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

• Published in: International journal of multiphase flow 2018-02, Vol.99, p.312-328
• Main Authors: Timoshevskiy, Mikhail V., Zapryagaev, Ivan I., Pervunin, Konstantin S., Maltsev, Leonid I., Markovich, Dmitriy M., Hanjalić, Kemal
• Format: Article
• Language: English
• Subjects: Cavitation; Continuous tangential injection; Flow control; Frequency spectra analysis; Guide vane model; High-speed imaging; Instabilities; Partial/cloud cavities; PIV measurements; Pressure pulsations; Time-resolved LIF visualization; Wall jet
• ISSN: 0301-9322; 1879-3533
• DOI: 10.1016/j.ijmultiphaseflow.2017.11.002

•The low-speed (Uinj/U0  1) injection is more preferable from the hydrodynamic standpoint but makes the flow more cavitation-prone.•In unsteady regimes, the wall jet turns out to be mostly ineffective to suppress flow instabilities but can substantially reduce the amplitude of pressure pulsations.•At high attack angles, an unsteady sheet cavity on the hydrofoil with the slot channel exhibits intermittent length variations due to superposition of instabilities. 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