Measurement of effective bulk modulus and its use in CFD simulation

The two-phase model using the Rayleigh-Plesset equation has recently been used for the CFD simulation of hydraulic components or systems. However, the CFD simulation with a two-phase model has yet a problem in accuracy and simulation time owed to the many inaccurate parameters in the Rayleigh- Plesset equation for hydraulic oil. In addition, a very fine mesh and time step size is required for stable convergence of the solver. Therefore, this study suggests a one-phase model using the effective bulk modulus for the CFD simulation of a hydraulic pump. Theoretical equations to compute the effective bulk modulus of hydraulic oil have not been yet experimentally proven and only poor measured data are available to calculate the effective bulk modulus at low pressure until now. Therefore, in this study the theoretical equation was first verified through the effective bulk moduli, which are based on measurements of the pressure change according to the volume change at low pressure on variation of temperature, entrained air content and type of state change. To apply the effective bulk modulus in a CFD simulation program, the effective bulk modulus was converted into the effective density depending on pressure. In this study, the effective density was modified to an alternative effective density which allows stable convergence of the solver. Subsequent, the accuracy of the CFD simulation using the alternative effective density is confirmed by measurements of the pressure change inside a cylinder and the delivery pressure pulsation of a test pump.