In fluid dynamics, a streamline is the path that an imaginary massless particle would make if it followed the flow of a fluid in which it was embedded.

Due to the way fluids interact in a flow, the flow is often complex. For instance, in a stream the water at the top middle is moving faster than the water at the bottom or sides. This is due to the no-slip condition and viscosity. In effect, the water in contact with the earth experiences force from the earth, slowing it down, and it then slows down the water beside it, and so on.

A streamline becomes more complex when such a flow is disturbed by some solid object, like a rock in the water. In this case the water moves around the rock in a complex fashion. However, for each point upstream from the rock, there is a particular path to the downstream side. This is the streamline.

Attempting to make as smooth a streamline as possible is a common task in engineering. A smooth streamline implies as little disturbance of the fluid as possible, and thus as little wasted energy as possible. For a vehicle moving through a fluid, careful design can lead to considerable improvements in fuel economy or performance.

Engineers often use dyes in water or smoke in air in order to see existing streamlines, and then modify the designs to reduce the drag. This task is known as streamlining, and the resulting design is referred to as being streamlined. Streamlined designs are often esthetically pleasing to the eye. The Streamline Moderne style, an 1930s and 1940s offshoot of Art Deco, brought flowing lines to architecture and design of the era.

The same terms have since become common vernacular to describe any process that smooths an operation. For instance, it is common to hear references to streamlining a business practice, or operation.

related terms: streakline, filament line, equipotential surface

See also: streamliner, drag coefficient