Turbulence (or turbulent flow) is characterized by chaotic, random property changes. Turbulence occurs with low momentum diffusion (spreading of atmospheric properties), high momentum convection (vertical transference of atmospheric properties), and rapid variation of pressure and velocity in both space and time. A flow that is not turbulent is known as laminar flow. A vortex moving at low speeds will most likely cause laminar flow, and as speeds increase a transition is made to turbulent flow. The Reynolds number is a system that characterizes whether flow becomes laminar or turbulent, and a Reynolds number above 4000 is typically turbulent.
In turbulence, unstable vortices have many different characteristics and react with each other in different ways. Depending on the structure of the boundary layer and where it is located, overall drag may be reduced or eliminated. Laminar-turbulent flow transition changes not only depending on air speed, but also the size of the object, the viscosity of fluid, or the density of fluid changes, which may cause the turbulence to increase or decrease.
Turbulence causes eddies to form. Eddies are currents of air, dust, or fluids that run counter to the main current, such as with whirlpools. Energy from turbulence is carried and transferred from large masses to smaller and smaller masses. As this process continues, a continuing trend of eddies are produced. Eventually these masses become so small that the energy dissipates through molecular diffusion. This transference of energy is known as the Kolmogorov length scale. The Kolmogorov length scale was named for the Russian mathematician Andrey Kolmogorov who first proposed the theory of turbulence and the notion of cascading energy.
The complete description of turbulence remains a mystery and is one of the unsolved phenomena in physics.
Some good examples of turbulence include the following:
Smoke rising from a cigarette. The smoke remains laminar for the first few inches. It quickly becomes unstable and turbulent as the rising hot air speeds up.
Warm air that mixes with cold air in the atmosphere through wind causes clear-air turbulence.
Mixing of oceanic and atmospheric boundary layers may cause intense turbulence in ocean currents.
Flow conditions in industrial equipment (pipes, ducts, heat exchangers, etc.) and machines (internal combustion engines, gas turbines, etc.) cause turbulence.
The external flow over moving vehicles, such as cars, airplanes, boats, etc.
Some possible theoretical models to describe the behavior of turbulence exist in our everyday world. These include how race cars are unable to follow each other through fast corners due to the lead car causing understeer and how trucks on a highway get tossed about by windy conditions.