Effective drag reduction by large-scale manipulation of streamwise vortices in near-wall turbulence

Wade Schoppa, Fazle Hussain

Research output: Chapter in Book/Report/Conference proceedingConference contribution

4 Scopus citations

Abstract

Using direct numerical simulations of turbulent channel flow, we present new insight into the regeneration dynamics and control of near-wall longitudinal vortices, which dominate turbulence production, drag, and heat transfer. Initially linear instability of lifted low-speed streaks, free from any initial vortex, is shown to generate these dominant streamwise vortices upon (nonlinear) saturation. The instability requires sufficiently strong streaks (y circulation per unit χ > 7.6) and is inviscid in nature, despite the proximity of the no-slip wall. Streamwise vortex formation (collapse) is dominated by stretching - caused by the positive (Formula Presented) (i.e. positive VISA) associated with streak waviness - rather than roll-up of cox sheets. Significantly, the 3D features of the instability-generated vortices are close to those of both instantaneous and ensemble-averaged flows, suggesting that this instability mechanism is prevalent in the (uncontrolled) fully turbulent flow. We develop effective new control approaches for turbulent boundary layers, via large-scale streak manipulation, which exploit this crucial role of streaks in vortex generation and hence turbulence production. Using control flows with no χ variation, a spanwise wavelength of 400 wall units, and a (frozen) amplitude of only 5% of the channel centerline velocity, we find a significant sustained drag reduction: 20% for imposed counterrotating streamwise vortices and 50% for colliding spanwise wall jet-like forcing. These results suggest promising new drag reduction strategies, e.g. passive vortex generators and spanwise jets from χ-aligned slots, involving large-scale (hence more durable) actuation and requiring no wall sensors or feedback logic.

Original languageEnglish
Title of host publication4th Shear Flow Control Conference
PublisherAmerican Institute of Aeronautics and Astronautics Inc, AIAA
ISBN (Print)9780000000002
StatePublished - 1997
Event4th AIAA Shear Flow Control Conference, 1997 - Snowmass Village, United States
Duration: Jun 29 1997Jul 2 1997

Publication series

Name4th Shear Flow Control Conference

Conference

Conference4th AIAA Shear Flow Control Conference, 1997
CountryUnited States
CitySnowmass Village
Period06/29/9707/2/97

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    Schoppa, W., & Hussain, F. (1997). Effective drag reduction by large-scale manipulation of streamwise vortices in near-wall turbulence. In 4th Shear Flow Control Conference [AIAA-97-1794] (4th Shear Flow Control Conference). American Institute of Aeronautics and Astronautics Inc, AIAA.