Extensive visual and quantitative studies of turbulent boundary layers are described. Visual studies reveal the presence of surprisingly well-organized spatially and temporally dependent motions within the so-called ‘laminar sublayer’. These motions lead to the formation of low-speed streaks in the region very near the wall. The streaks interact with the outer portions of the flow through a process of gradual ‘lift-up’, then sudden oscillation, bursting, and ejection. It is felt that these processes play a dominant role in the production of new turbulence and the transport of turbulence within the boundary layer on smooth walls.Quantitative data are presented providing an association of the observed structure features with the accepted ‘regions’ of the boundary layer in non-dimensional co-ordinates; these data include zero, negative and positive pressure gradients on smooth walls. Instantaneous spanwise velocity profiles for the inner layers are given, and dimensionless correlations for mean streak-spacing and break-up frequency are presented.Tentative mechanisms for formation and break-up of the low-speed streaks are proposed, and other evidence regarding the implications and importance of the streak structure in turbulent boundary layers is reviewed.

Improved flow-visualization methods based on the hydrogen-bubble technique are described. Use of “combined-time-streak markers” allows quantitative measurement of the instantaneous velocity field in a plane as a function of time in low-speed water flows. Adaptation to a great variety of situations using different probe techniques is possible. Disturbance to the flow is very small. Adequate accuracy is obtainable. The method offers the advantage of simultaneous visual observation of the flow structure and quantitative measurement of velocity over a finite region. It also allows some types of measurement not previously possible. Limitations of the method include its restriction to low-speed water flows and the observation of fluctuations only at low frequencies. Practical difficulties occur due to probe frangibility and problems of obtaining uniform bubble production. Part 1 describes the underlying concepts, summarizes the history of the method and describes operating experience to date at Educational Services, Incorporated, and Stanford University. Part 2 is a detailed analysis of the uncertainties in velocity measurements using combined-time-streak markers formed by hydrogen bubbles.

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