In hydrostatic we consider that fluids were at rest. Now we relax this consideration but with caution. We will discuss the flow of an real fluid. There are the assumptions about perfect fluids.
Here are the assumptions that we make about our ideal fluid:
1. Steady Flow: In steady flow the velocity of the moving fluid at any given point does not alter as time goes on either in direction or in magnitude. The gentle speed of water near the center of a quiet stream is steady; that in a chain of rapids is not.
2. Incompressible Flow: We consider, as we have already completed for fluids at rest, that our perfect fluid is incompressible. That is, its density has a constant magnitude.