An Integrated Experimental/Numerical Study of the Bubbly Wake Behind a Cavitating Hydrofoil

Since cavitation is often unavoidable, it is crucial to understand how fluid-handling machinery operates under cavitating conditions. The purpose of this research is to investigate two-phase flow structure in the wake of a hydrofoil undergoing unsteady partial cavitation using an integrated experimental and numerical approach. Experiments were conducted systematically in the high-speed water tunnel at the St. Anthony Falls Laboratory to capture the characteristics of the bubbly wake induced by unsteady partial cavitation. The velocity and bubble information were obtained using a TSI color burst LDA/PDA system with a beam splitter and IFA processor. A nominal sampling period of 30 seconds per coordinate point was chosen for the collection of all data sets of LDV measurements. Since the LDV data rate changes with position, the collection period was also controlled by the upper limit of samples, which were 3,000 samples per point. In order to get enough samples, a nominal sampling period of 180 seconds per coordinate point was selected for PDA measurements and 10,000 samples were used as the upper limit. The actual number of samples for each PDA measurement location was between 5,000 and 10,000 with an average data rate of 40 Hz. The averaged bubble size measured in the wake is obtained to be 240 ∼ 300 microns and the estimated maximum void fraction in the wake being in the order of one percent. A virtual single-phase natural cavitation model with the effect of incondensable gas was developed and implemented to better understand the cavitating wake physics. A number of parameters, such as time-averaged velocity distributions both on suction side of the hydrofoil and in the wake, spectral characteristics of unsteady lift oscillations etc., were computed and found in very good agreement with experimental data (Qin 2004), indicating this model can capture the main physics of unsteady cavitating flow. It was also found that the peak of time-averaged incondensable gas in the wake is in the order of one percent, indicating that the majority of vapor bubbles were condensed to water in the wake and hence the bubbles in the wake of unsteady cavitation are mainly gas bubbles.