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

Goudar, Manu; Breugem, Wim-Paul; Elsinga, Gerrit E.

doi:10.1063/1.4944048

Cited by 10 publications

(13 citation statements)

References 26 publications

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“…The initially small blob develops into a new arch vortex (H5) in just 3-4 wall units time by connecting to the leg of the upstream hairpin H4 (figure 10h). The fact that the head develops first is consistent with the auto-generation mechanisms reported in the literature (Zhou et al 1999;Goudar, Breugem & Elsinga 2016). The head of the new arch (H5) is located at 25-30 wall units distance from the wall.…”

Section: Auto-generationsupporting

confidence: 81%

“…The distorted shear layer then starts to roll up near the point marked X in figure 11(b) and develops the new head H5 ( figure 11c,d). The present observation of incoming high-velocity flow (in this case the sweep associated with vortex S1) deforming the shear layer upstream of an existing hairpin with subsequent roll-up of this shear layer is consistent with the auto-generation scenario outlined in Goudar et al (2016). Their scenario incorporates the effect of the environment around the initial hairpin, complementing models based mostly on self-induction by the hairpin vortex (Zhou et al 1999).…”

Section: Auto-generationsupporting

confidence: 76%

“…This peak strength is comparable to the ejection of the α = 2 conditional eddy at y + = 49.6, u + v + = −12.8 (Zhou et al 1999), which is above the threshold strength required for auto-generation in a disturbance-free environment (Zhou et al 1999;Kim et al 2008). Here, α denotes a multiple of the ejection velocity vector that contributes most to the mean Reynolds shear stress (Zhou et al 1999;Kim et al 2008;Goudar et al 2016). When introduced into a fully turbulent channel flow, however, a stronger α = 3 conditional eddy at y + = 51.3 was found insufficient to trigger the auto-generation of a detectable new hairpin (Kim et al 2008).…”

Section: Auto-generationmentioning

confidence: 84%

“…This ejection strength corresponds to an α = 1 conditional eddy, which is expected not to auto-generate in a clean background, but may auto-generate in the presence of added background noise (Kim et al 2008) or a downstream hairpin (Goudar et al 2016). Following the latter explanation, the auto-generation of H3 in figure 12 is promoted by the presence of the hairpin H1 downstream of the hairpin H2.…”

Section: Auto-generationmentioning

confidence: 88%

“…Additionally, the hairpin H3 located 100-120 wall units downstream of H4 may contribute to the ability of H4 to auto-generate. As shown by Goudar et al (2016), a packet of two weak conditional eddies, which do not auto-generate individually, can still auto-generate a third hairpin, provided the upstream smaller eddy is within 120 wall units distance from the downstream taller eddy. The present configuration of hairpins satisfies this criterion.…”

Section: Auto-generationmentioning

confidence: 95%

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Experimental observation of hairpin auto-generation events in a turbulent boundary layer

Jodai

Elsinga

2016

J. Fluid Mech.

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Time-resolved tomographic particle image velocimetry experiments show that new hairpin vortices are generated within a fully developed and unperturbed turbulent boundary layer. The measurements are taken at a Reynolds number based on the momentum thickness of 2038, and cover the near-wall region below y + = 140, where y + is the wall-normal distance in wall units. Instantaneous visualizations of the flow reveal near-wall low-speed streaks with associated quasi-streamwise vortices, retrograde inverted arch vortices, hairpin vortices and hairpin packets. The hairpin heads are observed as close to the wall as y + = 30. Examples of hairpin packet evolution reveal the development of new hairpin vortices, which are created upstream and close to the wall in a manner consistent with the auto-generation model (Zhou et al., J. Fluid Mech., vol. 387, 1999, pp. 353-396). The development of the new hairpin appears to be initiated by an approaching sweep event, which perturbs the shear layer associated with the initial packet. The shear layer rolls up, thereby forming the new hairpin head. The head subsequently connects to existing streamwise vortices and develops into a hairpin. The time scale associated with the hairpin auto-generation is 20-30 wall units of time. This demonstrates that hairpins can be created over short distances within a developed turbulent boundary layer, implying that they are not simply remnants of the laminar-to-turbulent transition process far upstream.

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Section: Auto-generationsupporting

confidence: 81%

Section: Auto-generationsupporting

confidence: 76%

Section: Auto-generationmentioning

confidence: 84%

Section: Auto-generationmentioning

confidence: 88%

Section: Auto-generationmentioning

confidence: 95%

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Experimental observation of hairpin auto-generation events in a turbulent boundary layer

Jodai

Elsinga

2016

J. Fluid Mech.

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Experimental study on dominant vortex structures in near-wall region of turbulent boundary layer based on tomographic particle image velocimetry

et al. 2019

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Vortex structures are very popular research objects in turbulent boundary layers (TBLs) because of their prime importance in turbulence modelling. This work performs a tomographic particle image velocimetry measurement on the near-wall region ( $y<0.1\unicode[STIX]{x1D6FF}$ ) of TBLs at three Reynolds numbers $Re_{\unicode[STIX]{x1D70F}}=1238$ , 2286 and 3081. The main attention is paid to the wall-normal evolution of the vortex geometries and topologies. The vortex is identified with swirl strength ( $\unicode[STIX]{x1D706}_{ci}$ ), and its orientation is recognized by using the real eigenvector of the velocity gradient tensor. The vortex inclination angles in the streamwise–wall-normal plane and in the streamwise–spanwise plane as functions of wall-normal positions are investigated, which provide useful information to speculate on the three-dimensional shape of the vortex tubes in a TBL. The difference between the orientations of vorticity and swirl is discussed and their inherent relationship is revealed based on the governing equation of vorticity. Linear stochastic estimation (LSE) is further deployed to directly extract three-dimensional vortex models. The LSE velocity fields for ejection events happening at different wall-normal positions shed light on the evolution of the topologies for the vortices dominating ejection events. LSE based on a centred prograde spanwise vortex provides a typical packet model, which indicates that the population density of the packets in a TBL is large enough to leave footprints in conditionally averaged flow fields. This work should help to settle the severe debate on the existence of packet structures and also lays some foundation for the TBL model theory.

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Precursors of backflow events and their relationship with the near-wall self-sustaining process

2021

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This study examines the precursors and consequences of rare backflow events at the wall using direct numerical simulation of turbulent pipe flow with a high spatiotemporal resolution. The results obtained from conditionally averaged fields reveal that the precursor of a backflow event is the asymmetric collision between a high- and a low-speed streak (LSS) associated with the sinuous mode of the streaks. As the collision occurs, a lifted shear layer with high local azimuthal enstrophy is formed at the trailing end of the LSS. Subsequently, a spanwise or an oblique vortex spontaneously arises. The dominant nonlinear mechanism by which this vortex is engendered is enstrophy intensification due to direct stretching of the lifted vorticity lines in the azimuthal direction. As time progresses, this vortex tilts and orientates towards the streamwise direction and, as its enstrophy increases, it induces the breakdown of the LSS located below it. Subsequently, this vortical structure advects as a quasi-streamwise vortex, as it tilts and stretches with time. As a result, it is shown that reverse flow events at the wall are the signature of the nonlinear mechanism of the self-sustaining process occurring at the near-wall region. Additionally, each backflow event has been tracked in space and time, showing that approximately 50 % of these events are followed by at least one additional vortex generation that gives rise to new backflow events. It is also found that up to a maximum of seven regenerations occur after a backflow event has appeared for the first time.

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

Cited by 10 publications

References 26 publications

Experimental observation of hairpin auto-generation events in a turbulent boundary layer

Experimental observation of hairpin auto-generation events in a turbulent boundary layer

Experimental study on dominant vortex structures in near-wall region of turbulent boundary layer based on tomographic particle image velocimetry

Precursors of backflow events and their relationship with the near-wall self-sustaining process

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