An experimental method to observe repetitive bubble jet collapse
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Under the proper conditions, bubbles formed near a solid surface can collapse, creating a high-velocity liquid jet as the bubble implodes. These jets have the potential to damage the surfaces on which they collapse, often after only brief exposure to cavitating flow. Since cavitation damage has been observed on propellers, hydrofoils, hydro power machinery, and bubble chambers used in high energy nuclear particle detection, the concept of bubble collapse has piqued the interest of researchers across multiple industries. Most laboratory experimental attempts to create the jetting collapse rely on transient events, and measurements are hampered by the short time scale associated with the collapse, the often unpredictable position and time of the event, the small size of the cavity during the final stages of the collapse, and the self-destructiveness of the event. The purpose of this work is to develop a method to generate repetitive jet collapse and rebound at temporal and spatial scales that render measurement and observation easy. The goal is also to expand on the previously detailed experimental methods by identifying and tolerancing the key dependent variables to define a robust and repeatable procedure. The experimental set up uses an acrylic test chamber mounted on an electromagnetic shaker with variable driving frequency near 60 Hz and amplitude of up to 2 mm peak. The atmospheric pressure in the test chamber is reduced between -26 and -30 in.-hg with a vacuum pump to decouple shape and volume oscillations of the bubbles. An analog camera is positioned to record bubbles formed at the bottom surface of the containers, and the driving amplitude and frequency of the shaker is controlled by a waveform generator. A key outcome of this study was the identification of a region in the parameter space of shaker amplitude and ambient pressure where stable volumetric or ‘breathing’ oscillations could be maintained. The maximum ambient pressure with observable breathing was 27.5 in Hg of vacuum at a range of 0.2 mm to 1.2 mm of peak vertical shaker amplitude. Near -30.5 in Hg breathing was observed at smaller shaker amplitudes. The parameter space was bounded by the threshold for the rapid onset of clouds of cavitation bubbles filling large volumes of the test chamber. Repetitive jetting was observed within this region for a bubble approximately 3 mm in diameter, at 0.5 in Hg above the vapor pressure being driven at 0.3 mm amplitude at 60.1 Hz. The jetting occurred near the bottom corner and the jet angle was approximately 50 degrees from the horizontal surface. The knowledge gained from this study points the way towards achieving a more robust process to generate repetitive bubble collapse. Suggestions for improvements to the experimental setup are presented in the concluding section.