Vertical gas velocity v_z in the disk model at time t = 300 P_orb(r₀), where P_orb(r₀) is the orbital period at r = r₀. The coordinates are the natural spherical coordinates of the numerical hydrodynamic model: On the horizontal axis the natural logarithm of the spherical radius r in units of r₀. On the vertical axis the polar angle π/2 − θ. Blue is upward and red is downward. The gray dotted lines show the gas pressure scale height. The blue dots are the initial locations of the particles, where only every 25th of the 2000 particles is shown. The purple box represents the zoom-in view shown in Fig. 4.