Water flow through a converging-diverging glass nozzle experiences a pressure drop and its velocity increases as it flows through the converging section. For an inviscid fluid, the pressure minimum occurs at the nozzle throat, where the cross-sectional area is minimum. If the minimum pressure is below the water vapor pressure, cavitation may occur. Viscous fluid flow through a converging-diverging nozzle experiences more complex flow patterns. Additionally, fluid through the nozzle may be driven into the metastable region and subsequently cavitate at a lower pressure than the vapor pressure.
The dynamic conditions that trigger cavitation in a converging-diverging nozzle are not well understood; moreover, direct measurements involving invasive probe insertion in the region of cavitation onset can induce cavitation. The study of a glass converging-diverging nozzle allowed for noninvasive flow visualization and quantitative observational measurements to be made. A high-speed digital camera was used to capture qualitative and quantitative information on the flow pattern inside the nozzle. The transient time period during cavitation onset was visualized at 35,000 frames per second. Video from the high-speed digital camera revealed that the cavitation front began approximately one nozzle throat diameter downstream from the nozzle throat. Small glass sphere seed particles and injected bubbles were used to trace flow through the nozzle and measure flow velocity at different locations in the nozzle. Small injected bubbles were tracked using the high-speed camera to measure the flow velocity in the nozzle inlet and converging sections. Glass spheres of 10 μm and 120 μm diameter were introduced to the flow to visualize the flow inside the nozzle and track flow velocity. The 120 μm glass spheres were visible using the high-speed camera and were tracked to measure flow velocity in the converging and throat regions of the nozzle. The 120 μm spheres were large enough to provide nucleation sites for cavitation and were seen to trigger cavitation near the nozzle throat. The cavitation induced by the glass spheres occurred upstream of the cavitation front previously observed in the absence of the spheres at identical nozzle inlet and outlet pressures. This shift in the cavitation front suggested the presence of metastable flow through the nozzle throat in the absence of seed particles. The 120 μm spheres also revealed that the flow had separated from the nozzle wall downstream of the nozzle throat. Tracking bubbles produced by the cavitation front also permitted flow visualization of the regions of separated flow, which first separated from the wall upstream of the cavitation front.
Flow visualization of cavitation in the converging-diverging glass nozzle obtained by the high-speed digital camera provided valuable information regarding the conditions that lead to cavitation. High-speed imaging revealed the dynamic fluid behaviors during the onset of cavitation. Bubble and seed particle tracking provided velocity information at several locations throughout the nozzle. Visualization of the entire region of cavitation allowed for the measurement of the cavitation region length, which varied depending upon the nozzle outlet pressure.