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The effects of tip gap size on the unsteady flow characteristics in an axial flow pump at design flow rate were investigated using large eddy simulation with sliding mesh technique. Five configurations with different tip gap sizes were considered, and the tip leakage vortexes and pressure pulsations of near tip zone on the blades were studied in detail. The results indicate that as the tip clearance increased from 0.001D2 to 0.003D2, the head and efficiency of the pump reduce by 6.2% and 5.6%, respectively. When the tip gap is larger than 0.001D2, the dominant tip-leakage vortex extend to the pressure side of adjacent blade and the secondary tip leakage vortexes formed in the tip-gap region. Affected by the tip leakage vortex, the amplitude of the pressure pulsation of the monitoring point PP6 (on the pressure side and near the middle of the blade tip edge) increases with the tip gap size. For different tip clearance configurations, the rotating frequency (fr) dominates the pressure pulsations of near tip zone on suction side of the blades, while the dominant frequency of near tip region on the pressure side changes with the tip-gap size.