Self-excited primary and secondary instability of laminar separation bubbles

Rodríguez, Daniel; Gennaro, Elmer M.; Souza, Leandro Franco de

doi:10.1017/jfm.2020.767

Cited by 22 publications

(30 citation statements)

References 103 publications

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“…As a consequence of this, the PSE integration ‘jumps’ from the downstream propagating disturbance wave to the upstream propagating one as it marches towards the saddle-point, and the results are meaningless. For u 0,rev ≥7.75%, the three-dimensional LSBs become absolutely unstable giving rise to self-sustained oscillations; ( 11 ) those cases are not of relevance for the present study. In the rest of the paper, only convectively unstable cases are discussed.…”

Section: Resultsmentioning

confidence: 88%

“…The streamwise, wall-normal and spanwise co-ordinates are denoted by x , y and z , respectively, and the corresponding velocity components by u , v and w . Following our previous works ( 9 , 11 , 12 ) , a family of LSBs on a flat-plate boundary layer is considered. Baseline LSBs are computed using an inverse non-similar boundary-layer formulation in which a streamwise distribution of the displacement thickness is prescribed, circumventing Goldstein’s singularity and allowing for the recovery of separated flows.…”

Section: Three-dimensional Laminar Separation Bubbles Resulting From mentioning

confidence: 99%

“…Rodríguez et al ( 11 ) . studied the non-linear evolution subsequent to the onset of the three-dimensional instability using direct numerical simulations, and determined that it corresponds to a supercritical pitchfork bifurcation: the non-linear interactions involving the fundamental wavenumber β c , its harmonics and the “mean flow distortion” (in this case the spanwise-uniform component β=0) result in the saturation of the disturbance growth and the formation of fully three-dimensional yet steady flows.…”

Section: Three-dimensional Laminar Separation Bubbles Resulting From mentioning

confidence: 99%

“…2). For higher reversed flows, an absolute instability of Kelvin–Helmholtz waves gives rise to self-excited oscillations for the three-dimensional LSBs ( 11 ) , and analysing the convective instability is no longer meaningful. These base flows have boundary-layer separation at x s ≈220–222 and reattachment at x r ≈320–325, which corresponds to Re L ≈37,600–38,200.…”

Section: Resultsmentioning

confidence: 99%

“…As a consequence of the centrifugal instability, LSBs develop a spanwise modulation of their properties (recirculation, streamwise and wall-normal extent). The ensuing non-linear evolution leads to the saturation of this primary centrifugal instability and results in fully three-dimensional flow fields ( 11 , 12 ) .…”

Section: Introductionmentioning

confidence: 99%

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Enhancement of disturbance wave amplification due to the intrinsic three-dimensionalisation of laminar separation bubbles

Rodríguez¹,

Gennaro

2018

Aeronaut. j.

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Previous studies demonstrated that laminar separation bubbles (LSBs) in the absence of external disturbances or forcing are intrinsically unstable with respect to a three-dimensional instability of centrifugal nature. This instability produces topological modifications of the recirculation region with the introduction of streamwise vorticity in an otherwise purely two-dimensional time-averaged flows. Concurrently, the existence of spanwise inhomogeneities in LSBs have been reported in experiments in which the amplification of convective instability waves dominates the physics. The co-existence of the two instability mechanisms is investigated herein by means of three-dimensional parabolised stability equations. The spanwise waviness of the LSB on account of the primary instability is found to modify the amplification of incoming disturbance waves in the linear regime, resulting in a remarkable enhancement of the amplitude growth and a three-dimensional arrangement of the disturbance waves in the aft portion of the bubble. Present findings suggest that the oblique transition scenario should be expected in LSBs dominated by the convective instability, unless high-amplitude disturbances are imposed.

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Section: Resultsmentioning

confidence: 88%

Section: Three-dimensional Laminar Separation Bubbles Resulting From mentioning

confidence: 99%

Section: Three-dimensional Laminar Separation Bubbles Resulting From mentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Enhancement of disturbance wave amplification due to the intrinsic three-dimensionalisation of laminar separation bubbles

Rodríguez¹,

Gennaro

2018

Aeronaut. j.

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The effect of compressibility on the primary global instability of unforced laminar separation bubbles

Gennaro

Souza

Rodríguez

2019

J Braz. Soc. Mech. Sci. Eng.

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Laminar separation bubbles present a three-dimensional self-excited instability mechanism which leads to the appearance of spanwise-periodic structures. In incompressible flow, this mechanism was found to become active at conditions in which wave-like, two-dimensional perturbations are only convectively unstable. In the absence of continuous external excitation, the three-dimensional instability is expected to dominate the flow dynamics and initiate the laminar-turbulent transition. This work extends previous analyses by incorporating the effect of compressibility at subsonic conditions. Two-dimensional numerical simulations of a flat-plate boundary layer with a prescribed free-stream deceleration and acceleration are carried out to obtain a set of model laminar separation bubbles. A linear stability analysis is applied then, to study the influence of Reynolds and Mach numbers on the three-dimensional instability.

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