Subcritical tube flow
= Subcritical tube flow
{title2=$0<u<c$}
A positive <collapsible tube flow> is subcritical when its <characteristic speeds> $u+c$ and $u-c$ have opposite signs. A downstream boundary can then send information upstream. For a quadratic pressure-area law and fixed dimensionless flux $q$, this corresponds to $A/A_0>\sqrt q$.