On the Secondary Instability of Forced and Unforced Laminar Separation Bubbles

Rodríguez, Daniel; Gennaro, Elmer M.

doi:10.1016/j.piutam.2015.03.026

Cited by 9 publications

(5 citation statements)

References 17 publications

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“…The first computations of three-dimensional linear instability eigenmodes (also referred to as tri-global instability analysis, Theofilis 2011) in the literature, either employing matrix-free (Tezuka & Suzuki 2006;Bagheri et al 2009;Feldman & Gelfgat 2010;Loiseau et al 2014) or matrix-forming (Gómez et al 2012;Rodríguez & Gennaro 2017) approaches, are relatively recent and still limited by the availability of computational resources. An alternative methodology is used here for the analysis of three-dimensional flows in which the streamwise variations take place on a scale which is large compared to that of the cross-stream plane (Rodríguez & Gennaro 2015;Siconolfi et al 2017), without simplifying assumptions on the in-plane shape of the disturbances. This is a natural extension of the classic weakly non-parallel approach that gave rise to the description of linear global oscillators (Chomaz, Huerre & Redekopp 1988;Huerre & Monkewitz 1990), but considering local (two-dimensional) cross-planes instead of (one-dimensional) velocity profiles.…”

Section: Scope and Outlinementioning

confidence: 99%

“…The result is a bifurcated state (with respect to the two-dimensional baseline LSB ) presenting a steady, three-dimensional laminar separation bubble. For baseline LSBs sufficiently close to the neutral conditions of the primary instability, this process can be studied by means of the weakly nonlinear stability theory, as done by Rodríguez & Gennaro (2015). The bifurcation is found to be a supercritical pitchfork one: two-dimensional LSBs with reversed flow below the critical value remain two-dimensional in the absence of external sustained excitation.…”

Section: Primary Instability: Steady Three-dimensionalizationmentioning

confidence: 99%

“…2012; Rodríguez & Gennaro 2017) approaches, are relatively recent and still limited by the availability of computational resources. An alternative methodology is used here for the analysis of three-dimensional flows in which the streamwise variations take place on a scale which is large compared to that of the cross-stream plane (Rodríguez & Gennaro 2015; Siconolfi et al. 2017), without simplifying assumptions on the in-plane shape of the disturbances.…”

Section: Introductionmentioning

confidence: 99%

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Self-excited primary and secondary instability of laminar separation bubbles

2020

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Section: Scope and Outlinementioning

confidence: 99%

Section: Primary Instability: Steady Three-dimensionalizationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Self-excited primary and secondary instability of laminar separation bubbles

2020

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“…Os escoamentos bifurcados,q 2D eq 3D , estáveis, recuperados pela expansão fracamente não-linear apresentados em [7] serão considerados como escoamentos base para análise do impacto da tridimensionalidade sobre a instabilidade do escoamento. A figura 1 mostra a evolução da frequência global complexa ω g e a localização do wavemaker X s correspondente ao oscilador global dominante encontrado como função de A, a amplitude da bifurcação.…”

Section: Resultsunclassified

Estudo da instabilidade secundária de uma bolha de separação laminar tridimensional.

Gennaro¹,

Rodríguez²,

Souza³

2017

Proceeding Series of the Brazilian Society of Computational and Applied Mathematics

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Resumo. As mudanças topológicas associadas com a tridimensionalidade das bolhas de separação laminares podem iniciar o processo de instabilidades naquelas bolhas em que o pico de escoamento reverso seja fraco demais para sustentar uma instabilidade absoluta bidimensional. O objetivoé investigar a rota precisa das instabilidades e a existência de instabilidade secundárias de escoamentos 3D bifurcados. Uma análise da instabilidade secundária das bolhas de separação laminar tridimensionais e estacionátias que aparecem como consequência da bifurcação do escoamento primária foi realizada. Uma nova metodologia foi proposta para esta análise e se baseia na análise fracamente não-paralela para escoamento 3D. Os resultados foram validados com os resultados obtidos pela análise de autovalor 3D. Os resultados obtidos indicam que a tridimensionalidade da bolha de separação originada pela instabilidade primária desempenha um papel fundamental na origem da instabilidade autossustentada das bolhas de separação na ausência de excitações externas.Palavras-chave. Instabilidade hidrodinâmica, separação de camada limite, problemas de autovalor IntroduçãoSeparação de escoamento laminar e a formação de bolhas de separação fechadaé um problema de importância tecnológica primária devidoà sua forte relação com as propriedades aerodinâmicas das superfícies de sustentação que operam em altosângulos de ataque. O comportamento das bolhas de separação laminar (LSB) acredita-se ser dominada por mecanismos de instabilidades hidrodinâmicas, e estes mecanismos descrevem o processo pelo qual um escoamento em regime permanente espontaneamente se torna transiente.A presente pesquisa tem por base as ideias de U. Dallmann apresentadas no final dos anos 80, que um mecanismo de instabilidade global pode existir agindo sobre os escoamentos de recirculação bidimensionais e que sob uma perturbação infinitesimalmente fraca 1 elmer.gennaro@sjbv.unesp.br 2

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“…3,8,18 In the complete absence of external forcing (zero freestream turbulence), several numerical studies have conjectured the presence of global instabilities, able to self-excite the transition process. [19][20][21] In cases of imposed external forcing (localized wall suction/blowing), increase of the forcing amplitude has a stabilizing effect and the LSB shrinks from both sides. 22,23 It is conjectured that this behaviour is a result of a feedback loop 23-25 that involves distortion of the mean flow.…”

Section: Introductionmentioning

confidence: 99%

Spatio-Temporal Response of a Laminar Separation Bubble Under Impulsive Forcing

Michelis

Kotsonis

2016

46th AIAA Fluid Dynamics Conference

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The transient response of a laminar separation bubble to impulsive disturbances of varying amplitudes is examined by means of time-resolved PIV. The impulse is two-dimensional and introduced in spanwise invariant fashion by means of a DBD plasma actuator. As a result, a wave packet emerges and amplifies while travelling through the separated shear layer. Laminar-turbulent transition is advanced, causing size reduction or elimination of the LSB. The latter are accompanied by the ceasing of vortex shedding. As the bubble recovers, it bursts and significantly elongates. The time interval until full recovery is independent of the forcing amplitude. The conjecture that bursting and flapping mechanisms are due to the same feedback loop is confirmed. A sufficiently high amplitude disturbance distorts the mean flow reducing the amplification rate of following disturbances, delaying transition to turbulence. Similarly, small amplitude disturbances mildly affect the amplification rate at an intermittent rate and correspondingly alter the transition characteristics leading to the flapping behavior. Nomenclaturea sampling frequency [Hz] δ * displacement thickness [m] f b burst frequency [Hz] ρ density [kg/m 3 ] f c carrier frequency [Hz] σ standard deviation h bubble height [m] τ ij Reynolds stress [kg/m 2 ] l bubble length [m] φ phase Re Reynolds number Φ POD spatial eigenfunction t time [s] ω vorticity [1/s] x, y, z space coordinates [m] u, v velocity components [m/s] Subscripts w wavelet coefficient s separation U 0 freestream velocity [m/s] r reattachment U ∞ local freestream velocity [m/s] ro reduced order φ phase-averaged 0 unforced

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On the Secondary Instability of Forced and Unforced Laminar Separation Bubbles

Cited by 9 publications

References 17 publications

Self-excited primary and secondary instability of laminar separation bubbles

Self-excited primary and secondary instability of laminar separation bubbles

Estudo da instabilidade secundária de uma bolha de separação laminar tridimensional.

Spatio-Temporal Response of a Laminar Separation Bubble Under Impulsive Forcing

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