Two-dimensional plot of the radial velocity u_r at t = 45 orbits. The direction of the unperturbed flow is from bottom to top, i.e. in the direction of growing ϕ. The dashed white lines are contour lines of the azimuthal velocity at the levels 0.15,0.3, and 0.7 (from left to right). All velocities are given in units of u_K(R_∗). The radial velocity develops a pattern with vortices in the BL and shocks at the top of the BL. This pattern most likely resembles the lower branch of the acoustic modes discussed in Belyaev et al. (2013a).