| الوصف: |
Floating tidal turbines (FTT) are gaining popularity in the field of tidal energy these past few years due to some advantages they had when compared to the bottom fixed devices. However, the floating motion of an FTT, due to waves and tidal current, leads to unsteady loadings on the rotor. This motivates the present study to pursue this topic to further understand and characterize the hydrodynamic of an FTT. To conduct a study on a device scale, a floating tidal turbine undergoing single degrees of freedom (DoF) is preferable to isolate the effect of each motion on the device performance. Blade resolved computational fluid dynamic (CFD) simulations of floating tidal turbines undergoing surge, heave, and pendulum (pitch) motions were carried out over a range of motion's amplitude and frequency and also using multiple rotor rotational speeds. The present study seeks to determine the effect of each motion on the floating turbine's performance. The rotor goes into stall at higher motion amplitude and frequency for the surge and pendulum motions. The mean power decrease while the mean thrust increase as the motion amplitude and frequency is increased, due to the loss in the lift at the maximum apparent velocity which increases the angle of attack on the blade, causing the flow to separate, thus stalling the rotor. This can be minimized when operating the rotor at a higher rotational speed, i.e., using speed control to reduce the stalling effect. A floating tidal turbine undergoing the heave motion can be viewed as a dynamic yawing motion case; there is evidence of the advancing and retreating phases and wake skewness formation at every heave motion cycle. The added mass and damping forces are calculated based on the blade resolved CFD results, which can be used as standardized hydrodynamic coefficients to be included in a simplified model, such as an actuator disc, for a fully coupled model simulation. Modifications to include the added mass and damping forces into an existing fully coupled model are presented in this thesis. The result shows good improvement from the original model when verified against the blade resolved simulations. The model is then tested under various wave frequencies, including the combined wave-current conditions. The present model increases the mean thrust on the rotor while decreasing the loading amplitude compared to the original model. |