Dynamics of Cavitating Flow and Flexural Waves in Fluid-Filled Tubes Subject to Axial Impact

We have experimentally studied the coupling of flexural waves in water-filled tubes with cavitating flow. To examine the cavitation events in the tube, we used a transparent polycarbonate (PC) tube. The flexural waves are generated by stress waves in the water propagating along the tube axis. A steel impactor is accelerated by gravity and strikes a polycarbonate buffer placed on top of the water surface within the tube. Strain gages measure hoop strain along the polycarbonate tube and a piezoelectric pressure gage measures the pressure at the bottom of the tube. The events were visualized by a high-speed video camera synchronized with the strain and pressure measurements. The impact of the projectile creates a stress wave propagating along the tube and coupled to the pressure wave in water. A sequence of traveling waves results and when the pressure in the waves drops below the vapor pressure, cavitation occurs in the tube. When cavitation bubbles are present, the tube vibrates at the natural frequency of the second mode of the circular ring. The duration of the cavitation can be estimated from a balance between the buffer kinetic energy and the work done in accelerating the fluid. Cavitation does not occur uniformly or simultaneously; cavitation events are observed near the bottom surface of the buffer, the middle of the tube, and at the bottom (closed) end. High-speed video of the cavitation events reveals that the cavities are clusters of bubbles that have a rosary or grape-bunch appearance.