Structural changes of laminar separation bubbles induced by global linear instability

Rodríguez, Daniel; Theofilis, Vassilios

doi:10.1017/s0022112010000856

Cited by 87 publications

(81 citation statements)

References 50 publications

(73 reference statements)

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“…Since the leading global eigenmode is unstable at the conditions monitored, the perturbation amplitude will grow exponentially and the flow field will undergo all the topology bifurcations described herein, until non-linear effects will appear and satúrate further growth. Similar stall-cell stmctures have been shown to exist in a laminar separation bubble embedded in adverse-pressure-gradient boundary-layers over a flat píate [20]. There too the origin of the topological changes lies with the global flow eigenmode.…”

Section: Discussionsupporting

confidence: 57%

“…In addition to the critical points known from the fíat píate analysis [20], new critical points appear on the suction side of the stalled airfoil, partly as an effect of the presence of the trailing edge. New topology bifurcations are possible then, as the critical points merge and disappear with increasing disturbance amplitudes.…”

mentioning

confidence: 86%

“…The chordwise component has a constant sign at each z-plane, decelerating the flow in fíz e (0, n/2) and accelerating it in fíz e (n/2, TT). The consequence The geometry of the airfoil introduces a new issue to the surface streamlines topology of separated flow, in comparison to its flat-plate counterpart [20]. The Kutta condition imposes flow stagnation at both the leading edge and the trailing edge.…”

Section: Topological Changes Exerted By the Stationary Global Modementioning

confidence: 99%

“…The presence of the threedimensional global mode of the separation bubble is therefore expected to give rise to new three-dimensional flow topologies, regardless of the disturbance amplitude. The different topological bifurcations arising from the linear superposición of a fíat píate boundary-layer flow with an embedded laminar separation bubble and its leading global mode have been discussed in [20]. There, it was shown that for disturbance amplitudes small enough to remain linear, the amplification of the stationary global mode leads to the well-known U-separation pattern, first introduced in the literature in work classifying separated flow from a topological point of view [12].…”

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confidence: 99%

See 3 more Smart Citations

On the birth of stall cells on airfoils

Rodríguez

Theofilis

2010

Theor. Comput. Fluid Dyn.

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Critical point theory asserts that two-dimensional topologies are deflned as degeneracies and any three-dimensional disturbance of a two-dimensional flow will lead to a new fhree-dimensional flowfleld topology, regardless of the disturbance amplitude. Here, the topology of the composite flowflelds reconstructed by linear superposición of the two-dimensional flow around a stalled airfoil and the leading stationary threedimensional global eigenmode has been studied. In the conditions monitored the two-dimensional flow is steady and laminar and is separated over a fraction of the suction side, while the amplitudes considered in the linear superposition are small enough for the linearization assumption to be valid. The múltiple topological bifurcations resulting have been analysed in detail; the surface streamlines generated by the leading stationary global mode of the separated flow have been found to be strongly reminiscent of the characteristic stall cells, observed experimentally on airfoils just beyond stall in both laminar and turbulent flow.

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Section: Discussionsupporting

confidence: 57%

mentioning

confidence: 86%

Section: Topological Changes Exerted By the Stationary Global Modementioning

confidence: 99%

mentioning

confidence: 99%

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On the birth of stall cells on airfoils

Rodríguez

Theofilis

2010

Theor. Comput. Fluid Dyn.

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“…The structure of the LSB was later examined by Rodríguez and Theofilis. 51 The recirculation bubble may possibly act as an amplifier for environment disturbances, or as an oscillator to sustain the instability waves propagated into it. 52 For the present airfoil at a high angle of attack, it is shown in Figures 5 and 6 that the recirculation bubbles elongate in the streamwise direction so that the base flow induces significant strain in the wake.…”

Section: Itemmentioning

confidence: 99%

BiGlobal linear stability analysis on low-Re flow past an airfoil at high angle of attack

Zhang

Samtaney

2016

Physics of Fluids

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CitationBiGlobal linear stability analysis on low-Re flow past an airfoil at high angle of attack 2016, 28 (4) We perform BiGlobal linear stability analysis on flow past a NACA0012 airfoil at 16• angle of attack and Reynolds number ranging from 400 to 1000. The steady-state two-dimensional base flows are computed using a well-tested finite difference code in combination with the selective frequency damping method. The base flow is characterized by two asymmetric recirculation bubbles downstream of the airfoil whose streamwise extent and the maximum reverse flow velocity increase with the Reynolds number. The stability analysis of the flow past the airfoil is carried out under very small spanwise wavenumber β = 10 −4 to approximate the two-dimensional perturbation, and medium and large spanwise wavenumbers ( β = 1-8) to account for the three-dimensional perturbation. Numerical results reveal that under small spanwise wavenumber, there are at most two oscillatory unstable modes corresponding to the near wake and far wake instabilities; the growth rate and frequency of the perturbation agree well with the two-dimensional direct numerical simulation results under all Reynolds numbers. For a larger spanwise wavenumber β = 1, there is only one oscillatory unstable mode associated with the wake instability at Re = 400 and 600, while at Re = 800 and 1000 there are two oscillatory unstable modes for the near wake and far wake instabilities, and one stationary unstable mode for the monotonically growing perturbation within the recirculation bubble via the centrifugal instability mechanism. All the unstable modes are weakened or even suppressed as the spanwise wavenumber further increases, among which the stationary mode persists until β = 4. C 2016 AIP Publishing LLC. [http://dx

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Special issue on global flow instability and control

Theofilis

Colonius

2011

Theor. Comput. Fluid Dyn.

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This special issue is intended to provide a snapshot of current research in the área of "Global Flow Instability and Control". The original papers, and to a certain extent the topic itself, are intimately linked with the series of symposia by the same ñame that were held in Crete, Greece, between 2001 and 2009. As members of the organizing committees of the Crete symposia, we invited all past participants to contribute, and all papers were reviewed following the strict standards of the journal. This preface gives a brief historical account of events that have shaped ideas in the fleld over the past decade, followed by a synopsis of the papers published herein. Keyword Global flow instability and control The Crete meetingsIn July 1998, Peter Duck and Anatoly Ruban organized the EUROMECH 384 CoUoquium on "Steady and Unsteady SeparatedFlows" in Manchester, with Sir James Lighthill as the keynote speaker. The proceedings of this colloquium were published as a Research Frontiers issue, volume 358, of the Philosophical Transactions of the Royal Society London journal. The preface of that issue contains the colourful hand-written transparencies of Sir James, which were bound to be the last scientiflc document written by this towering fluid mechanics figure. A summary versión of another set of hand-written colourful drawings, corresponding to the hour-long presentation of Uwe Dallmann on flow topology [6], was included as the last chapter of the three-part paper by Theofilis, Hein and Dallmann [39] and provided a qualitative description of the first quantitative discovery, via a partial-differential-equation (PDE)-based eigenvalue problem (EVP) solution, of what is now commonly called global mode of a laminar separation bubble. One of the highlights in the prematurely ended scientiflc career of Uwe Dallmann was the enthusiastic appreciation of his flow topology work by Sir James, which is only natural, given that the first appearance of the association between flow separation and flow topology was the work of Lighthill himself in the classic textbook of Rosenhead [19].

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Structural changes of laminar separation bubbles induced by global linear instability

Cited by 87 publications

References 50 publications

On the birth of stall cells on airfoils

On the birth of stall cells on airfoils

BiGlobal linear stability analysis on low-Re flow past an airfoil at high angle of attack

Special issue on global flow instability and control

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