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Streak instability in turbulent channel flow: the seeding mechanism of large-scale motions

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Title: Streak instability in turbulent channel flow: the seeding mechanism of large-scale motions
Authors: De Giovanetti, M
Sung, HJ
Hwang, Y
Item Type: Journal Article
Abstract: It has often been proposed that the formation of large-scale motion (or bulges) is a consequence of successive mergers and/or growth of near-wall hairpin vortices. In the present study, we report our direct observation that large-scale motion is generated by an instability of an ‘amplified’ streaky motion in the outer region (i.e. very-large-scale motion). We design a numerical experiment in turbulent channel flow up to Reτ '2000 where a streamwise-uniform streaky motion is artificially driven by body forcing in the outer region computed from the previous linear theory (Hwang & Cossu, J. Fluid Mech., vol. 664, 2015, pp. 51–73). As the forcing amplitude is increased, it is found that an energetic streamwise vortical structure emerges at a streamwise wavelength of λx/h '1–5 (h is the half-height of the channel). The application of dynamic mode decomposition and the examination of turbulence statistics reveal that this structure is a consequence of the sinuous-mode instability of the streak, a subprocess of the self-sustaining mechanism of the large-scale outer structures. It is also found that the statistical features of the vortical structure are remarkably similar to those of the largescale motion in the outer region. Finally, it is proposed that the largest streamwise length of the streak instability determines the streamwise length scale of very-largescale motion.
Issue Date: 26-Oct-2017
Date of Acceptance: 18-Sep-2017
URI: http://hdl.handle.net/10044/1/51434
DOI: https://dx.doi.org/10.1017/jfm.2017.697
ISSN: 0022-1120
Publisher: Cambridge University Press (CUP)
Start Page: 483
End Page: 513
Journal / Book Title: Journal of Fluid Mechanics
Volume: 832
Copyright Statement: The final publication is available via Cambridge Journals Online at http://dx.doi.org/10.1017/jfm.2017.697
Sponsor/Funder: Engineering and Physical Sciences Research Council
Engineering & Physical Science Research Council (EPSRC)
Funder's Grant Number: EP/N019342/1
EP/N019342/1
Keywords: Science & Technology
Technology
Physical Sciences
Mechanics
Physics, Fluids & Plasmas
Physics
instability
turbulent boundary layers
turbulence simulation
NEAR-WALL TURBULENCE
SELF-SUSTAINING PROCESS
BOUNDARY-LAYER STREAKS
LOW-REYNOLDS-NUMBER
ATTACHED EDDIES
SHEAR-FLOW
ENERGY AMPLIFICATION
COHERENT STRUCTURE
TRANSIENT GROWTH
VORTICES
01 Mathematical Sciences
09 Engineering
Fluids & Plasmas
Publication Status: Published
Appears in Collections:Faculty of Engineering
Aeronautics



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