High-resolution measurements of two-dimensional instabilities and turbulence transition in plane mixing layers

Estevadeordal, Jordi; Kleis, Stanley J.

doi:10.1007/s003480050362

Cited by 17 publications

(13 citation statements)

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“…Once the spanwise-oriented vortices grow to a sufficient size, a subharmonic pairing instability then develops where two sequential vortices begin to orbit and agglomerate, through m utual induction of their vorticity fields, into a larger vortex, from which a subharmonic of th e fundamental instability frequency develops. The vorticity roll-up and pairing process is shown schematically in Figure 2.1, as observed by Estevadeordal and Kleis (1999). As was pointed out by Ho and Huang (1982), the change of the local length scale of the shear layer as a result of the vortex merging makes the original subharmonic become neutrally stable, and the new subharmonic becomes th e wave of most rapid amplification.…”

Section: Free Shear Layersmentioning

confidence: 69%

“…Through analysis based on linear stability theory, Rist and Maucher (2002) unstable to subharmonic disturbances (Kelly, 1967). This subharmonic instability leads to the vortex pairing phenomenon as observed in numerous experimental and numerical studies of such shear layers (Ho and Huerre, 1984;Huang and Ho, 1990;Estevadeordal and Kleis, 1999). This pairing of vortices is suggested as the dominant mechanism associated with growth of planar free-shear layers (Ho and Huerre, 1984).…”

Section: Instability M Echanismsmentioning

confidence: 96%

“…Through receptivity of a free shear layer to small disturbances, the K-H instability gives rise to the roll-up of vorticity into discrete two-dimensional vortices, often called "Kelvin cat-eyes" , connected by a line of concentrated vorticity called a "braid" , as described by Estevadeordal and Kleis (1999). The frequency of maximum amplification with which this instability develops has been identified by Ho and Huerre (1984) to correspond to a Strouhal number of 0.016, based on the mom entum thickness and the velocity difference across the shear layer (Sre -f ■ 8/AU).…”

Section: Free Shear Layersmentioning

confidence: 99%

“…Huang and Ho (1990) th a t the generation of small scale turbulence, hence transition, occurs at a location where both streamwise vorticity and an agglomerating vortex pair are first present. Despite the small-scale breakdown to turbulence, the large-scale vortical structures remain coherent for a significant distance downstream (Ho and Huerre, 1984;Estevadeordal and Kleis, 1999).…”

Section: Free Shear Layersmentioning

confidence: 99%

“….1: K elvin-H elm holtz in stab ility roll-up and pairing process in a free shear layer (adapted from E stevadeordal andKleis, 1999) …”

mentioning

confidence: 99%

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Transition in separation bubbles: physical mechanisms and passive control techniques

McAuliffe¹

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A thesis subm itted to the Faculty of G raduate Studies and Research in partial fulfillment of the requirements for the degree of D o cto r o f P h ilo so p h y in Aerospace Engineering Direction du Patrimoine de I'edition Bien que ces formulaires aient inclus dans la pagination, il n'y aura aucun contenu manquant. i * i Canada Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.Dedicated to m y wife Rakhi and to m y daughter P riya E m m a.This thesis documents complementary experimental and com putational studies aimed at identifying th e physical mechanisms associated with lam inar-to-turbulent transition in separation bubbles, and at examining passive techniques for m anipulation of this process.Measurements were made over the suction surface of two airfoil models using particle image velocimetry (PIV), and were performed in a low-Reynolds-number tow -tank facility. One of the models, which consists of an unconventional airfoil design, was developed specifically for the purpose of examining passive m anipulation of separation-bubble transition. The com putational studies were performed using direct numerical simulation (DNS), providing detailed resolution of the spatial and tem poral scales of the flow. Separation-bubble transition is observed to occur in a different manner depending on the level of freestream turbulence. In a low-disturbance environment, transition is shown to be initiated by the inviscid Kelvin-Helmholtz instability mechanism. This instability results in the roll-up and subsequent shedding of vorticity in the separated shear layer, downstream of which break-down to small-scale turbulence occurs in the region of high shear between sequential rollers and initiates reattachm ent of the shear layer to the surface. Under low-Reynolds-number conditions, a vortex-pairing phenomenon is identified and associated with a subharmonic instability of the separated shear layer. A possible link between the inviscid Kelvin-Helmholtz and the viscous Tollmien-Schlichting instabilities is identified. Under elevated-freestream-turbulence conditions, separation-bubble transition is shown to occur through the production, growth, and merging of turbulent spots. The spots, which consist of a series of vortex loops, are shown to be initiated through a secondary inviscid instability, promoted by the presence of streamwise streaks th a t originate in the upstream laminar boundary layer disturbed by the freestream turbulence. i Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Two-dimensional surface modifications were made to the unconventional airfoil to provide passive m anipulation of the separation-bubble-transition process. Most of the configurations examined are shown to provide reduced boundary-layer losses downstream of reattachm ent. The spatially-resolved PIV results identify various means through which the transition process can be altered to improve th e performance of airfoils over which separation bubbl...

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Section: Free Shear Layersmentioning

confidence: 69%

Section: Instability M Echanismsmentioning

confidence: 96%

Section: Free Shear Layersmentioning

confidence: 99%

Section: Free Shear Layersmentioning

confidence: 99%

“….1: K elvin-H elm holtz in stab ility roll-up and pairing process in a free shear layer (adapted from E stevadeordal andKleis, 1999) …”

mentioning

confidence: 99%

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Transition in separation bubbles: physical mechanisms and passive control techniques

McAuliffe¹

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DPIV study of near-stall wake-rotor interactions in a transonic compressor

et al. 2002

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Influence of vortex-pairing location on the three-dimensional evolution of plane mixing layers

Estevadeordal

Kleis

2002

J. Fluid Mech.

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Detailed three-dimensional measurements of the first vortex pairing of a large plane mixing layer reveal excitation of several three-dimensional instability modes. Time evolution in three-dimensional space (x, y, z, t) shows how the two-dimensional rollers become three-dimensional as they approach each other and that the linear growth of at least two instability waves leads to a spanwise periodic pairing. The results are based on phase-locked measurements made in three-dimensional spatial grids, with a mesh spacing of 8.5% of the fundamental instability wavelength. Spanwise-uniform, periodic acoustic excitation stabilizes the most probable two-dimensional natural features – roll-up and first pairing. The second subharmonic is added to study the effect of alternate streamwise pairing locations on the three-dimensional characteristics of vortex pairing. Velocities are measured using hot-wire anemometry, and the coherent structures are reconstructed from the ensemble-averaged vorticity field.Vortex pairing is shown to initiate through local ‘bridging’ at the maxima of periodic spanwise undulations. The undulations result from linear amplification of various instability modes on pairing rollers having different strengths. Bridging results from the change of the relative phase between the spanwise undulations of the pairing rollers from in-phase (due to the initial translative mode) to out-of-phase (due to the amplification of bulging-like and non-axisymmetric modes). It is found that when pairing occurs sufficiently far upstream, only axisymmetric waves are amplified and the evolution results in axisymmetric merging. In contrast, when pairing occurs sufficiently far downstream, both axisymmetric and non-axisymmetric waves are amplified and the evolution results in non-axisymmetric merging.The results indicate that vortex pairing is accompanied by the counter-rotating pairs of secondary structures (‘streamwise vortices’ or ‘ribs’) located in the mixing-layer braids and residing in the valleys of the spanwise-roller waves. Time evolution of these secondary structures shows that they move in the transverse direction, following the rollers.

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High-resolution measurements of two-dimensional instabilities and turbulence transition in plane mixing layers

Cited by 17 publications

References 11 publications

Transition in separation bubbles: physical mechanisms and passive control techniques

Transition in separation bubbles: physical mechanisms and passive control techniques

DPIV study of near-stall wake-rotor interactions in a transonic compressor

Influence of vortex-pairing location on the three-dimensional evolution of plane mixing layers

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