The Effect of Gas Content on Cavitation Shedding and Test Facility Dynamics

This study investigates experimentally the coupling between cavitation shedding, considered as the source of tunnel dynamics, and the system response for various degrees of tunnel gas content. The experimental work took place in the high-speed water tunnel at the St. Anthony Falls Laboratory at the University of Minnesota. The test object was a c=81 mm NACA 0015 hydrofoil spanning the whole test section, 190mm by 190mm, at speeds around 10 m/s. Gas content was controlled by either comprehensive degassing, by allowing different gas saturations and eventually by continuously injecting air directly into the flow downstream of the test object. Most of the gas was removed by a gas collector, unique to the facility, upstream the test section. By combining simultaneous readings of pressure dynamics at three locations in the tunnel loop, together with measured shedding dynamic loads in parallel with high frequency impacts using the same externally mounted accelerometers, the research team aimed at concluding the effect of gas content on several cavitation characteristics. The results presented, and upon which discussion and conclusions are made, include excitation strength as a function of test section gas saturation level and the frequency response between the various pressure transducers and observed shedding strength.