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Flow is the steady or continuous movement of a fluid, whether it is through a pipe, across an airfoil or in the open, as in the case of the open air or sea. This type of movement falls into one of three general categories: laminar flow, turbulent flow or transitional flow. Supercomputers and specialty software are necessary in considering complex mathematical models of such flow behavior.
Laminar flow through a pipe can be visualized as a series of very thin layers — nested cylinders of fluid — from a pipe’s inside surface to its center. Characteristics favoring laminar flow are a smooth pipe wall and high fluid viscosity. The outermost layer of fluid clings to the pipe. The other layers glide inside each other at slightly increasing velocities as the center is approached. Laminar flow is moderately predictable and orderly.
Turbulent flow is flow in turmoil — flow complete with eddies and vortices. The word "turbulence" implies a mixing action, increasing or maintaining uniform fluid composition throughout the pipe’s contents. Turbulent flow is favored by a rough pipe interior and fluids of low viscosity. It also is favored by increasing fluid density. One example of turbulent flow is that of blood through the arteries.
Transitional flow is between the first two types. It combines laminar flow along the inside edge of a pipe with turbulent flow down the middle. Engineers of specialty mixers design impellers that introduce transitional flow in highly viscous fluids that ordinarily would exhibit laminar flow. This saves mixing time and money.
Turbulent flow is of extreme importance to aviation in two ways. Turbulence of the surrounding air, produced by weather systems, is beyond the control of the designer. The other is turbulence generated by the aircraft itself, particularly its wings. This sort of controllable turbulence comes from wings and can be divided into wake turbulence and airfoil turbulence.
Wake turbulence is the flow generated by the wingtips of aircraft, and it often takes the form of a circular pattern of air perpendicular to the direction of flight. Airfoil turbulence is generated across the surface of the wing and causes aircraft drag and loss of lift. Methods to reduce wake and airfoil turbulence are the subject of much theory and design.