TY - CONF
T1 - Formation of near-wall streamwise vortices by streak instability
AU - Schoppa, Wade
AU - Hussain, Fazle
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. Supercomputer time was provided by the NASA Ames Research Center.
Publisher Copyright:
© 1998 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
Copyright:
Copyright 2017 Elsevier B.V., All rights reserved.
PY - 1998
Y1 - 1998
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 (y circulation per unit x > 7.6) and is inviscid in nature, despite the proximity of the no-slip wall. Streamwise vortex formation (collapse) is dominated by stretching, rather than rollup, of instability generated ωx 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 3D 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. Fundamental differences in the regeneration dynamics of minimal channel and Couette flows are revealed regarding (nonlinear) streak instability, vortex formation and evolution, and wall shear behavior.
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 (y circulation per unit x > 7.6) and is inviscid in nature, despite the proximity of the no-slip wall. Streamwise vortex formation (collapse) is dominated by stretching, rather than rollup, of instability generated ωx 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 3D 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. Fundamental differences in the regeneration dynamics of minimal channel and Couette flows are revealed regarding (nonlinear) streak instability, vortex formation and evolution, and wall shear behavior.
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U2 - 10.2514/6.1998-3000
DO - 10.2514/6.1998-3000
M3 - Paper
AN - SCOPUS:84959332328
SP - 1
EP - 11
T2 - 29th AIAA Fluid Dynamics Conference
Y2 - 15 June 1998 through 18 June 1998
ER -