Did I see recently a **Reynolds** number that was **NOT fluid mechanics?**

Anyway, in fluid mechanics, this number **compares the effect of viscosity and momentum**. It tells if a flow is more likely turbulent or laminar, by comparison with a "critical Reynolds number".

Fluid mechanics is essentially experimental, and its theory rudimentary. A handful of cases are documented with curves allowing to make predictions by hand, the rest is finite elements methods, with all the necessary caution. In this context, a dozen of dimensionless numbers have been defined besides the most famous Reynold's, with the aim of reducing the number of variables when plotting experimental curves used to predict other cases. Then, empirical relations mix observed fractional powers of these numbers. There is little more than that.

So this number is not related just with a material, but with the conditions of a flow. It's used at extrusion, injection... of polymers, which is usually done with heat, not at room temperature.