Effect of immersion depth on hydrodynamic characteristics of a horizontal-axis tidal turbine

• Out-of-water blade exposure produces extra power loss beyond swept-area reduction. • Blade emergence induces blade-surface interaction at passing frequency and its harmonics. • Free-surface effects under submergence cause 3.0 % power loss and 1.8 % axial-force change. • Greater submergence reduces surface effects, yielding near-sinusoidal loads. As the scale of horizontal-axis tidal turbines increases, their blades are increasingly likely to approach or pierce the free surface during operation, particularly under tidal variations. Such proximity inevitably introduces free surface effects that may influence turbine performance and structural loading. Although this issue has been recognized, most existing studies emphasize engineering aspects while lacking detailed physical interpretation, especially for cases involving blade emergence from the water surface. In this study, a computational fluid dynamics model of a horizontal-axis tidal turbine at various immersion depths is developed and validated against experimental data. Turbine performance, free surface deformation, and wake vortex dynamics are systematically analyzed. Results show that when blade tips emerge above the surface, performance losses substantially exceed those expected from swept-area reduction alone. For blade tip exposures of 0.125 D and 0.25 D , swept-area decreases of 7.2 % and 19.6 % correspond to power coefficient reductions of 19.8 % and 35.3 %, respectively. These losses are driven primarily by flow asymmetry induced by the free surface, with further contributions from tip vortex ventilation and wake unsteadiness. When fully submerged, the turbine’s performance is less affected by the free surface, with maximum impacts on power and thrust coefficient of 3.0 % and 1.8 %, respectively. Although the interaction between the turbine wake and free surface becomes negligible beyond 0.5 D immersion depth, slight changes in the water surface persist. Load fluctuations near the surface are dominated by interactions between the blades and the free surface. In contrast, at greater depths, these fluctuations are governed by the coupling between the blades and the turbine wake. In this deeper regime, harmonic content decays rapidly, and waveforms tend to become more sinusoidal.