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On the growth of turbulent regions in laminar boundary layers

Published online by Cambridge University Press:  20 April 2006

Mohamed Gad-El-Hak ,
Ron F. Blackwelderf  and
James J. Riley
Mohamed Gad-El-Hak
Affiliation:
Flow Research Company, Kent, Washington 98031
Ron F. Blackwelderf
Affiliation:
Flow Research Company, Kent, Washington 98031 Permanent address: Department of Aerospace Engineering. University of Southern. California, Los Angeles, California 90007.
James J. Riley
Affiliation:
Flow Research Company, Kent, Washington 98031
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Abstract

Turbulent spots evolving in a laminar boundary layer on a nominally zero pressure gradient flat plate are investigated. The plate is towed through an 18 m water channel, using a carriage that rides on a continuously replenished oil film giving a vibrationless tow. Turbulent spots are initiated using a solenoid valve that ejects a small amount of fluid through a minute hole on the working surface. A novel visualization technique that utilizes fluorescent dye excited by a sheet of laser light is employed. Some new aspects of the growth and entrainment of turbulent spots, especially with regard to lateral growth, are inferred from the present experiments. To supplement the information on lateral spreading, a surbulent wedge created by placing a roughness element in the laminar boundary layer is also studied both visually and with probe measurements. The present results show that, in addition to entrainment, another mechanism is needed to explain the lateral growth characteristics of a turbulent region in a laminar boundary layer. This mechanism, termed growth by destabilization, appears to be a result of the turbulence destabilizing the unstable laminar boundary layer in its vicinity. To further understand the growth mechanisms, the turbulence in the spot is modulated using drag-reducing additives and salinity stratification.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 110 , September 1981 , pp. 73 - 95
Copyright
© 1981 Cambridge University Press

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