Lamb surfaces are everywhere tangent to this quantity and the velocity.  
Crocco's theorem relates this quantity to the temperature and velocity fields and to the streamline-normal gradients of the entropy and the stagnation enthalpy.  Lamb surfaces are everywhere tangent to this quantity and the velocity.  In the analogy between ideal MHD and incompressible fluid dynamics this quantity is paired with the magnetic field, in part because it is solenoidal.  The volume integral of this quantity's dot product with the velocity equals the (*) helicity, and in general this quantity equals the curl of the velocity.  For 10 points, name this quantity that vanishes in irrotational flow and whose line integral equals the circulation.
Crocco's theorem relates this quantity to the temperature and velocity fields and to the streamline-normal gradients of the entropy and the stagnation enthalpy.  
{vorticity}