Turbulence is a flow condition within a medium – such as air or water – in which local speed and pressure changes unpredictably within an overall stable fl ow direction. As such, turbulence is characterised by rapid changes of momentum diffusion and convection, as well as velocity. It is the result of a medium like air having unseen, intermingling layers moving at varying speeds and in many directions. In general we are most familiar with turbulence while travelling on aeroplanes, when the cabin experiences a period of buffeting. This in-air bumpiness is referred to as clear-air turbulence (CAT) and is caused when atmospheric warm and cold air are mixed by high winds. Often occurring near jet streams, the segments of air have varying pressures due to their heat differential. CAT is invisible and so can occur suddenly without warning. The severity of turbulence is measured in fl uid mechanics by its Reynolds number. For example, if the medium’s Reynolds number is less than 2,000, the medium’s fl ow is laminar (ie steady, parallel layers), while if the number exceeds 2,000, then its fl ow is generally described as turbulent (ie disruptive, intermixing layers).
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