Documentation of the role of large-scale structures in the bursting process in turbulent boundary layers

The structure of turbulent boundary layers which develop with zero pressure gradient on a smooth wall and a k-type rough wall was examined using arrays of X-wires. Although the data were obtained only on two orthogonal planes, the technique provides some information on the three-dimensionality of the large-scale structures. The major effect of the roughness is to tilt the inclination of the structures towards the wall-normal direction. This is caused by the reduced damping of the wall-normal velocity fluctuations close to the rough surface and the break-up of structures whose scales are comparable to the size of the roughness elements. Both effects cause a reduction in the streamwise lengthscales, as suggested by all the measured two-point correlations. The correlations also show that the roughness tends to reduce the overall anisotropy of the large-scale motion. There is evidence to suggest that the magnitude of the vorticity field is larger over the rough wall.

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0810 Invariant assumption of velocity PDF and its relation to large scale structures in High-Reynolds number turbulent boundary layers

The Proceedings of the Fluids engineering conference ◽

10.1299/jsmefed.2014._0810-1_ ◽

2014 ◽

Vol 2014 (0) ◽

pp. _0810-1_-_0810-2_

Author(s):

Yuki Wada ◽

Yoshiyuki Tsuji

Keyword(s):

Reynolds Number ◽

Boundary Layers ◽

High Reynolds Number ◽

Large Scale ◽

Turbulent Boundary Layers ◽

Large Scale Structures ◽

Scale Structures ◽

High Reynolds

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The Role of Large-scale Structures on Crackle Noise

22nd AIAA/CEAS Aeroacoustics Conference ◽

10.2514/6.2016-3027 ◽

2016 ◽

Cited By ~ 1

Author(s):

David Buchta ◽

Jonathan B. Freund

Keyword(s):

Large Scale ◽

Large Scale Structures ◽

Scale Structures

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Cosmic Inflation, Quantum Information and the Pioneering Role of John S Bell in Cosmology

Universe ◽

10.3390/universe5040092 ◽

2019 ◽

Vol 5 (4) ◽

pp. 92 ◽

Cited By ~ 7

Author(s):

Jérôme Martin

Keyword(s):

Quantum Mechanics ◽

Quantum Information ◽

Large Scale ◽

Gravitational Instability ◽

Cosmic Inflation ◽

Large Scale Structures ◽

Scale Structures ◽

Accuracy Data ◽

Made In

According to the theory of cosmic inflation, the large scale structures observed in our Universe (galaxies, clusters of galaxies, Cosmic Background Microwave—CMB—anisotropy...) are of quantum mechanical origin. They are nothing but vacuum fluctuations, stretched to cosmological scales by the cosmic expansion and amplified by gravitational instability. At the end of inflation, these perturbations are placed in a two-mode squeezed state with the strongest squeezing ever produced in Nature (much larger than anything that can be made in the laboratory on Earth). This article studies whether astrophysical observations could unambiguously reveal this quantum origin by borrowing ideas from quantum information theory. It is argued that some of the tools needed to carry out this task have been discussed long ago by J. Bell in a, so far, largely unrecognized contribution. A detailled study of his paper and of the criticisms that have been put forward against his work is presented. Although J. Bell could not have realized it when he wrote his letter since the quantum state of cosmological perturbations was not yet fully characterized at that time, it is also shown that Cosmology and cosmic inflation represent the most interesting frameworks to apply the concepts he investigated. This confirms that cosmic inflation is not only a successful paradigm to understand the early Universe. It is also the only situation in Physics where one crucially needs General Relativity and Quantum Mechanics to derive the predictions of a theory and, where, at the same time, we have high-accuracy data to test these predictions, making inflation a playground of utmost importance to discuss foundational issues in Quantum Mechanics.

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The meandering behaviour of large-scale structures in turbulent boundary layers

Journal of Fluid Mechanics ◽

10.1017/jfm.2019.131 ◽

2019 ◽

Vol 865 ◽

Cited By ~ 11

Author(s):

Kevin Kevin ◽

Jason Monty ◽

Nicholas Hutchins

Keyword(s):

Boundary Layers ◽

Large Scale ◽

Flow Direction ◽

Main Flow ◽

Turbulent Structures ◽

Velocity Fluctuations ◽

Large Scale Structures ◽

Main Flow Direction ◽

Scale Structures ◽

Spanwise Velocity

This paper quantifies the instantaneous form of large-scale turbulent structures in canonical smooth-wall boundary layers, demonstrating that they adhere to a form that is consistent with the self-sustaining streak instability model suggested by Flores & Jiménez (Phys. Fluids, vol. 22, 2010, 071704) and Hwang & Cossu (Phys. Fluids, vol. 23, 2011, 061702). Our motivation for this study stems from previous observations of large-scale streaks that have been spatially locked in position within spanwise-heterogeneous boundary layers. Here, using similar tools, we demonstrate that the randomly occurring large-scale structures in canonical layers show similar behaviour. Statistically, we show that the signature of large-scale coherent structures exhibits increasing meandering behaviour with distance from the wall. At the upper edge of the boundary layer, where these structures are severely misaligned from the main-flow direction, the induced velocities associated with the strongly yawed vortex packets/clusters yield a significant spanwise-velocity component leading to an apparent oblique coherence of spanwise-velocity fluctuations. This pronounced meandering behaviour also gives rise to a dominant streamwise periodicity at a wavelength of approximately $6\unicode[STIX]{x1D6FF}$. We further statistically show that the quasi-streamwise roll-modes formed adjacent to these very large wavy motions are often one-sided (spanwise asymmetric), in stark contrast to the counter-rotating form suggested by conventional conditionally averaged representations. To summarise, we sketch a representative picture of the typical large-scale structures based on the evidence gathered in this study.

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