The performance of the turbine model under the simultaneous influence of shear and waves was studied by simulating case w1 together with a shear velocity profile using a power law of
γ = 7 and 9 in two hub immersion depths of 0.31 m and 0.76 m, as outlined in Table 5.1. No significant change was seen in the time-averaged values of CP and CT, calculated using the equivalent flow velocity, when the submergence depth reduced from 0.76 m to 0.31 m, similar to what was seen in the wave condition without shear. Correcting the simulation data for Stokes drift resulted in lower values for the depth of 0.31 m. Comparing the wave simulation results for each depth before and after adding shear velocity profile, either using power law of 7 or 9, showed little change in the time-averaged and phase-averaged values of CP and CT, provided the equivalent flow velocity be utilised in analysis. As can be seen in Table 5.2, the fluctuation of resultant torque on the rotor is higher than the case w1 with uniform inlet velocity for the two depths.
5.5. Conclusion
The effect of free surface proximity, shear flow velocity profile and surface waves on the hydrodynamic characteristics of a two-bladed horizontal axis marine current turbine was investigated using numerical approach. A CFD model was developed to account for shear and waves in the performance evaluation of the turbine. The volume of fluid and the single-phase model were the two numerical approaches utilised to simulate the turbine performance. The CFD models were validated against the experimental results from the CWC and the towing tank. Comparison between the two CFD models showed that both the single-phase model and the VOF model were accurate enough in predicting the turbine performance under the sheared flow condition, but the VOF comes with higher computational costs. However, the VOF model
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in wave conditions had closer agreement with the experiments than the single-phase model. Moreover, the single-phase model was not able to account for the effect of free surface proximity on the turbine performance. Therefore, the VOF was selected to study the turbine performance under the effect of wave and shear in various submergence depths.
The effect of reducing the free surface proximity to less than half of the radius was a reduction in the CP and CT values only for the condition without waves. In the presence of waves, changing the turbine submergence depth made no significant change to the mean CP and CT values, however resulted in a slight increase in the phase-averaged values. Different outcomes obtained from the simulations with and without waves when the free surface proximity was reduced can be attributed to the effect of Stokes drift in wave conditions. Correcting the wave case data for Stokes drift resulted in smaller mean values of CPand CT for smaller proximity to the free surface. For all the study cases, the cyclic fluctuations of torque and thrust experienced by the blades and rotor were increased when the proximity to the free surface was reduced.
Introducing the shear velocity profiles, with two different power law (γ), had little impact on the mean values of power and thrust coefficients when the equivalent flow velocity was utilised for the performance analysis. However, ranges of torque and thrust on the blades are higher over a rotation compared to the condition with uniform flow.
For 0.76 m of depth where the influence of the free surface was less, the maximum torque and thrust on an individual blade occurred at an angular position of 90o where the velocity values are higher in the profile. The maximum torque and thrust on a blade over a complete rotation occurred in an angular position before 270o in uniform inflow velocity.
The different applied wave cases had negligible impact on the time-averaged values of CP and CT. Increasing wave period and wave height, performed in separate simulation cases, significantly raised the phase-average values of CP and CT. The maximum and minimum of CP and CT coincide with the wave peak and wave trough respectively.
Provided the equivalent flow velocity is employed in the calculations and the data is corrected for Stokes drift, the time averaged and phase-averaged values of CP and CT from the turbine simulations under waves and shear velocity profile matched with the wave case without shear.
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Overall, there was no change on the mean power and thrust coefficients of the turbine as a result of shear velocity profile or small proximity to the free surface or when surface waves are applied. However, these conditions increase the fluctuations of torque and thrust on the individual blades and the rotor, which is important in terms of higher risk of structural failure in the blades due to fatigue and reduction of the power output quality.