# Analysis of Bulb Turbine Hydrofoil Cavitation
## 5 Conclusions  
In future power grids, waterpower flexibility will be an important parameter for ensuring network stability. Water turbines will need to operate at wide flow intervals and in transient regimes, and also allow for fast response times. For this reason, among others, further research on cavitation is required.  
The goal of this research was to demonstrate the relation between the hydrofoil shape and cavitation phenomena. Existing and modified hydrofoils were extracted from runner blades on the 95% runner blade radius and modified. The modifications were made on the hydrofoil's leading edge, as well as at the points of maximum thickness and curvature. The leading-edge radius was enlarged to avoid flow separation. Maximum thickness and curvature were moved in the direction of the leading edge to stabilize the flow around the hydrofoil towards the trailing edge. Both hydrofoils were implemented in a bulb turbine and measured under equal energy numbers. Measurements of both turbine runners were performed on the measuring station according to the standard for acceptance tests of water turbines.  
The results of this study show improved cavitation characteristics. However, we show here only a single modification of the hydrofoil and the results only demonstrate the efficiency of this modification. Of particular benefit for future design would be a comprehensive study, providing for a hydrofoil and runner shape optimization algorithm, based on selection and later minimization of the objective functions, including the manufacture and measurements of a selection of hydrofoils and runners.  
To evaluate the cavitation properties, the flow was visualized in the cavitation tunnel on the suction side. The modified hydrofoil exhibited improved cavitation properties and we chose to evaluate the intensity of the cavitation using the cavitation cloud length. A regression model prediction of the cavitation length shows that modified hydrofoil features improved cavitation characteristics in comparison with the existing hydrofoil.  
The primary goal of this study was to provide for improved cavitation characteristics on the modified hydrofoil. Using the modified hydrofoil invariably changes runner characteristics and this resulted in operating points shifting to the left in the hill diagram. This leftward shift resulted in increased efficiency as shown in [6].  
We believe that future requirements for water turbines will emphasize flexibility. For this reason, the future research on cavitation in water turbines will focus on the minimization of cavitation occurrence and erosion, while at the same time providing adequate energy output and efficiency.  
Conceptualization, A.P, M.H. and M.D.; methodology, A.P., M.H., L.N. and M.D.; software, A.P. and L.N.; validation, A.P., M.H., L.N. and M.D.; formal analysis, A.P., M.H. and L.N.; investigation, A.P., M.H., L.N. and M.D.; resources, A.P., M.H. and M.D.; data curation, A.P., M.H. and L.N.; writing--original draft preparation, A.P. and M.H.; writing--review and editing, A.P., M.H., L.N. and M.D.; visualization, A.P. and M.H.; supervision, M.H. and M.D.; project administration, A.P. and M.H.; funding acquisition, M.H. and M.D. All authors have read and agreed to the published version of the manuscript.  
This study was performed with financial support of Research Program P2-0401 Energy Engineering from Slovenian Research Agency ARRS.  
Not applicable.  
The authors declare no conflict of interest.