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Wall-layer structure and drag reduction

Published online by Cambridge University Press:  20 April 2006

W. G. Tiederman ,
T. S. Luchik  and
D. G. Bogard
W. G. Tiederman
Affiliation:
School of Mechanical Engineering, Purdue University, W. Lafayette. Indiana 47907 USA
T. S. Luchik
Affiliation:
School of Mechanical Engineering, Purdue University, W. Lafayette. Indiana 47907 USA
D. G. Bogard
Affiliation:
School of Mechanical Engineering, Purdue University, W. Lafayette. Indiana 47907 USA
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Abstract

When drag-reducing additives are confined entirely to the linear sublayer of a turbulent channel flow of water, both the spanwise spacing and bursting rate of the wall-layer structure are the same as those for a water flow and there is no evidence of drag reduction. Drag reduction is measured downstream of the location where the additives injected into the sublayer begin to mix in significant quantities with the buffer region (10 < y+ < 100)The superscript + denotes a dimensionless quantity scaled with the kinematic viscosity ν and the wall shear velocity v* = (τw/ρ)½. of the channel flow. At streamwise locations where drag reduction does occur and where the injected fluid is not yet uniformly mixed with the channel flow, the dimensionless spanwise streak spacing increases and the average bursting rate decreases. The decrease in bursting rate is larger than the corresponding increase in streak spacing. The wall-layer structure is like the structure in the flow of a homogeneous, uniformly mixed, drag-reducing solution. Thus, the additives have a direct effect on the flow processes in the buffer region and the linear sublayer appears to have a passive role in the interaction of the inner and outer portions of a turbulent wall layer.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 156 , July 1985 , pp. 419 - 437
Copyright
© 1985 Cambridge University Press

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