Consider a class of flow termed as parallel flow in which only one velocity term is nontrivial and all the fluid particles move in one direction only.
- We choose
to be the direction along which all fluid particles travel , i.e.
. Invoking this in continuity equation, we get
 |
which means 
- Now. Navier-Stokes equations for incompressible flow become
 | |
 | |
 |
So, we obtain
which means
and  | (25.11) |
