A numerical simulation method based on overlap grid, i.e. dynamic fluid body interaction(DFBI), was applied to study effects of diffuser on hydrodynamic performance of a marine current turbine in complex current environments in this paper. Since the overlap grid can remove topological constraints between objects and meshes, DFBI can simulate passive motion of a rigid body under action of fluid to realize the simulation of unsteady performance of a passively rotating current turbine. Particularly, the influences of diffuser on the hydrodynamic performance and the surrounding flow field around the turbine were analyzed under different flow conditions comparatively. The result shows that the met-hod used in the paper can accurately monitor the processes such as speed acceleration and rotor adaptive rotation and so on caused from the changes in current flow field around the turbine, and consequently investigating into unsteady performance of the turbine becomes realizable. An installed diffuser can increase the current velocity around the turbine by around 1.2 times, raise the turbine speed by 1.2-fold and improve the turbine power coefficient by about 35%. Under unsteady current flow conditions, the averaged power coefficient is reduced by about 33%. The diffuser can increase the turbine average rotational speed, but also can raise the average power coefficient by approximate 30%.
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