# Stratford's separation criterion

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Stratford's separation criteria is an old classical analytical way to assess if a boundary layer is likely to separate or not. Stratford's criteria says that from the start of the pressure recovery (max velocity and minimum static pressure) the boundary layer is on the verge of separation when:

$C'_p \cdot \sqrt{x' \frac{dC'_p}{dx}} = k \cdot \left( \frac{Re}{10^6} \right) ^ {0.1}$

Where

$C'_p$ is the canonical pressure distribution defined by:

$C'_p = 1 - \left( \frac{U}{U_{max}} \right) ^ 2$
$U$ is the local velocity and $U_{max}$ is the maximum velocity at the start of the pressure recovery.

$x'$ is the effective length of the boundary layer. Note that computing $x'$ can be a bit tricky. If the boundary layer is first accelerated up to the start of the recovery a turbulent boundary layer can be assumed to have the followig effective length: