A proper name; also refers to the Reynolds number, a dimensionless quantity in fluid dynamics measuring the ratio of inertial to viscous forces.
As a name, from Old Norse 'Ragnarvaldr' composed of 'regin' (advice) + 'valdr' (power); in physics, named after Osborne Reynolds, an Irish engineer who developed the concept in the 1880s.
The Reynolds number is why engineers can test tiny airplane models in wind tunnels and predict what happens to full-size planes—it's one of physics' most elegant shortcuts that lets you scale predictions up or down.
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