Early tidal turbine deployment is expected to occur in relatively shallow waters, where monopile support structures are widely adopted due to their structural simplicity, cost-effectiveness, and ease of installation. However, the influence of tip to-tower clearance and symmetry boundary
conditions on turbine performance and structural loading remains insufficiently understood. This study presents a combined analysis of the effects of tip-to-tower clearance and symmetry boundary conditions on the performance of a horizontal-axis tidal-current turbine (HATCT), using high-fidelity, transient computational fluid dynamics (CFD) simulations. Simulations were performed in ANSYS CFX by solving the Reynolds-Averaged Navier-Stokes (RANS) equations using the Shear Stress Transport (SST) turbulence model and a transient rotor–stator
interface. Turbine performance is evaluated using two key coefficients: the coefficient of performance (Cp) and the coefficient of thrust (Ct). A tower diameter of 2 m was considered, with blade tip-to-tower clearances of 7 m, 5 m, 3 m, and 2 m. The corresponding mean cycle Cp values are 0.468, 0.476, 0.470, and 0.472, respectively, while Ct values remain largely unaffected, ranging between 0.86 and 0.87. Although average power shows minimal variation, power output fluctuates notably (approximately 3–4%) as clearance decreases. Additionally, substantial unsteady forces are generated on the tower, which may contribute to increased fatigue loading. The use of symmetry boundary conditions shows negligible impact on prediction accuracy while significantly reducing computational cost. These findings demonstrate that, when structural elasticity effects are accounted for, a blade tip-to-tower clearance approximately equal to the tower diameter is optimal for HATCT systems, providing practical design guidance for tidal turbine support structures.