Interaction between surface blowing and re-entrant jet in active control of hydrofoil cavitation

Numerically simulations were performed to study the effect of active flow control (through steady blowing) on unsteady cavitation over a modified scaled-down Francis turbine (hydrofoil). Large eddy simulations (LES) using a one-equation subgrid-scale model and the Sauer & Schnerr cavitation model, all available in OpenFOAM, served as the main numerical tool for the study. An angle of attack of 9° and a cavitation number σ=1.56 were chosen for study. The dynamics of the re-entrant jet and its role in the shedding of the cavitation sheet were elucidated. For active flow control, wall jet injection through a slot on the top surface of hydrofoil was considered for two different injection velocities of U_inj=0.88U₀ and U_inj=1.42U₀, where U₀ is the inflow velocity. Quantitative comparisons to previously published measured data were used for model validation. It was found that the interaction of the re-entrant jet with the wall jet causes large fluctuations in pressure, thereby counteracting the local rise in pressure due to the wall jet. Thus, the effectiveness of wall jet injection to mitigate cavitation is limited under the operating conditions studied here.