• Could you provide me with a simple procedure to visualise a 3D velocity field with streamlines?

The easiest way to define a stream line in a 3D mesh is to select a point by giving its coordinates. The stream line that goes through this point is drawn. In the contextual menu there are other ways to select several points at once. Tipically, a cut is made in the middle of the volume mesh and the points are selected in the cut.