Instantaneous streamlines and surface density map of the simulation with Σ₀ = 30 M_⊙ pc⁻² and speed of sound c_s = 10 km s⁻¹. At the shocks the gas is compressed and loses a great part of its kinetic energy. Its velocity becomes subsonic and the gas is pushed to lower radii until it again reaches sufficient velocity to balance centrifugal and gravitational forces. This process can drive the gas far into the inner disk within a few orbits.