Gap flow has been shown to be the primary aspect of an integrated motor pump-jet (IMP) thruster that cannot be disregarded. In this study, the gap flow was divided into three areas: the rim’s front and rear end faces, internal surfaces, and external surfaces. According to the boundary layer theory and semi-empirical formulas, the influence of the gap geometry on the frictional torque of the external surfaces and end faces was analyzed when the gap flow lying in non-pressure difference. RANS solver was employed to calculate the gap flow of a selected gap to validate the turbulent model SST \(k - \omega\) and grid structure by comparing the velocity distribution in the radial gap with experimental results. The gap flow of different gap cases with/without the pressure difference was calculated, and the influence of the variation of parameters such as radial gap ratio and axial gap ratio on the hydrodynamic performance was analyzed, and the variable law of axial and radial gap on the friction torque was obtained. The change of the gap on the gap vortex is one of the causes of the duct thrust and \(\eta\) . Reducing the relative pressure at the gap inlet and outlet is beneficial for improving the efficiency of the IMP thruster. In addition, the relative Cp for different axial gap dimensions had a greater impact on the efficiency of the IMP thruster. When the rim length is fixed, decreasing the radial, increasing the axial gap ratio, or increasing the Re, etc., will decrease the friction torque coefficient of the rim. These findings shed light on the design of the gap geometry to improve the IMP thruster hydrodynamic performance.
