TY - JOUR
T1 - Genesis of longitudinal vortices in near-wall turbulence
AU - Schoppa, W.
AU - Hussain, F.
N1 - Funding Information:
This research was supported by AFOSR grant F49620-97-1-0131 and the NASA Graduate Fellowship grant NGT-51022 of W.S.
Copyright:
Copyright 2004 Elsevier Science B.V., Amsterdam. All rights reserved.
PY - 1998/10
Y1 - 1998/10
N2 - Using direct numerical simulations of turbulent channel flow, we present new insight into the formation mechanism of near-wall longitudinal vortices. Instability of lifted, vortex-free low-speed streaks is shown to generate, upon nonlinear saturation, new streamwise vortices, which dominate near-wall turbulence production, drag, and heat transfer. The instability requires sufficiently strong streaks (the wall-normal circulation on either side of a streak exceeding 7.6) and is inviscid in nature, despite the proximity of the no-slip wall. Streamwise vortex formation (collapse) is dominated stretching, rather than Kelvin-Helmholtz rollup, of instability-generated ω-x$/ sheets. In turn, direct stretching results from the positive ∂u/∂x (i.e. positive VISA) associated with streak waviness in the (x, z) plane, generated upon finite-amplitude evolution of the sinuous instability mode. Significantly, the three-dimensional features of the (instantaneous) instability-generated vortices agree well with the coherent structures educed (i.e. ensemble averaged) from fully turbulent flow, suggesting the prevalence of this instability mechanism. These results suggest promising new drag reduction strategies, involving large-scale (hence more durable) control of near-wall flow and requiring no wall sensors of feedback logic.
AB - Using direct numerical simulations of turbulent channel flow, we present new insight into the formation mechanism of near-wall longitudinal vortices. Instability of lifted, vortex-free low-speed streaks is shown to generate, upon nonlinear saturation, new streamwise vortices, which dominate near-wall turbulence production, drag, and heat transfer. The instability requires sufficiently strong streaks (the wall-normal circulation on either side of a streak exceeding 7.6) and is inviscid in nature, despite the proximity of the no-slip wall. Streamwise vortex formation (collapse) is dominated stretching, rather than Kelvin-Helmholtz rollup, of instability-generated ω-x$/ sheets. In turn, direct stretching results from the positive ∂u/∂x (i.e. positive VISA) associated with streak waviness in the (x, z) plane, generated upon finite-amplitude evolution of the sinuous instability mode. Significantly, the three-dimensional features of the (instantaneous) instability-generated vortices agree well with the coherent structures educed (i.e. ensemble averaged) from fully turbulent flow, suggesting the prevalence of this instability mechanism. These results suggest promising new drag reduction strategies, involving large-scale (hence more durable) control of near-wall flow and requiring no wall sensors of feedback logic.
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U2 - 10.1023/A:1004320610285
DO - 10.1023/A:1004320610285
M3 - Article
AN - SCOPUS:0032179835
SN - 0025-6455
VL - 33
SP - 489
EP - 501
JO - Meccanica
JF - Meccanica
IS - 5
ER -