The colour contours correspond to the magnitude of the axial velocity component, the black arrows indicate the direction and magnitude of the transverse flow velocities. The white lines are the transverse streamlines and give an overall impression of the secondary flow.
The flow field was computed with a Finite Element solver for the Stokes equations (zero Reynolds number; slow viscous flow) which was specifically developed for this problem.
The problem was solved as a fully coupled fluid-structure interaction problem (i.e. the wall was allowed to deform in response to the fluid traction).