Effects of platform induced pitching motion on the aerodynamics of a FOWT

Abstract

The hunt for new methods to harness wind energy is at an all-time high as wind energy quickly overtakes other renewable energy sources. Deep waters are promising alternatives with higher average wind speed (thus, higher yields) and also due to lesser availability of land on shore. Even though the design of the blades and rotor of a FOWT is largely similar to that of onshore counterparts, they are mounted on floating platforms, unlike the fixed platforms on onshore wind turbines. Due to the excitation from wind and waves, these foundations are made to oscillate in all six degrees of freedom. Pitch and surge motions have the most significant effects out of all the possible types of motions. Since surging motion comes closest to the pitching motion, this study attempts to compare the aerodynamics of these two motions by simulating a pitching wind turbine using an actuator disk CFD model. This study can also be considered a continuation of the actuator disk study of the surging motion.

The primary objective of this thesis is to understand the differences between the aerodynamics of a surging and a pitching wind turbine using an actuator disk model. The actuator disk is made to pitch sinusoidally with the prescribed pitching frequency and amplitude. In order to identify the effects of motion and thrust prescription separately, three different types of actuator disk models were created. Using the first model, which simulates a pitching actuator disk with constant thrust, the effects of pitch motion are captured. The second model which simulates a still actuator disk with dynamic loading gives the effects of change in thrust on the rotor. The last model, which is a combination of the first two with a pitching actuator disk under dynamic loading, attempts to simulate a more realistic pitching actuator.

On comparing the surge and pitch motions of the actuator disk, the stark difference observed is the meandering in wake due to flow shear and asymmetric tip vortex shedding. At high thrust and low-frequency cases, turbulent wake
states are also visible in the wake. At higher frequencies, they are nearly as strong as the tip vortices and introduce non-linear effects on the induction field. They also tend to introduce varied dynamic responses based on the radial location. It was also observed that the motion introduces a phase delay that varies with the radial position on the rotor.

Thus, this project provides a comprehensive view of the effects of platform pitching motion on a FOWT and it differences with respect to the surging dynamics. This project also confirms that an actuator disc model is capable of reproducing the effects of pitching in a FOWT. Using the insights provided in this thesis, a better dynamic inflow can be developed for use in industries