Quantifying the hydrodynamic performance of a twin-rotor vertical axis tidal turbine under coupled rotation–surge–yaw dynamic excitations
Kong Ming, Renwei Ji, Xiaojie Zhang, Ke Sun, Jianhua Zhang, Sheng Xu, Renqing Zhu, Ratthakrit Reabroy
Abstract
This study investigates the hydrodynamic and wake characteristics of a twin-rotor vertical axis tidal stream turbine (TR-VATST) under dynamic coupled excitation of rotation–surge–yaw. A three-dimensional computational fluid dynamics (CFD) model of the multi-degree-of-freedom (DoF) coupled motion of TR-VATST was developed using the improved delayed detached eddy simulation method. The effects of coupled motion at both co-frequency and different frequencies on the performance of TR-VATST were analyzed. Additionally, least squares method was used to fit the time histories of hydrodynamic coefficients, aiming to explore the influence of damping force and added mass force associated with surge and yaw motions. The study reveals the following findings: (1) In the coupled surge and yaw motion, the peak envelope of the hydrodynamic coefficient of TR-VATST exhibits significant fluctuations. The amplitude of these fluctuations is influenced by the motion frequency. Specifically, a combination of a higher surge frequency and a lower yaw frequency notably increases the dynamic loading amplitude of the hydrodynamic coefficients. (2) The wake of the TR-VATST experiences interpenetration and superimposition of axial stretching from surge motion and transverse shear from yaw motion. This leads to enhanced mixing between the two rotors and increased asymmetry in vortex distribution. The wake vortex structure becomes severely twisted and deformed. (3) In high-frequency co-frequency coupled motion, the added mass or damping force associated with surge motion is greater. In different frequency coupled motions, however, the damping force related to both surge and yaw are larger. The fitted results show good agreement with CFD calculations. The study's results provide valuable insight for the hydrodynamic analysis, structural design, and operational control of TR-VATST with multi-DoF.