Water Quality and the Periodicity of Sheet/Cloud Cavitation

Cloud cavitation is the rapid formation and shedding of vaporous clouds from a cavitating hydrofoil. This type of cavitation occurs under certain conditions that are characterized by the cavitation number [σ] and the angle of attack [α]. Associated with cloud cavitation are large, abrupt changes in surface pressure caused by the shedding of the attached cavity. Our experimental data display trends that are contained in the linearized flat plate theories of Acosta and Tulin. Near values of σ/2α equal to 4, a singularity exists in the flat plate theory. Experimental results and numerical simulations indicate that in this region a transition between competing mechanisms of cavity shedding occurs. A new finding is that water quality appears to have a significant effect on cavitation behavior. It is well known that nuclei content plays an important role in cavitation inception. However, a recent investigation made possible by high-speed video reveals that the cloud shedding is periodic and that, for each cycle, the cavitating surface becomes fully wetted. Thus, inception physics come into play for a fraction of each cycle. Experimental data shows that the fraction of time in each period that the hydrofoil is fully wetted varies with gas content. In addition, the spectral characteristics of lift and surface pressure measurements show a strong dependence on gas content. Numerical simulations made to incorporate gas content effects show surprisingly close agreement with experimental data. This is also factor that may be of importance in comparing the results from different experimental facilities since comparisons are often made without considering gas content as a factor.