Cavitation Measurements and Flow Visualization on a Hydrofoil With Different Tip Gaps

Many types of hydraulic turbomachinery must often operate in the cavitating regime. Cavitation on the hydraulic turbomachinery can lead to unwanted effects such as noise, vibrations, turbomachine efficiency drop, and erosion on surfaces. As far as turbomachinery performance is concerned, the main research interest is focused on the flow over the runner blades, which can be evaluated as hydrofoils for an analysis. In spite of considerable research, there are still many features of the cavitation problem that have not been explored or understood, if especially the effects of the tip gap are considered. This paper investigates the behavior of cavitation on a hydrofoil. A NACA0012 profile hydrofoil is investigated at incidence angles ranging from 0° through 8° and with varying cavitation numbers, σ, between 0.2 and 2.5. The Reynolds number defined with the chord length is 106. Flow visualization is carried out using a high-speed camera with recordings around the hydrofoil from the top and from one side. With flow visualization, the dynamics and spatial structures of the cavities are explored. The cavitation behavior around the hydrofoil is especially investigated for different tip gaps in order to determine the effect of the tip gap variation on the cavitation. With the pressure measurements for different tip gaps, the influence of the tip gap size change on the hydrofoil cavitation characteristics are determined for different cavitation numbers and angle of incidences. Using the flow visualization, the development of the tip leakage and tip separation vortices are also explored.