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The performance and flow field of hydropower turbine in shallow water can be affected by the presence of free surface.Therefore,the influence of free surface on the turbine performance was studied through transient computational fluid dynamics (CFD)analysis.The turbine included three blades made ofNACA0018 hydrofoil.Submerged depths and different tip speed ratio (TSR) were two main variables in this numerical simulation.A comprehensive simulation of quantities was performed for submerged depths ranging from h / R =1.2 to 2.0,and tip speed ratio from λ =1.0 to 30.The results revealed that the presence of the free surface decreased the power coefficient by 19.05％ for the closest submerged depth of h / R =1.2 at optimal tip speed ratio of λ =2.5.The wave breaking also occurred when the submerged depth was smaller than h / R =2.0.In the case of variable pitch,the top speeds for free-to-spin cases were investigated by 6DOF method.The top speed had a 4.24％ drop by comparing the largest and the smallest submerged depths.Variable pitch also improved the power coefficient by 28.09％ when the free surface was far from the hydropower turbine.However,the power coefficient improvement became significantly small when the hydropower turbine was placed close to the free surface.