Dynamic response of the fluid force acting on an axially oscillating balance piston was numerically investigated. The balance piston of a rocket engine turbopump was modeled and forced to oscillate in the axial direction. The flow field around the rotor was simulated using unsteady CFD simulation changing the oscillation frequency. By using the simulation results, the fluid force acting on the balance piston was found to be described by the equation of motion with the additional mass, the additional damping coefficient, and the additional stiffness coefficient. From the analysis of these coefficients, the fluid force on the balance piston in the present model was also found to work to stabilize for the axial oscillation.
