Using adjoint-based optimization to enhance ignition in non-premixed jets

Qadri, Ubaid Ali; Magri, Luca; Ihme, Matthias; Schmid, Peter J.

doi:10.1098/rspa.2020.0472

Cited by 4 publications

(4 citation statements)

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“…Magri (2019) and Qadri et al. (2021) for applications in reactive flows), and has furnished quantitative stability measures beyond the time-asymptotic limit and without the need for simplifying assumptions.…”

Section: Introductionmentioning

confidence: 99%

Optimal energy growth in pulsatile channel and pipe flows

Pier

Schmid

2021

J. Fluid Mech.

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Pulsatile channel and pipe flows constitute a fundamental flow configuration with significant bearing on many applications in the engineering and medical sciences. Rotating machinery, hydraulic pumps or cardiovascular systems are dominated by time-periodic flows, and their stability characteristics play an important role in their efficient and proper operation. While previous work has mainly concentrated on the modal, harmonic response to an oscillatory or pulsatile base flow, this study employs a direct–adjoint optimisation technique to assess short-term instabilities, identify transient energy-amplification mechanisms and determine their prevalence within a wide parameter space. At low pulsation amplitudes, the transient dynamics is found to be similar to that resulting from the equivalent steady parabolic flow profile, and the oscillating flow component appears to have only a weak effect. After a critical pulsation amplitude is surpassed, linear transient growth is shown to increase exponentially with the pulsation amplitude and to occur mainly during the slow part of the pulsation cycle. In this latter regime, a detailed analysis of the energy transfer mechanisms demonstrates that the huge linear transient growth factors are the result of an optimal combination of Orr mechanism and intracyclic normal-mode growth during half a pulsation cycle. Two-dimensional sinuous perturbations are favoured in channel flow, while pipe flow is dominated by helical perturbations. An extensive parameter study is presented that tracks these flow features across variations in the pulsation amplitude, Reynolds and Womersley numbers, perturbation wavenumbers and imposed time horizon.

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Section: Introductionmentioning

confidence: 99%

Optimal energy growth in pulsatile channel and pipe flows

Pier

Schmid

2021

J. Fluid Mech.

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“…(2016), Qadri et al. (2021) and Sayadi & Schmid (2021). Modal analysis has also been conducted for hot jets (Coenen et al.…”

Section: Introductionmentioning

confidence: 95%

“…In a recent study, we used resolvent analysis to identify the optimal forcing structures in a slot flame, leading to maximal heat release (Wang et al 2022). For linear analysis of diffusion jet flames, one may refer to Nichols & Schmid (2008), Qadri, Chandler & Juniper (2015), Moreno-Boza et al (2016), Qadri et al (2021) and Sayadi & Schmid (2021). Modal analysis has also been conducted for hot jets (Coenen et al 2017;Chakravarthy, Lesshafft & Huerre 2018) and in turbulent swirling flames with the presence of a precessing vortex core (Oberleithner et al 2015a,b).…”

Section: Introductionmentioning

confidence: 99%

Global linear stability analysis of a flame anchored to a cylinder

2022

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This study investigates the linear stability of a laminar premixed flame, anchored on a square cylinder and confined inside a channel. Many modern linear analysis concepts have been developed and validated around non-reacting bluff-body wake flows, and the objective of this paper is to explore whether those tools can be applied with the same success to the study of reacting flows in similar configurations. It is found that linear instability analysis of steady reacting flow states accurately predicts critical flow parameters for the onset of limit-cycle oscillations, when compared to direct numerical simulation performed with a simple one-step reaction scheme in the low Mach number limit. Furthermore, the linear analysis predicts a strong stabilising effect of flame ignition, consistent with documented experiments and numerical simulations. Instability in ignited wake flows is, however, found to set in at sufficiently high Reynolds number, and a linear wavemaker analysis characterises this instability as being driven by hydrodynamic mechanisms of a similar nature as in non-reacting wake flows. The frequency of nonlinear limit-cycle flame oscillations in this unstable regime is retrieved accurately by linear eigenmode analysis performed on the time-averaged mean flow, under the condition that the full set of the reacting flow equations is linearised. If, on the contrary, unsteadiness in the density and in the reaction rate are excluded from the linear model, then the congruence between linear and nonlinear dynamics is lost.

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“…A brief state-of-the-art survey is given for such studies on premixed flames. The reader may want to refer to [19,20,21,22,23] for work on diffusion flames.…”

Section: Introductionmentioning

confidence: 99%