Critical Reynolds Number

1) How useful is the concept of critical Reynolds number?
I find it is not really universal, and seems to change with
experimental geometry. In fact, there may be several types of critical
Reynolds number given an experimental configuration. Can any physicist
or engineer tell me how useful he found the concept?
2) Can anyone recommend a reference which both defines the Reynolds
number and evaluates the critical Reynolds number for different
geometries?
The Reynolds number equation seems to vary a lot with
experimental conditions. The "standard length" and "standard velocity"
concepts seem to fluctuate a great deal, making the definition of
Reynolds number more situational than universal. I would think someone
would write a table of formulas for Reynolds number, and values of the
critical Reynolds number..
I am particularly interested in a reference that gives an
equation for the critical Reynolds number of an open channel of water
(as in a shallow stream). However, I would welcome any discussion on
the "critical Reynolds number" concept.
.

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