# Analysis of Bulb Turbine Hydrofoil Cavitation
## 3 Experiment
### Cavitation Tunnel Measurements  
The flow visualization measurements were taken on the cavitation measuring station. The approach to investigating the turbine blade flow properties in a separate tunnel experiment was similar to that used in [24]. A closed-type cavitation measuring station was used as shown in Figure 7. The flow was provided by a centrifugal pump driven  
Figure 6: Turbine measuring points. by a frequency variable drive. The volumetric flow measurement was performed using an electromagnetic transducer ABB COPA-XL. Absolute pressure p\({}_{\text{abs}}\) was measured on the suction side using an ABB 2600T 264 NS pressure transducer. The temperature was measured by a Pt-100 4 wire temperature transducer, connected to the Agilent 34970A data acquisition unit. Valves in front of and behind the test section were fully open during experiments and they were only used during installation. Suction pressure was set using a vacuum pump, located on the top of a tank with a free water surface. The cavitation tunnel test section cross-section was 50 cm \(\times\) 100 cm. Hydrofoil width was 50 mm and chord length was l = 60 mm. The hydrofoil was installed at the angle of attack 8\({}^{\circ}\). A high-speed Photron SA-Z camera was used together with a Nikkor 105 mm 1:2.8 lens to visualize the cavitation in the cavitation section. The camera was mounted from the side. The camera was operated in a free-running mode. Images were recorded using PFV software in 8-bit BMP black and white mode at a resolution of 640 \(\times\) 280. The framerate used was 25,000 frames/s. Shutter speed was equal to 10 \(\upmu\)s. For each operating point, 2000 frames were recorded. For all experiments, all other camera settings were the same. A high intensity continuous LED illumination was provided by many CREE XLamp XM-L LED modules. LED modules were powered by an Instek DC power supply. Illumination was provided from the top of the test section above the hydrofoil on its suction side and arranged such that as few reflections as possible were present in the recorded sequences. Illumination intensity was not the same for all experiments; with the modified hydrofoil it was necessary to increase illumination intensity to visualize weaker cavitation structures. Keeping the illumination constant was impossible due to the low 8-bit depth of recorded images. We have selected two dimensionless numbers to describe the cavitation in the cavitation tunnel, namely the Reynolds number Re and cavitation number \(\sigma\). In Reynolds number Re  
\[\text{Re }=\ \frac{\text{L}\cdot\text{v}}{\text{v}} \tag{4}\]  
L is hydrofoil length, v (m/s) is flow velocity in front of the hydrofoil and v is the kinematic viscosity of water. To obtain the hydrofoil cavitation characteristics, the cavitation numbers \(\sigma\) were evaluated. Cavitation number \(\sigma\) was calculated according  
\[\sigma\ =\ \frac{\text{P}_{\text{abs}}-\text{P}_{\text{v}}}{\text{\rho} \cdot\text{v}^{2}} \tag{5}\]  
Figure 7: Cavitation measuring station with measuring and visualization equipment. where \(\rm p_{v}\) (N/m\({}^{2}\)) is the water evaporation pressure and \(\rm p_{abs}\) (N/m\({}^{2}\)) is absolute pressure. Water evaporation pressure was estimate using the Equation  
\[\rm P_{v}\ =\ 10^{2.7862+0.0312\cdot T_{w}-0.000104\cdot T_{w}^{2}} \tag{6}\]  
where \(\rm T_{w}\) (\({}^{\circ}\)C) is water temperature. Equation (6) provides for a maximum difference of 0.2%, with a negligible influence on the measurements. The cavitation \(\rm\sigma\)-\(\rm\eta\) curve is used to estimate the cavitation properties of bulb turbines. The cavitation point for the highest cavitation numbers is measured at ambient pressure, while for subsequent cavitation points suction pressure is decreased. For the tunnel measurements the cavitation number \(\rm\sigma\) was used to observe the cavitation's development on both hydrofoils. For each experiment, the absolute pressure \(\rm p_{abs}\) was set as constant at the inlet of the cavitation tunnel.