Manipulating cavitation by a wall jet: experiments on a 2D hydrofoil

We report on the experimental investigation of cavitating flow control over a 2D model of guide vanes of a Francis turbine by means of a continuous tangential injection of liquid along the foil surface. The generated wall jet, providing supplementary mass and momentum, issues from a nozzle chamber inside the hydrofoil through a spanwise slot channel on its upper surface. High-speed imaging was used to distinguish cavity flow regimes, study the spatial patterns and time dynamics of partial cavities, as well as to evaluate the characteristic integral parameters of cavitation. Time-resolved LIF visualization of the jet discharging from the nozzle was employed to check if the generated wall jet is stable and spanwise uniform. Hydroacoustic measurements were performed by a hydrophone to estimate how the amplitudes and frequencies of pressure pulsations associated with cavity oscillations change with the injection rate. A PIV technique was utilized to measure the mean velocity, its fluctuations and the dominant turbulent shear stress component, which were all compared for different flow conditions and with the results for the unmodified (standard) foil. The effect of injection rate on cavitation and flow dynamics was examined for three attack angles, 0, 3 and 9°, and a range of cavitation numbers corresponding to different regimes. The low-speed injection was shown to lead to an intensification of turbulent fluctuations in the boundary layer and shrinking of the attached cavity length by up to 25% compared to the case without injection. The injection with a high velocity, in turn, causes a rise of the local flow velocity and a reduction of turbulent fluctuations near the wall, which, consequently, increases the foil hydrodynamic quality at a relatively low energy consumption for generation of the wall jet. However, in this case the vapor cavity becomes longer. Thus, the low-speed injection turns out to be effective to mitigate cavitation but the injection at a high velocity is more preferable from the standpoint of the flow hydrodynamics. In the whole, the implemented control method showed to be quite an efficient tool to manipulate cavitation and hydrodynamic structure of the flow and, thereby, under certain conditions, to suppress the cavitation-caused instabilities.