Time rate of change of some physical quantity A(t, {\bf r}) (like heat or momentum) of a material element that is subjected to a space-and-time-dependent macroscopic velocity field variations of that physical quantity:
| (1) | \frac{D A}{Dt} = \frac{\partal A}{\partial t} + {\bf u} \nabla A |
where
( t, {\bf r} ) | time and spatial variables |
{\bf u}(t, {\bf r}) | velocity of matter |
A(t, {\bf r}) | velocity of matter |