Streamlines of the external field
Visualization of the external field is particularly interesting when there is separation of the boundary layer or in general when the flow is threedimensional. If the flow is unsteady, it can be defined as:
■ Streamlines: lines tangent to the direction of the instantaneous speed; these cannot be visualized.
■ Pathlines: all points crossed in time by each particle; these can be visualized with long exposure photography.
■ Streaklines: instantaneous positions of all the particles that have passed through given points; these can be visualized with low exposure photography.
Only if the flow is steady do streamlines, pathlines and streaklines overlap, and the streamlines can be visualized.
The techniques for displaying streamlines in a plane containing the velocity vector make use of particles carried by the stream:
■ in air: smoke from wood, charcoal, incense or tobacco, vapor of mineral oil or steam produced by dry ice + hot water;
■ in water: hydrogen bubbles, inks, particles of aluminum, lycopodium or coffee.
To visualize the streamlines in a plane orthogonal to the direction of the asymptotic velocity, flakes of wool or silk linked to screens are placed across the stream or smoke or vapor illuminated by a light sheet normal to the direction of motion is used.