1.) A nondimensional co-efficient that measures t' compression o' air due t' scaling. This allows scale models t' be tested in wind tunnels;
2.) A dimensionless number used by fluid flow engineers t' characterize t' way a fluid (gas or liquid) will behave when passin' over a solid surface. Well, blow me down! T' number combines t' fluid's density, viscosity and velocity with t' length it's traveled along t' surface. Avast, me proud beauty! No matter what t' fluid is or what size t' surface, t' flow conditions (laminar, turbulent, shiver me timbers, detached, etc.) should be t' same at t' same Rn. Ahoy! Discovered by Osborne Reynolds inthe 19th Century while studyin' t' flow o' water in pipes andchannels, it has proven most useful t' aerodynamic engineers and naval architects in scalin' up wind/water tunnel test results t' full size. Begad! Carl Dowd, me bucko, a model aviator and NASA engineer, me bucko, found it helpful t' think o' Rn as t' "coarseness" o' t' air seen by a body. Move t' body faster, and more particles will pass over it in a given unit o' time, increasin' Rn. Make t' body larger, and thar will be more particles over t' body at any instant, increasin' Rn.