In the course of one revolution, the tidal turbine blade will encounter a non-uniform inflow velocity, with implications for the design of the blade. Such a blade has the potential to suffer cavitation, depending on local inflow speed, forces (pressure reductions) and depth of immersion [75]. Cavitation occurs where the local pressure on the blade falls below that of the vapour pressure of the fluid and bubbles of gas form on the surface of the blade. The cavitation phenomenon was first observed and reported scientifically on a marine propeller over one hundred years ago [88, 89].
It can be assumed that cavitation will develop at any point on a blade where the pressure level is reduced to the level of the saturated vapour pressure of the ambient fluid. Different types of cavitation may occur – tip, vortex, sheet, bubble, cloud – depending on the effects of operating conditions, blade geometry, and water quality. Some undesirable effects may be allied with the type of cavitation incurred including;
erosion of the blade due to imploding cavities near to the blade surfaces, associated noise as well as structural vibration,
performance degradation depending on the extent, volume and fluctuating nature of the developed cavitation.
Cavitation tends to occur towards the ends of the blades on the face and near the tip, reducing the efficiency the turbine as a whole. Experimental evidence suggests that tidal
53
turbines may experience strong and unstable sheet and cloud cavitation, and tip vortices at a shallow depth of shaft submergence [75, 90]. Figure 3-9 illustrates a model turbine in a cavitation tunnel exhibiting both sheet and cloud cavitation, and tip vortices.
Figure 3-9: Cavitation experienced by a model turbine in a cavitation tunnel [75].
For cavitation analyses a cavitation number, σ, is defined as:
2 2 0.5 0.5 o v AT v P P P gl P v v (3.25)
Where Po is the reference static pressure, PAT is the atmospheric pressure, and Pv is the vapour pressure of the fluid. Cavitation inception may be predicted from the pressure distribution, since cavitation will occur when PL = Pv (where PL is the local pressure), or the minimum negative pressure coefficient, –Cp, is equal to σ, Figure 3-10.
Figure 3-10: The relationship between pressure coefficient and chord length, illustrating cavitation inception [75]
54
The cavitation characteristics for a particular section can be described by a minimum pressure envelope, or cavitation-free bucket, as a function of the section cavitation number. Since the lift coefficient of the section is a function of the pressure distribution then, for a particular section, the cavitation-free bucket can be represented as a limiting CL envelope to a base of σ. A schematic outline of a cavitation bucket, together with some likely types of cavitation, may be seen in Figure 3-11.
Figure 3-11: The cavitation bucket [75]
The width of the bucket, represented vertically in Figure 3-11, is a measure of the tolerance of the section to cavitation-free operation, i.e. if a section has a wider bucket it will be able to tolerate a much greater variation in angle of attack without cavitating. The width and shape of the bucket are dependent on the section characteristics such as thickness, camber, overall shape and nose shape. For example, an increase in section thickness tends to widen the bucket, while an increase in section camber tends to move the same bucket width and shape vertically to higher values of CL.
Accepted levels of cavitation on tidal turbines are not yet clear. Marine propellers are capable of withstanding a reasonably high amount of cavitation (up to 20% of the blade surface) without significant loss in performance; it is not known whether tidal turbines may cope with a similar level of cavitation. Another factor to consider is the erosion of the blade surface due to cavitation; how do different blade materials respond to cavitation erosion? The phenomenon of cavitation erosion can be defined as a loss of material from a solid surface due to damaging effects from nearby collapsing cavitation bubbles. Bhagat [91] states that in
55
ductile matrix/brittle fibre composites, fibres do not contribute toward increasing the cavitation erosion resistance of the composites. The brittle fibres break and accelerate the erosion of composites. Cracks develop easily causing removal of large chunks of matrix material. Therefore, the energy absorption capability of these composites is drastically reduced and the materials have poor resistance to cavitation erosion. Hammond et al [92] examined the response of several different polymer matrix composite systems subject to vibratory cavitation. It was concluded that the mechanical response of the matrix material dominated erosion resistance. Saturation was found to decrease the damage resistance of composite materials by disrupting the interface, degrading the fibres, and swelling the matrix. Carbon fibre/epoxy composites were found to be least affected by saturation, whilst woven E- glass/epoxy and carbon fibre/thermoplastic matrix composites were most affected. They also noted that observation of the damaged surfaces over time indicated that the topography of the eroded surface significantly influenced the erosion behaviour. Due to the evident lack of performance of GRP with respect to cavitation erosion, protection alternatives were evaluated by Light [93]. Results showed that a GRP composite system could be designed to increase the cavitation erosion resistance of the material; however, the performance still remained below that of common metallic materials. Mention is also made of the low resistance of composite materials to cavitation erosion by the Specialist Committee on Cavitation Erosion on Propellers and Appendages on High Powered/High Speed Ships [94]. Whilst the majority of literature suggests that the cavitation erosion performance of composites is lower than that of metals, there are conflicting opinions [95]. Recent published research on the subject is limited and this could be an area of potential further work. The noise associated with cavitation is also a point for discussion; some theories suggest that it is beneficial for the device to create some noise in order to provide a sort of warning signal to marine mammals. It is undesirable, however, for such noise to be heard above the water surface.