To investigate the hydrodynamic characteristics of the surge motion of the tidal stream turbine (TST) under free surface conditions

this paper establishes a hydrodynamic analysis model

utilizing the improved delayed detached eddy simulation (IDDES) and the overset grid method. The hydrodynamic characteristics of the TST under different operating conditions (surge amplitude

surge period) are studied. The results indicate that: 1) An increased surge amplitude leads to greater instability in the wake velocity

resulting in wake turbulence around the TST. 2) As the surge period increases

the dynamic loading on the TST becomes more prolonged

thereby elevating the risk of fatigue fractures over long-term service. 3) The unsteady characteristics of the instantaneous load value are mainly dominated by the viscous damping term

which is approximately 1.5 and 0.8 times of the Cp0 value and Ct0 value respectively. These findings provide significant insights into the mechanisms of wake generation and diffusion during the surge motion of the TST.