INSTABILITY OF LOW-SPEED STREAKS LEADING TOWALL TURBULENCE

The dynamics of the vortex structures appearing in an oscillatory boundary layer (Stokes boundary layer), when the flow departs from the laminar regime, is investigated by means of flow visualizations and a quantitative analysis of the velocity and vorticity fields. The data are obtained by means of direct numerical simulations of the Navier–Stokes and continuity equations. The wall is flat but characterized by small imperfections. The analysis is aimed at identifying points in common and differences between wall turbulence in unsteady flows and the well-investigated turbulence structure in the steady case. As in Jimenez & Moin (1991), the goal is to isolate the basic flow unit and to study its morphology and dynamics. Therefore, the computational domain is kept as small as possible.The elementary process which maintains turbulence in oscillatory boundary layers is found to be similar to that of steady flows. Indeed, when turbulence is generated, a sequence of events similar to those observed in steady boundary layers is observed. However, these events do not occur randomly in time but with a repetition time scale which is about half the period of fluid oscillations. At the end of the accelerating phases of the cycle, low-speed streaks appear close to the wall. During the early part of the decelerating phases the strength of the low-speed streaks grows. Then the streaks twist, oscillate and eventually break, originating small-scale vortices. Far from the wall, the analysis of the vorticity field has revealed the existence of a sequence of streamwise vortices of alternating circulation pumping low-speed fluid far from the wall as suggested by Sendstad & Moin (1992) for steady flows. The vortex structures observed far from the wall disappear when too small a computational domain is used, even though turbulence is self-sustaining. The present results suggest that the streak instability mechanism is the dominant mechanism generating and maintaining turbulence; no evidence of the well-known parent vortex structures spawning offspring vortices is found. Although wall imperfections are necessary to trigger transition to turbulence, the characteristics of the coherent vortex structures, for example the spacing of the low-speed streaks, are found to be independent of wall imperfections.

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Transfer, Segregation and Deposition of Heavy Particles in Horizontal and Vertical Turbulent Channel Flows

Volume 1: Fora, Parts A, B, C, and D ◽

10.1115/fedsm2003-45737 ◽

2003 ◽

Author(s):

Cristian Marchioli ◽

Fabio Sbrizzai ◽

Alfredo Soldati

Keyword(s):

Coherent Structures ◽

Particle Distribution ◽

Outer Region ◽

Turbulent Channel Flow ◽

Transfer Mechanism ◽

Wall Turbulence ◽

Wall Region ◽

Turbulence Structure ◽

Low Speed ◽

Low Speed Streaks

Particle transfer in the wall region of turbulent boundary layers is dominated by the coherent structures which control the turbulence regeneration cycle. Coherent structures bring particles toward the wall and away from the wall and favour particle segregation in the viscous region giving rise to nonuniform particle distribution profiles which peak close to the wall. In this work, we focus on the transfer mechanism of different size particles and on the influence of gravity on particles deposition. By tracking O(105) particles in Direct Numerical Simulation (DNS) of a turbulent channel flow at Reτ = 150, we find that particles may reach the wall directly or may accumulate in the wall region, under the low-speed streaks. Even though low-speed streaks are ejection-like environments, particles are not re-entrained into the outer region. Particles segregated very near the wall by the trapping mechanisms we investigated in a previous work [1] are slowly driven to the wall. We find that gravity plays a role on particle distribution but, for small particles (τp+ < 3), the controlling transfer mechanism is related to near-wall turbulence structure.

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Low-speed streaks in drag-reduced turbulent flow

Physics of Fluids ◽

10.1063/1.869469 ◽

1997 ◽

Vol 9 (8) ◽

pp. 2397-2404 ◽

Cited By ~ 41

Author(s):

G. Hetsroni ◽

J. L. Zakin ◽

A. Mosyak

Keyword(s):

Turbulent Flow ◽

Low Speed ◽

Low Speed Streaks

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Growth and breakdown of low-speed streaks leading to wall turbulence

Journal of Fluid Mechanics ◽

10.1017/s002211200700688x ◽

2007 ◽

Vol 586 ◽

pp. 371-396 ◽

Cited By ~ 24

Author(s):

MASAHITO ASAI ◽

YASUFUMI KONISHI ◽

YUKI OIZUMI ◽

MICHIO NISHIOKA

Keyword(s):

Boundary Layer ◽

Turbulent Boundary Layer ◽

Low Frequency ◽

Wall Turbulence ◽

Linear Stability Theory ◽

Transition Flow ◽

Wall Suction ◽

Low Speed ◽

Low Speed Streaks ◽

Near Wall

Two-dimensional local wall suction is applied to a fully developed turbulent boundary layer such that near-wall turbulence structures are completely sucked out, but most of the turbulent vortices in the original outer layer can survive the suction and cause the resulting laminar flow to undergo re-transition. This enables us to observe and clarify the whole process by which the suction-surviving strong vortical motions give rise to near-wall low-speed streaks and eventually generate wall turbulence. Hot-wire and particle image velocimetry (PIV) measurements show that low-frequency velocity fluctuations, which are markedly suppressed near the wall by the local wall suction, soon start to grow downstream of the suction. The growth of low-frequency fluctuations is algebraic. This characterizes the streak growth caused by the suction-surviving turbulent vortices. The low-speed streaks obtain almost the same spanwise spacing as that of the original turbulent boundary layer without the suction even in the initial stage of the streak development. This indicates that the suction-surviving turbulent vortices are efficient in exciting the necessary ingredients for the wall turbulence, namely, low-speed streaks of the correct scale. After attaining near-saturation, the low-speed streaks soon undergo sinuous instability to lead to re-transition. Flow visualization shows that the streak instability and its subsequent breakdown occur at random in space and time in spite of the spanwise arrangement of streaks being almost periodic. Even under the high-intensity turbulence conditions, the sinuous instability amplifies disturbances of almost the same wavelength as predicted from the linear stability theory, though the actual growth is in the form of a wave packet with not more than two waves. It should be emphasized that the mean velocity develops the log-law profile as the streak breakdown proceeds. The transient growth and eventual breakdown of low-speed streaks are also discussed in connection with the critical condition for the wall-turbulence generation.

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On the role of the outer region in the turbulent-boundary-layer bursting process

Journal of Fluid Mechanics ◽

10.1017/s0022112094000170 ◽

1994 ◽

Vol 259 ◽

pp. 345-373 ◽

Cited By ~ 13

Author(s):

ROY Y. Myose ◽

Ron F. Blackwelder

Keyword(s):

Boundary Layer ◽

Turbulent Boundary Layer ◽

Large Scale ◽

Outer Region ◽

Low Speed ◽

Moderate Reynolds Number ◽

Eddy Structures ◽

Low Speed Streaks ◽

Bursting Process

The dynamics and interaction of turbulent-boundary-layer eddy structures was experimentally emulated. Counter-rotating streamwise vortices and low-speed streaks emulating turbulent-boundary-layer wall eddies were generated by a Görtler instability mechanism. Large-scale motions associated with the outer region of turbulent boundary layer were emulated with — ωzspanwise vortical eddies shed by a periodic non-sinusoidal oscillation of an airfoil. The scales of the resulting eddy structures were comparable to a moderate-Reynolds-number turbulent boundary layer. Results show that the emulated wall-eddy breakdown was triggered by streamwise acceleration associated with the outer region of turbulent boundary layer. This breakdown involved violent mixing between low-speed fluid from the wall eddy and accelerated fluid associated with the outer structure. Although wall eddies can break down autonomously, the presence of and interaction with outer-region — ωzeddies hastened their breakdown. Increasing the — ωzeddy strength resulted in further hastening of the breakdown. Conversely, + ωzeddies were found to delay wall-eddy breakdown locally, with further delays resulting from stronger + ωzeddies. This suggests that the outer region of turbulent boundary layers plays a role in the bursting process.

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