Streamlines on the surface of a body

In three-dimensional flows, streamlines near a solid surface can be very different from those of the external stream. Surface streamlines can also be very complicated in the zone of the boundary layer, separation and near obstacles on the surface and below a vortex.

Particles or wool tufts can be used as for the external stream but it is much more appropriate to spread a non-volatile liquid (oil), possibly with the addition of dyes, on the surface: oil moves under the action of tangential stresses and then in the direction of streamlines that appear as streaks in the oil film.