Streak instability in turbulent channel flow: the seeding mechanism of large-scale motions
File(s)
Accepted version
Author(s)
de Giovanetti, M
Sung, HJ
Hwang, Y
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.
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.
Date Issued
2017-10-26
Date Acceptance
2017-09-18
Citation
Journal of Fluid Mechanics, 2017, 832, pp.483-513
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
Engineering and Physical Sciences Research Council
Engineering & Physical Science Research Council (EPSRC)
Grant Number
EP/N019342/1
EP/N019342/1
Subjects
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