation feedback to upstream locations. The manifestation of such feedback in the eigenmode spectrum is analysed here for two simple configurations. First, explicitly prescribed feedback in a Ginzburg-Landau model is shown to produce a spurious eigenmode branch, named the 'arc branch', that strongly resembles a characteristic family of eigenmodes typically present in open shear flow calculations. Second, corresponding mode branches in the global spectrum of an incompressible parallel jet in a truncated domain are examined. It is demonstrated that these eigenmodes of the numerical model depend on the presence of spurious forcing of a local k+ instability wave at the inflow, caused by pressure signals that appear to be generated at the outflow. Multiple local k+ branches result in multiple global eigenmode branches. For the particular boundary treatment chosen here, the strength of the pressure feedback from the outflow towards the inflow boundary is found to decay with the cube of the numerical domain length. It is concluded that arc-branch eigenmodes are artifacts of domain truncation, with limited value for physical analysis. It is demonstrated, for the example of a non-parallel jet, how spurious feedback may be reduced by an absorbing layer near the outflow boundary.Comment: to appear in Theoretical and Computational Fluid Dynamics (2017 or 2018

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“…This picture ( Fig. 2d) resembles the spectrum of the slowly developing jet of Coenen et al [7], reproduced in our Fig. 1.…”

Section: A Ginzburg-landau Model Problemsupporting

confidence: 88%

Section: Possible Remedies Tested For a Non-parallel Jetsupporting

confidence: 81%

Section: A Ginzburg-landau Model Problemsupporting

confidence: 63%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Artificial eigenmodes in truncated flow domains

Lesshafft

2017

Theor. Comput. Fluid Dyn.

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A Numerical Study of the Global Instability in Counter-Current Homogeneous Density Incompressible Round Jets

Wawrzak

Bogusławski

Tyliszczak

2021

Flow Turbulence Combust

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The paper focuses on a global instability phenomenon in counter-current round jets issuing from co-axial nozzles. Three different configurations that differ in a way of the counter flow generation are investigated. Besides typical configurations used in experimental and numerical research performed so far, in which suction applied in an annular nozzle is a driving force for the counterflow, a novel set-up is proposed where the annular nozzle is oriented in the opposite direction and placed above the main one. Such a configuration eliminates the suction of fluid from the main jet, which in previous research was found to have a destructive impact on the occurrence of the global flow instability. The research is performed using a large eddy simulation (LES) method and the computations are carried out applying a high-order numerical code, the accuracy of which has been proven in previous works and also in the present research through comparisons with available experimental data. The research is complemented by the linear stability analysis which supports the LES results and formulated conclusions. In agreement with a number of the previous works it has been shown that the global modes can be triggered only when the velocity ratio (I) between the main jet velocity and the velocity of the jet issuing from the annular nozzle is above a certain threshold level ($$I_{\text {cr}}$$ I cr ). It has been shown that in the classical configurations of the co-axial nozzles the range of $$I\ge I_{\text {cr}}$$ I ≥ I cr for which the global instability phenomenon exists is very narrow and it disappears for larger velocity ratios. Reasons for that have been identified through detailed scrutiny of instantaneous flow pictures. In the new set-up of the nozzles the global instability persists for a significantly wider range of I. It has been shown that $$I_{\text {cr}}$$ I cr depends on both the momentum thickness of the mixing layer formed between the counter-current streams and the applied configuration of the nozzles. The LES results univocally showed that the latter factor decides on the type of the instability mode (Mode I or Mode II) that emerges in the flow, as it directly influences on a length of the region where the counter-current streams are parallel allowing the growth of short or long wave disturbances characteristic for Mode I and Mode II, respectively.

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On the critical conditions for pool-fire puffing

Moreno-Boza

Coenen

Carpio

et al. 2018

Combustion and Flame

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Pool fires are known to undergo a bifurcation to a globally unstable puffing state driven by baroclinic and buoyant vorticity production. Although the supercritical puffing regime away from the bifurcation has been studied extensively in the literature, no detailed account has been given of the critical conditions for its onset, that being the purpose of the present paper. For the relevant canonical case of round liquid pools without swirl, aside from the inherent thermochemical and transport parameters associated with the fuel, pool-fire puffing is governed by a single dimensionless number, the Rayleigh number, which scales with the cube of the pool diameter. Consequently, for a fixed fuel and under fixed ambient conditions, there is a critical fuel pool diameter, associated with a critical value of the Rayleigh number, above which the flame starts puffing. A global linear stability analysis that accounts for the axisymmetry of the prevailing instability mode is developed here to describe the bifurcation. The mathematical formulation employs the limit of infinitely fast reaction, with account taken of the nonunity Lewis number and vaporization characteristics of typical liquid fuels. Predictions of critical puffing conditions, including critical diameters and puffing frequencies, are provided for methanol and for heptane pool fires, and the results are compared with results of new small-scale experiments under controlled laboratory conditions, reported here, yielding reasonably good agreement.

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Global instability of low-density jets

Cited by 42 publications

References 32 publications

Artificial eigenmodes in truncated flow domains

Artificial eigenmodes in truncated flow domains

A Numerical Study of the Global Instability in Counter-Current Homogeneous Density Incompressible Round Jets

On the critical conditions for pool-fire puffing

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