Jet pumps are used in a great number of engineering applications. In the present study, jet shapes, mixing chamber shapes, and numerical methods for predicting the performance of an annular-type jet pump are investigated to determine the optimal turbulence model. The flow fields are simulated by solving the momentum and the continuity equations with the standard ${kappa}-{epsilon}$ and the RNG ${kappa}-{epsilon}$ turbulence models at different Reynolds numbers. After that, they are compared with the corresponding experimental data to determine the optimal model. Next, various calculations are conducted to find an optimal shape using the selected turbulence model. The study shows us that the RNG ${kappa}-{epsilon}$ model predicts the performance more exactly, and also shows that the most effective performance can be achieved with $12^{circ}$ reducing angle and 130mm throat length.