The aim of this study is to investigate gases discharged from a high-pressure vessel numerically. To simulate this subject more realistically, the viscosity and compressibility of the gas are taken into consideration simultaneously. The methods of the Roe scheme, preconditioning, and dual time-stepping matching the lower-upper symmetric-GaussSeidel method are adopted to solve compressible flow problems during gaseous discharge processes. The nonreflecting boundary condition is used to prevent flowfields from being polluted by the reflection of the pressure wave induced by the compressible flow at the boundary. Computing procedures are performed on the compute unified device architecture computation platform, which was recently developed and is a highly effective technology for accelerating computational speed. Results show that the mass flow rate of this work is consistent with the existing experimental work. Because of a sudden expansion at a small opening, the phenomena of an alternating variation of the pressures of gases, rapid decrements of the temperature of gases, and a quick acceleration of the velocities of gases are remarkably observed in the mainstream direction. The ratio of the thrust caused by the gases released to the reaction force is less than 1 because of the dissipation of entropy generation. Nomenclature a o = opening area, m 2 C = discharge coefficient c = speed of sound, m∕s e = internal energy, J∕kg F = reaction force, N k = thermal diffusivity, W∕m · k L = channel width (Fig. 1), m L 2 = square opening width, m M = Mach number m = mass, kg P = pressure, Pa Pr = Prandtl number R = gas constant, 287 · J∕kg · k s = entropy, J∕K T = temperature, K t = physical time, s u i = velocities in x i directions, m∕s V 1 = vessel volume, m 3 x i = Cartesian coordinate, m Y = expansion factor (Eq. 46) β = volumetric thermal expansion coefficient, K −1 γ = specific heat ratio, 1.4 δ x , δ y , δ z = central difference operators μ = absolute viscosity, kg∕m · s ρ = density, kg∕m 3 τ = artificial time, s Ω = immersed boundary Subscript o = opening Superscript = dimensionless forms