Biot Number Calculator
The Biot number calculator helps you compute the Biot number that determines how quickly heat transfers from the surface of a body to its interior.
The formula
each group is a ratio of two competing effects
Why physics is done in ratios
A dimensionless group compares two competing effects, and its value says which one is winning. That is far more useful than either quantity alone, because it means a model aircraft in a wind tunnel and a real one in flight behave the same way provided their groups match. This is dynamic similarity, and it is the entire basis of scale testing.
The thresholds are where the physics changes character. Reynolds 2300 in a pipe is where smooth flow gives way to turbulence. Mach 0.3 is where air stops behaving as incompressible. Froude 1 is where a free surface can support a hydraulic jump, exactly as Mach 1 is where a shock becomes possible. Knudsen 0.01 is where treating a gas as a continuous fluid stops being valid at all.
Two cautions. The characteristic length is a convention, not a measurement — pipe diameter, aerofoil chord, plate length — so two Reynolds numbers are only comparable when both used the same one. And Nusselt and Biot look identical as formulas; the difference is that Nusselt uses the fluid's conductivity and Biot the solid's, which makes them answer completely different questions.