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Reynolds number
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<p>In <a href="/facts/Fluid_dynamics/y7uzSCfg">fluid dynamics</a>, the Reynolds number (Re) is a <a href="/facts/Dimensionless_quantity/TINrpQIq">dimensionless quantity</a> that helps predict <a href="/facts/Fluid_flow/y7uzSCfg">fluid flow</a> patterns in different situations by measuring the ratio between <a href="/facts/Inertia/bzK9uzI2">inertial</a> and <a href="/facts/Viscous/QLVfrdCz">viscous</a> forces. At low Reynolds numbers, flows tend to be dominated by <a href="/facts/Laminar_flow/RFXN4eAC">laminar (sheet-like) flow</a>, while at high Reynolds numbers, flows tend to be <a href="/facts/Turbulence/NpgFUhG5">turbulent</a>. The turbulence results from differences in the fluid's speed and direction, which may sometimes intersect or even move counter to the overall direction of the flow (<a href="/facts/Eddy_(fluid_dynamics)/V0hTLblX">eddy currents</a>). These eddy currents begin to churn the flow, using up energy in the process, which for liquids increases the chances of <a href="/facts/Cavitation/RFSxKS41">cavitation</a>. </p><p>The Reynolds number has wide applications, ranging from liquid flow in a pipe to the passage of air over an aircraft wing. It is used to predict the transition from <a href="/facts/Laminar%25E2%2580%2593turbulent_transition/NKeUj2qM">laminar to turbulent</a> flow and is used in the scaling of similar but different-sized flow situations, such as between an aircraft model in a wind tunnel and the full-size version. The predictions of the onset of turbulence and the ability to calculate scaling effects can be used to help predict fluid behavior on a larger scale, such as in local or global air or water movement, and thereby the associated meteorological and climatological effects. </p><p>The concept was introduced by <a href="/facts/Sir_George_Stokes%2c_1st_Baronet/7RHJJ5Lj">George Stokes</a> in 1851, but the Reynolds number was named by <a href="/facts/Arnold_Sommerfeld/cJtpVHvn">Arnold Sommerfeld</a> in 1908 after <a href="/facts/Osborne_Reynolds/Q1BvNrKk">Osborne Reynolds</a> who popularized its use in 1883 (an example of <a href="/facts/Stigler%2527s_law_of_eponymy/hqkn97eh">Stigler's law of eponymy</a>). </p>