Cavitating flow around a tandem of 2D hydrofoils: high-speed imaging and PIV measurements

The research is aimed at an experimental study of cavitating flow around a tandem of 2D hydrofoils representing a scaled-down model of the stay vane (SV) - guide vane (GV) system of a Francis turbine distributor. The investigation was carried out for a wide range of flow conditions on the attack angles of both profiles and cavitation number. Using high-speed imaging, the influence of a turbulent wake of the SV section on cavitation patterns on the GV profile was considered. A PIV technique was applied to measure the flow mean velocity and turbulent characteristics over and behind the hydrofoils. As a result, noticeable discrepancies between the flows around the single GV model and the one in tandem were discovered. It was revealed that cavitation patterns on the single hydrofoil and the one in tandem are absolutely different. Cavitation development on the GV profile is hampered substantially due to the forced flow turbulization by SV. Cloud cavitation regimes do not occur on the GV section in the tandem configuration at moderate attack angles, the cavitation surge instability governs the flow dynamics instead. Under stationary flow conditions, the flow separation region on the GV model is shifted to the trailing edge and the intensity of turbulent fluctuations in the GV wake is noticeably reduced.