The effects of surge motion on the dynamics and wake characteristics of a floating tidal stream turbine under free surface condition

•A CFD code is established for the surge motion of the HATT under free surface condition.•The Fourier series expansion and the least square method are employed to analyze the hydrodynamic coefficients.•Surge effect increases turbine performance unsteadiness and reduces the length of the wake.•Surge-induced toroidal vortex results more sensitive with amplitude rather than period.•Wake proximity to the free surface, peak and cavitation effects rise with surge.