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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