The key to the definition of the transition location lies in the formulation of the transition onset criterion . To explain the concept, the correlation generically has the form:

(4–206)

when using the default constants, which is essentially a 1/ relation with limiter. The function FPG contains the dependence on the pressure gradient which will be discussed later.

In more detail, the correlation reads:

(4–207)

The quantities , , and are constants and is a blended coefficient (blending between low and high Re number situations):

(4–208)

with and being constants. The blending function between low and high Re number transition reads:

(4–209)

with being a constant. This function is 0 for low and 1 for high Re numbers. The purpose of this function is to allow for a different calibration for the different Re number regimes (with low Re typically being bypass transition and high Re number being natural transition).