Among the diverse demands on useful methods of flow measurement, there is often a requirement for not disturbing the flow under test, especially in cases of flows near a limit of stability. An acoustic method based on the measurement of flow-induced variations of the travel time of sonic pulses or, equivalently, of the phase of continuous sound waves has been developed that fulfills this requirement perfectly. Moreover, it offers further valuable advantages including much shorter measuring times and higher accuracy than several other methods of measurement commonly in use. The apparatus is relatively inexpensive, with no calibration needed. It has now been applied to a number of investigations made possible by these capabilities. This article presents a survey of a variety of cases where the acoustic method has proven valuable for flow research.
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Engler et al. (1989) studied this question.