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Entropy Layer

For hypersonic flows, the entropy layer can be an important outer-layer feature distinct from the viscous boundary layer.

Entropy Difference

flow_props computes a profile-based entropy difference relative to an edge reference state:

\[ \Delta s = c_p \ln\left(\frac{T}{T_e}\right) - R \ln\left(\frac{p}{p_e}\right) \]

where \(T_e\) and \(p_e\) are taken from the profile edge unless the user supplies reference values explicitly.

Current Criterion

The current implementation uses a normalized wall-based criterion:

\[ \frac{\Delta s}{\Delta s_w} = \text{threshold} \]

with a default threshold of \(0.25\).

That means the reported delta_entropy is the first wall-normal location where the normalized entropy difference decays to \(25\%\) of its wall value.

Interpretation

This makes the entropy-layer thickness a profile-reduction quantity, much like the boundary-layer edge location. It is useful for comparing how quickly the entropy excess decays with downstream position.