On the near-wall vortical structures at moderate Reynolds numbers

Schlatter, Philipp; Li, Q.; Örlü, Ramis; Hussain, Fazle; Henningson, Dan S.

doi:10.1016/j.euromechflu.2014.04.011

Cited by 67 publications

(64 citation statements)

References 81 publications

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“…The inflectional instability at the sides of the low-speed region leads to the formation of new hairpin vortices at x/h = 38, and is associated with the onset of a turbulent wedge (Ye et al 2016a). The correlation between hairpin vortices and low-speed regions has also been observed in transitional (Brinkerhoff & Yaras 2014) and fully turbulent boundary layers (Adrian 2007;Schlatter et al 2014). The observation highlights the important role of the low-speed streaks produced by the secondary vortex pair.…”

Section: Pod Analysismentioning

confidence: 62%

On Reynolds number dependence of micro-ramp-induced transition

Schrijer

Scarano

2018

J. Fluid Mech.

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The variation of transitional flow features past a micro-ramp is investigated when the Reynolds number is decreased approaching the critical regime. Experiments are conducted in the incompressible flow spanning from supercritical to subcritical roughness-height-based Reynolds number (Re h = 1170, 730, 460 and 320) with tomographic particle image velocimetry. The effect of Re h on three-dimensional flow behaviour is analysed in a domain encompassing 73 ramp heights in the streamwise direction. Above the critical Re h , the primary vortex pair and induced central low-speed region in the mean flow field are active over longer range when decreasing Re h . In the instantaneous flow, at Re h < 1000, the hairpin vortices induced by Kelvin-Helmholtz (K-H) instability progress gradually from close to the micro-ramp into the region where the overall shear layer is destabilized, indicating the correlation between the K-H instability and the onset of transition. The breakdown of K-H vortices as observed at Re h = 1170, does not occur at lower Re h . Decreasing Re h , the secondary vortex structures make their first appearance significantly downstream, postponing the formation of sideward disturbances, which destabilize the local shear layer by ejection events. Two major types of eigenmodes with symmetric and asymmetric spatial distribution of velocity fluctuations in the near wake are clearly identified by proper orthogonal decomposition. The symmetric and asymmetric modes correspond to the presence of vortex shedding and a sinuous wiggling motion respectively. It is found that Re h is the key factor determining the importance of the symmetric mode. At Re h = 1170, the disturbance energy of the symmetric mode decays before the onset of transition, suggesting that it is relatively insignificant in the process. However, decreasing Re h to 730 and 460, the symmetric mode produces continuous growth of high level disturbance energy, leading to transition.

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Section: Pod Analysismentioning

confidence: 62%

On Reynolds number dependence of micro-ramp-induced transition

Schrijer

Scarano

2018

J. Fluid Mech.

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“…Also, in Zhou et al, a symmetric hairpin vortex with two legs was used to explain the auto-generation mechanism 8 which is not often found in actual turbulent flows. 10 Zhou et al later added that the non-symmetric initial hairpins also auto-generated hairpin packets. These packets are more complicated but bear resemblance with the idealized symmetric case, showing long low-momentum zones and similar growth angles.…”

Section: Introductionmentioning

confidence: 99%

“…This is one of the advantages of the present interpretation as it does not rely on the presence of two stream-wise vortex legs which are not often found in actual turbulent flows. 10 It also further strengthens the existing hairpin eddy model.…”

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confidence: 99%

Auto-generation in wall turbulence by the interaction of weak eddies

2016

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For channel flow, we explore how commonly found weak eddies can still autogenerate and produce new eddies. Before, only strong eddies (above a threshold strength) were considered to auto-generate. Such strong eddies are rarely observed in actual turbulent flows however. Here, the evolution of two weak conditional eddies with different initial strengths, initial sizes, and initial stream-wise spacing between them is studied. The numerical procedure followed is similar to Zhou et al.["Mechanisms for generating coherent packets of hairpin vortices in channel flow," J. Fluid Mech. 387, 353 (1999)]. The two eddies are found to merge into a single stronger eddy when the initial upstream eddy is taller than the downstream eddy, which further auto-generates when the initial stream-wise separation is small (<120 wall units). However, it is observed that non-merging cases with small initial stream-wise separation also auto-generated. In the initial condition, the two conditional eddies are placed near to each other so their velocity fields (low-speed streaks and ejection events) get superimposed and amplified as a function of stream-wise spacing. To examine this effect, a divergence free low-speed streak is superimposed on an eddy. It is found that these low-speed streak simulations do not auto-generate. On the other hand, a rapid lift-up of an eddy by ejection events plays a role in the onset of auto-generation, which also leads to a modified interpretation of auto-generation mechanism. It differed from the existing auto-generation mechanism at the later stages of auto-generation where blockage of mean flow and shear layer deformation is considered instead of vortex dynamics. C 2016 AIP Publishing LLC.

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“…As in the case of Qs, vortex clusters separate into wall-attached and detached families, but, although the averaged flow field of an attached vortex cluster contains a long conical low-speed streamwise-velocity 'wake' reminiscent of streaks, headed by a shorter pair of ejections, the instantaneous shapes are irregular and very different from hairpins. The question of whether hairpin vortices persist in high-Reynolds-number wall-bounded turbulence, and of whether they should be understood as instantaneous structures (Wu & Moin 2009;Schlatter et al 2014) or as conditional statistical constructs (AJZM06; Tanahashi et al 2004;Flores & Jiménez 2006;Jiménez 2013b), remains controversial, and is the subject of much current debate.…”

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confidence: 99%

Coherent structures in statistically stationary homogeneous shear turbulence

et al. 2017

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The three-dimensional vortex clusters, and the structures based on the quadrant classification of the intense tangential Reynolds stress (Qs), are studied in direct numerical simulations of statistically stationary homogeneous shear turbulence (HST) at Taylor microscale Reynolds number Re λ ≈ 50-250, with emphasis on comparisons with turbulent channels (CHs). The Qs and vortex clusters in HST are found to be versions of the corresponding detached (in the sense of del Álamo et al. (J. Fluid Mech., vol. 561 (2006), pp. 329-358)) structures in CHs, although statistically symmetrised with respect to the substitution of sweeps by ejections and vice versa. In turn, these are more symmetric versions of the corresponding attached Qs and clusters. In both flows, only co-gradient sweeps and ejections larger than the local Corrsin scale are found to couple with the shear. They are oriented anisotropically, and are responsible for carrying most of the total Reynolds stress. Most large eddies in CHs are attached to the wall, but it is shown that this is probably a geometric consequence of their size, rather than the reason for their dynamical significance. Most small Q structures associated with different quadrants are far from each other in comparison to their size, but those that are close to each other tend to form quasi-streamwise trains of groups of a sweep and an ejection paired side by side in the spanwise direction, with a vortex cluster in between, generalising to three dimensions the corresponding arrangement of attached eddies in CHs. These pairs are organised around an inclined large-scale conditional vortex 'roller', and it is shown that the composite structure tends to be located at the interface between high-and low-velocity streaks, as well as in strong 'co-gradient' shear layers that separate streaks of either sign in which velocity is more uniform. It is further found that the conditional rollers are terminated by 'hooks' reminiscent of hairpins, both upright and inverted. The inverted hook weakens as the structures approach the wall, while the upright one changes little. At the same time, the inclination of the roller with respect to the mean velocity decreases from 45 • in HST to quasi-streamwise for † Email address for correspondence: jimenez@torroja.

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On the near-wall vortical structures at moderate Reynolds numbers

Cited by 67 publications

References 81 publications

On Reynolds number dependence of micro-ramp-induced transition

On Reynolds number dependence of micro-ramp-induced transition

Auto-generation in wall turbulence by the interaction of weak eddies

Coherent structures in statistically stationary homogeneous shear turbulence

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