The study investigates gas-dynamics in the working volume of an experimental stand of the discharge chamber with a movable electrode for the purpose of developing arc systems. The free opensource OpenFOAM package connected with swak4foam is used for numerical simulation. Some model assumptions are formulated. The gas is injected into the chamber due to the difference in pressure at the initial supersonic velocities. The task is solved in the continuum mechanics approximation. The movable contact moves due to the pressure of the gas supplied to the chamber. The methods of creating a three-dimensional model with a dynamic mesh are described in detail. The study gives the time dependences of pressure, velocity, temperature in the region where the arc discharge develops for different inlet pressures, as well as the time-dependency of the moving contact velocity on arc ignition time. Numerical simulation shows the mechanism of pressure equalization in the working volume and achieving the piston velocity necessary for effective arc quenching in the range of 20–25 m/s by the time moment of 4 ms with the inlet gas pressure of 2–3 MPa.