Abstract: The transient response of internal pressure following a sudden openning was described by a second-order, non-liner, ordinary differential equation. Because of the complexity of the geometry and the opening configuration, it is quite difficult for wind tunnel test to correctly estimate the constant coefficients in the equation. The transient response of flow pattern at sudden opening is simulated by Computational Fluid Dynamics method. Both the Helmholtz frequency and equivalent viscous losses are obtained. The gain function obtained by using these parameters coincides with the result of wind tunnel test fairly accurately. It is shown that the numerical computation can simulate the flow pattern at sudden opening correctly. Finally the flow dynamical parameters are derived by fitting the differential equation to the numerical response.