# Analysis of Bulb Turbine Hydrofoil Cavitation
## 2 Hydrofoil
The hydrofoil properties that most affect the cavitation on the blade suction and pressure side are the leading edge radius, the location of maximum thickness, and the curvature; we may also assume that the blade angle in bulb turbines is always optimal due to the turbine's dual regulation. We want to design a leading edge such that the flow around it is smooth so as to prevent any flow separation. As discussed above, we will focus on the hydrofoil at 95% of the blade radius, where we assume that in addition to the mentioned leading edge, maximum thickness and curvature influence, the cavitation properties are also influenced also by the presence of the gap flow. At 95% of the blade radius, the flow properties (velocity and pressure) are more pronounced because of higher blade velocity than near the hub. The sharp leading edges may introduce flow separation [23] and deteriorate the flow properties further downstream from the leading edge. Downstream from the leading edge the flow is determined by the camber shape, the position of maximum curvature, maximum thickness, etc. These parameters, among others, determine the velocity distribution in the blade channel from the pressure to the suction side. Velocity is related to absolute pressure; an increase in flow velocity decreases the local pressure [7], thus, the velocity distribution affects the cavitation properties. The pressure distribution may cause local cavitation inception, and both can influence boundary layer separation. The hydrofoils in the bulb or any other water turbines are the base elements of the turbine runner blades and are thus rotating. The bulb turbine features pressure distribution with the pressure normally decreasing from the leading edge towards the trailing edge. According to the bulb turbine's triangles of velocity, downstream from the hydrofoil the relative velocity increases and reaches its maximum near the trailing edge [7], and the situation is remarkably different in comparison with the single hydrofoil in the water tunnel. Furthermore, the pressure distribution around the hydrofoil in the cavitation tunnel cannot fully represent the pressure properties around the bulb turbine blade. The situation in the bulb turbines is therefore very complex and the flow properties around individual hydrofoils cannot be directly correlated around the bulb turbine runner blades. Nevertheless, we will assume [6] that bulb runner cavitation can be related to the cavitation analysis of a single hydrofoil in the water tunnel. Hydrofoil ModificationsThe turbine blade hydrofoils are the primary component of the turbine blades. To improve performance, the turbine hydrofoil profiles should be smoothly and consistently remodelled in a spatial 3D redesign of the entire bulb turbine runner blade. The new blade differs from the old in the following parameters: the leading-edge radius has increased, the maximum thickness location has moved forward, and the location of the maximum hydrofoil curvature has moved forward. The characteristics of both turbine hydrofoils are shown in Table 1.  
The modified hydrofoil as compared to the unmodified one features reduced absolute velocity in a section from the location of the maximum hydrofoil thickness and curvature to the trailing edge. The cavitation properties of the modified hydrofoil as compared to the unmodified one are in this section favorable. Because the points of maximum thickness and curvature for the modified hydrofoil profile were moved forward towards the leading edge, this section is relatively long. The unmodified hydrofoil in a region from the leading edge to the location of the maximum thickness is very sensitive to the optimum angle of attack due to its sharp leading edge. Away from the optimum attack angle, the unmodified hydrofoil has disadvantageous cavitation characteristics in comparison with the modified one [6]. Contemporary water turbines often operate away from the best efficiency points due to the introduction of renewable energy sources and environmental changes.