Wow, takes me back to my old Chemical Engineering days.
Buckingham's Pi Theorum suggests that a certain system can be described by 'dimensionless groups' and that to create a correct model of that system the value of these groups must be the same in the model as the modelled system. Reynolds number is one of many dimensionless groups (Mach number is another one).
These dimensionless groups are technically not in themselves fudge factors as suggested (they are not false numbers pulled out of the air to make a correlation agree), however they are often used in engineering correlations because it is then easy to pull out meaningful quantities for any system.
Re = fluid speed x scale x density
-----------------------------
viscosity
where scale is a chosen dimension in m e.g. flow width, pipe diameter etc.
Wow, I've impressed myself that I remember all that junk - I standby for my memory to be proved wrong
Oops, looking at the first post, too much detail me thinks... sorry
[This message has been edited by foghorn (edited 14 September 2000).]