# Analysis of Bulb Turbine Hydrofoil Cavitation
## 4 Results and Discussion
### Cavitation Dependence on the Cavitation Number \(\sigma\)  
The visual comparison and analysis of the two hydrofoils as a direct result of decreasing cavitation number \(\sigma\) is shown in Figures 10-13. Cavitation numbers from high to low were selected such that a marked difference in cavitation structures occurrence was observed. The selected images represent to the greatest extent possible the most typical conditions for selected cavitation number \(\sigma\). The analysis was limited to the hydrofoil suction side. A decrease in the cavitation number increases the intensity and amount of the cavitation structures. We will demonstrate that shape modification is essential for cavitation to occur on hydrofoils. A modified hydrofoil will demonstrate a decrease in cavitation phenomena intensity in comparison to an existing hydrofoil. The main area of visualization is over the hydrofoil's leading edge and the entire suction side of the hydrofoil. Figure 10: \(Cont.\)Figure 11: Visualization results of cavitation structures at \(\mathrm{p_{abs}}\) = 0.45 bar. Figure 10: Visualization results of cavitation structures at \(\mathrm{p_{abs}}\) = 0.65 bar. Figure 12: Visualization results of cavitation structures at p\({}_{\text{abs}}\) = 0.30 bar. Figure 13: Visualization results of cavitation structures at p\({}_{\text{abs}}\) = 0.15 bar. Figures 10-13 are limited to the intervals of the cavitation number that we were able to achieve in the cavitation tunnel with relevant cavitation intensities. The measurements at absolute pressure \(\mathrm{P_{abs}}\) = 0.65 bar are shown in Figure 10. Cavitation structures (void fraction) start to appear on the modified hydrofoil at cavitation number around \(\sigma\) = 3.65. The cavitation starts behind the leading edge as an attached cavitation. With the further decrease of the cavitation number to around \(\sigma\) = 3.1 we observed an increase in the cavitation intensity and the quasiperiodic shedding of cavitation clouds. On the other hand, cavitation was observed on the original hydrofoil at cavitation numbers around \(\sigma\) = 4.45. At cavitation number \(\sigma\) = 3.65 we observed some cavitation cloud shedding, and at cavitation \(\sigma\) = 3.1 more than two-thirds of the hydrofoil was cavitating. At cavitation \(\sigma\) = 3.1, with the modified hydrofoil, the cavitation length was only 1/3 of the hydrofoil length. Since the hydrofoils in both cases were attached at the same angle of attack, we assume that the difference in cavitation occurrence is due to the shape and curvature of the leading edge. A sharp leading edge cuts through the flow such that it cannot follow the hydrofoil shape immediately downstream from the leading edge. The resulting flow separation induces cavitation inception due to the low absolute pressure and high velocity. The measurements at absolute pressure \(\mathrm{P_{abs}}\) = 0.45 bar are shown in Figure 11. The cavitation on the modified hydrofoil profile starts behind the leading edge as an attached cavitation around \(\sigma\) = 3.45, and \(\mathrm{P_{abs}}\) = 0.65 bar. At cavitation \(\sigma\) = 3 we again observed cavitation cloud shedding as in Figure 10. The cavitation is much more intense for the case of the existing hydrofoil. We observed cavitation inception already at around \(\sigma\) = 4.5, while almost the entire hydrofoil was cavitating around \(\sigma\) = 3. The measurements at the absolute pressure \(\mathrm{P_{abs}}\)= 0.30 bar are shown in Figure 12. Measurements commenced around \(\sigma\) = 3.55; at this operating point the incipient cavitation is already present. Incipient cavitation here is present as an attached cavitation. For the modified hydrofoil, the attached cavitation formed only at around \(\sigma\) = 3; it covered up to around half of the hydrofoil length. With the original hydrofoil, the cavitation inception began around the same cavitation number (\(\sigma\) = 3.55) as for the modified hydrofoil, although with slightly higher intensity.