1.) A nondimensional co-efficient that measures the compression of air due to scaling. This allows scale models to be tested in wind tunnels;
2.) A dimensionless number used by fluid flow engineers to characterize the way a fluid (gas or liquid) will behave when passing over a solid surface. The number combines the fluid's density, viscosity and velocity with the length it's traveled along the surface. No matter what the fluid is or what size the surface, the flow conditions (laminar, turbulent, detached, etc.) should be the same at the same Rn. Discovered by Osborne Reynolds inthe 19th Century while studying the flow of water in pipes andchannels, it has proven most useful to aerodynamic engineers and naval architects in scaling up wind/water tunnel test results to full size. Carl Dowd, a model aviator and NASA engineer, found it helpful to think of Rn as the "coarseness" of the air seen by a body. Move the body faster, and more particles will pass over it in a given unit of time, increasing Rn. Make the body larger, and there will be more particles over the body at any instant, increasing Rn.