Formation of Near-Wall Streamwise Vortices by Streak Instability

Schoppa, Wade; Hussain, Fazle

doi:10.1007/978-94-011-4601-2_6

Cited by 10 publications

(10 citation statements)

References 16 publications

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“…For instance, the low Re causes (x-averaged) streamwise vortices to completely fill the gap between walls, much like Taylor-Couette 'roll' vortices. Our own analysis indicates fundamental differences in the vortex regeneration dynamics of minimal Couette and channel flows (Schoppa & Hussain 1998a).…”

Section: Streak Instabilitymentioning

confidence: 75%

“…Thus, this class of base flows does not exhibit any y symmetries, unlike prior instability studies of streaks bounded closely by both walls in low-Re plane Couette and channel flow. Compared to single-walled streaks, the influence of a second no-slip wall immediately above the streak is twofold: (i) additional y symmetry is imposed on the linear eigenmodes and (ii) the subsequent nonlinear evolution is fundamentally altered (discussed in Schoppa & Hussain 1998a). As an example, thin sheets of ω x (reflecting local jet-like w(y) flow) are generated in narrow-gap Couette flow, compared to collapsed streamwise vortices generated from single-walled streaks in channel flow.…”

Section: Computational Approachmentioning

confidence: 99%

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Coherent structure generation in near-wall turbulence

2002

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We present a new mechanism for generation of near-wall streamwise vortices -which dominate turbulence phenomena in boundary layers -using linear perturbation analysis and direct numerical simulations of turbulent channel flow. The base flow, consisting of the mean velocity profile and low-speed streaks (free from any initial vortices), is shown to be linearly unstable to sinuous normal modes only for relatively strong streaks, i.e. for wall inclination angles of streak vortex lines exceeding 50• . Analysis of streaks extracted from fully developed near-wall turbulence indicates that about 20% of streak regions in the buffer layer exceed the strength threshold for instability. More importantly, these unstable streaks exhibit only moderate (twofold) normalmode amplification, the growth being arrested by self-annihilation of streak-flank normal vorticity due to viscous cross-diffusion. We present here an alternative, streak transient growth (STG) mechanism, capable of producing much larger (tenfold) linear amplification of x-dependent disturbances. Note the distinction of STG -responsible for perturbation growth on a streak velocity distribution U(y, z) -from prior transient growth analyses of the (streakless) mean velocity U(y). We reveal that streamwise vortices are generated from the more numerous normal-mode-stable streaks, via a new STG-based scenario: (i) transient growth of perturbations leading to formation of a sheet of streamwise vorticity ω x (by a 'shearing' mechanism of vorticity generation), (ii) growth of sinuous streak waviness and hence ∂u/∂x as STG reaches nonlinear amplitude, and (iii) the ω x sheet's collapse via stretching by ∂u/∂x (rather than rollup) into streamwise vortices. Significantly, the three-dimensional features of the (instantaneous) streamwise vortices of x-alternating sign generated by STG agree well with the (ensemble-averaged) coherent structures educed from fully turbulent flow. The STGinduced formation of internal shear layers, along with quadrant Reynolds stresses and other turbulence measures, also agree well with fully developed turbulence. Results indicate the prominent -possibly dominant -role of this new, transient-growth-based vortex generation scenario, and suggest interesting possibilities for robust control of drag and heat transfer.

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Section: Streak Instabilitymentioning

confidence: 75%

Section: Computational Approachmentioning

confidence: 99%

Coherent structure generation in near-wall turbulence

2002

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“…However, Baron et al [4] and Dhanak et al [6] figured out that the mechanism of the drag reduction was the movement of low-speed streaks related to the longitudinal vortices, which disturbed the spatial coherence between the longitudinal vortices and the low-speed streaks, and caused the weakening of the streaks. Based on the turbulence regeneration mechanisms explained by Jimenez et al [22] and Schoppa et al [23][24][25] , velocity streaks have crucial effects in the regeneration of the vortices. Thus, the weakening of velocity streaks would weaken the generation of coherent structures, which would in turn cause the reductions of turbulence intensities and drag.…”

mentioning

confidence: 99%

Large eddy simulation of compressible turbulent channel flow with spanwise wall oscillation

Fang

Shao

2009

Sci. China Ser. G-Phys. Mech. Astron.

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The influences of the modification of turbulent coherent structures on temperature field and heat transfer in turbulent channel flow are studied using large eddy simulation (LES) of compressible turbulent channel flows with spanwise wall oscillation (SWO). The reliability of the LES on such problems is proved by the comparisons of the drag reduction data with those of other researches. The high consistency of coherent velocity structures and temperature structures is found based on the analyses of the turbulent flow field. When the coherent velocity structures are suppressed, the transportations of momentum and heat are reduced simultaneously, demonstrating the same trend. This shows that the turbulent coherent structures have the same effects on the transportations of momentum and heat. The averaged wall heat flux can be reduced with appropriate oscillating parameters.large eddy simulation, spanwise wall oscillation, compressible, temperature field, heat transportation

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“…3). This discrepancy reflects other fundamental differences in the underlying instability mechanisms of minimal channel and Couette turbulence (see Schoppa & Hussain 1998a for details).…”

Section: Linear Instabilitymentioning

confidence: 99%

“…Note that the streaks are localized to a single wall (via g(y) in (1)), and hence are essentially decoupled from the second wall. Compared to single-walled streaks, the influence of a second noslip wall immediately above the streak is twofold: (i) additional y symmetry is imposed on the linear eigenmodes and (ii) the subsequent nonlinear evolution is fundamentally altered (Schoppa & Hussain 1998a).…”

Section: Linear Instabilitymentioning

confidence: 99%

A Robust Scheme for Control of Skin Friction and Heat Transfer in Turbulent Boundary Layers via New Instability Mechanism

Hussain¹

1998

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Public reporting burden for this collection of information is estimated to average 1 hour per respc gathering and maintaining the data needed, and completing and reviewing the collection of inforr collection of information, including suggestions for reducing this burden, to Washington Headqu; Davis Highway, Suite 1204, Arlington, VA 22202-4302, and to the Office of Management and R. REPORT DATE f3.ng data sources, ier aspect of this ,1215 Jefferson I 20503. AGENCY USE ONLY (Leave blank)03 Nov 00 REPORT TYPE AND DATES COVEREDFinal Report 01 Jan 98 To 31 Mar 00 TITLE AND SUBTITLEA Robust Scheme for Control of Skin Friction and Heat Transfer in Turbulent Boundary Layers VIA a New Instability Mechanism AUTHOR(S)Fazle Hussain PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)Department of Mechanical Engineering University of Houston Houston TX 77204-4792 Using direct numerical simulations of turbulent channel flow, we present new insight into the generation of streamwise vortices near the wall, aud an associated drag reduction strategy. Growth of x-dependent spanwise velocity disturbances w(x) is shown to occur via two mechanisms: (i) linear transient growth, which dominates early-time evolution, and (ii) linear normal-mode instability, dominant asymptotically at late time (for frozen base flow streaks). Approximately 25% of streaks extracted from near-wall turbulence are shown to be strong enough for linear instability (above a critical vortex line Lift .'■:■ angle). However, due to viscous annihilation of streak normal vorticity COy, normal mode growth ceases after a factor of two energy growth. In contrast, the linear transient disturbance produces a 20-fold amplification, due to its rapid, early-time growth before significant viscous streak decay. Thus, linear transient growth of w(x) is revealed as a new, apparently dominant, generation mechanism of x-dependent turbulent energy near the wall. Research performed under this grant comprised two aspects, namely: (i) direct numerical simulations to understand the instability mechanism generating drag-enhancing near-wall vortices, and (ii) experiments to develop drag control techniques aiming to suppress the drag-enhancing vortices. These two aspects of our research are separately described below. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)AFOSR SUBJECT TERMS I. Direct Numerical SimulationsSummary. Using direct numerical simulations of turbulent channel flow, we present new insight into the generation of streamwise vortices near the wall, and an associated drag reduction strategy. Growth of ^-dependent spanwise velocity disturbances w{x) is shown to occur via two mechanisms: (i) linear transient growth, which dominates earlytime evolution, and (ii) linear normal-mode instability, dominant asymptotically at late time (for frozen base flow streaks). Approximately 25% of streaks extracted from nearwall turbulence are shown to be strong enough for linear instability (above a critical vortex line lift angle). However, due to viscous annihilation of streak normal vort...

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Formation of Near-Wall Streamwise Vortices by Streak Instability

Cited by 10 publications

References 16 publications

Coherent structure generation in near-wall turbulence

Coherent structure generation in near-wall turbulence

Large eddy simulation of compressible turbulent channel flow with spanwise wall oscillation

A Robust Scheme for Control of Skin Friction and Heat Transfer in Turbulent Boundary Layers via New Instability Mechanism

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