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The experiment involves concurrent flow of air and water in a transparent tube of rectangular cross-section. The tube is 1 meter long and can be mounted vertically, horizontally, or at an angle. The water is circulated by a gear pump equipped with a variable speed controller. A Web camera is focused on the tube, showing the flow pattern at about 3 frames per second. The remote operator can view the experiment using a standard (Netscape) browser. The air flow rate and the water pump speed are controlled by stepper motors, and the operator can set both flows using a browser. Sensors (which detect bubbles) measure the optical density at two points in the back-lit tube, The operator can trigger the creation of data files (typically 2500 points in 50 seconds) at any time. The camera frames and the optical sensor data both show clearly a transition from slug flow (discrete bubbles) to churn (turbulent) flow as the air and water flow rates are varied. A program calculates power spectra for the sensor signals, and these spectra show, for example, bubble coalescence as the flow moves up the tube, and strong periodicity under certain conditions.
Choy et al. (Sat,) studied this question.
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