Study of the Structure of Turbulence in Accelerating Transitional Boundary Layers
Study of the Structure of Turbulence in Accelerating Transitional Boundary Layers

Author: Michael F. Blair

Publisher:

Published: 1987

Total Pages: 219

ISBN-13:

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A combined experimental and analytical program has been conducted to examine transitional, accelerating boundary layer flows with high levels of freestream turbulence. An earlier program focused on measurement of transitional heat transfer distributions for four combinations of streamwise acceleration and freestream turbulence. The present program was designed to document the boundary layer turbulence structure and spectral distributions for the same four test conditions. The results from the present program have shown that transition in accelerating flows consists of an acceleration dominated stage of slowly developing intermittency followed by a second stage with the same general characteristics as zero-pressure-gradient transition. Conditionally sampled fluctuating velocity profile measurements indicated that the boundary layer turbulence was highly anisotropic in the early stages of transition. Conditionally sampled mean velocity measurements showed that within the intermittent turbulent patches the mean velocity profiles were very similar to those of an equilibrium turbulent boundary layer. Spectral distribution data indicated that preferred amplification of the most unstable (as predicted by linear stability theory) frequencies occurred upstream of the onset of transitional bursting. In addition to the experimental portion of this investigation, numerical experiments were undertaken to assess the ability of currently existing methods to predict heat transfer during transition in accelerating flows.

Similarity Behavior in Transitional Boundary Layers Over a Range of Adverse Pressure Gradients and Turbulence Levels
Similarity Behavior in Transitional Boundary Layers Over a Range of Adverse Pressure Gradients and Turbulence Levels

Author: J. P. Gostelow

Publisher:

Published: 1990

Total Pages: 0

ISBN-13:

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Boundary layer transition has been investigated experimentally under low, moderate, and high free-stream turbulence levels and varying adverse pressure gradients. Under high turbulence levels and adverse pressure gradients a pronounced sub-transition was present. A strong degree of similarity in intermittency distributions was observed, for all conditions, when the Narasimha procedure for determination of transition inception was used. Effects of free-stream turbulence on the velocity profile are particularly strong for the laminar boundary layer upstream of the transition region. This could reflect the influence of the turbulence on the shear stress distribution throughout the layer and this matter needs further attention. The velocity profiles in wall coordinates undershoot the turbulent wall layer asymptote near the wall over most of the transition region. The rapidity with which transition occurs under adverse pressure gradients produces strong lag effects on the velocity profile; the starting turbulent boundary layer velocity profile may depart significantly from local equilibrium conditions. The practice of deriving integral properties and skin friction for transitional boundary layers by a linear combination of laminar and turbulent values for equilibrium layers is inconsistent with the observed lag effects. The velocity profile responds sufficiently slowly to the perturbation imposed by transition that much of the anticipated drop in form factor will not have occurred prior to the completion of transition. This calls into question both experimental techniques, which rely on measured form factor to characterize transition, and boundary layer calculations, which rely on local equilibrium assumptions in the vicinity of transition.