Fractional-flow tangent construction
= Fractional-flow tangent construction
{title2=$F'(s_s)=[F(s_s)-F(s_0)]/(s_s-s_0)$}
When a <rarefaction wave> attaches to a saturation shock, its terminal <characteristic speed> must match the <Rankine-Hugoniot condition> speed. This gives a tangent from the initial state $(s_0,F(s_0))$ to the fractional-flow curve at $s_s$. It selects the upstream shock saturation; the actual dimensional shock speed still includes total flux divided by <porosity>.