Fluid dynamics is the study of fluids in motion. Velocity vectors are often used to illustrate fluid motion in applications like meteorology. For example, wind—the fluid motion of air in the atmosphere—can be represented by vectors indicating the speed and direction of the wind at any given point on a map. Another method for representing fluid motion is a streamline. A streamline represents the path of a small volume of fluid as it flows. When the flow pattern changes with time, the streamlines do not coincide with the flow lines. The velocity is always tangential to the streamline.

Fluid flow can be either laminar or turbulent. Laminar flow (sometimes described as a steady flow) is represented by smooth, parallel streamlines, whereas in turbulent flow, the streamlines are irregular and change over time. In turbulent flow, the paths of the fluid flow are irregular as different parts of the fluid mix together or form small circular regions that resemble whirlpools. This can occur when the speed of a fluid reaches a certain critical speed.

For example, smoke rises from incense sticks smoothly for a while and then begins to form swirls and eddies. The smooth flow is called laminar flow, whereas the swirls and eddies typify turbulent flow. In another example, normal blood flow in the human aorta is laminar, but a small disturbance, such as a heart pathology, can cause the flow to become turbulent. Additionally, the flow of water from a faucet is laminar at low speeds but becomes turbulent at sufficiently high speed.

This text is adapted from Openstax, College Physics, Section 12.4: Viscosity and Laminar Flow; Poiseuille's Law and Openstax, University Physics Volume 1, Section 14.5: Fluid Dynamics.