In hydrostatic we consider that fluids were at rest. Now we relax this consideration but with caution. We will discuss the flow of an actual fluid. There are four assumptions about perfect fluids.
Here are four assumptions that we create about our ideal fluid:
1. Steady Flow: In steady flow the velocity of the moving fluid at any given point does not change 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 done for fluids at rest, that our perfect fluid is incompressible. That is, its density has a constant magnitude.
3. Non-viscous Flow: Viscosity in fluids is the analog of friction in solids. Both are methods by which the kinetic energy of moving bodies can be converted into thermal power. In the absence of friction force, a part of block could slide at constant speed along a horizontal surface in the similar way; an object going through a no viscous fluid would experience no viscous drag friction. Lord Rayleigh has noticed out that, in a perfect fluid, a ship's propeller would not work but, on the other hand, a ship could not require a propeller.
4. Irrotational Flow: Although it requires not concern us further, we also consider that the flow is irrotational. To check for this property, let a tiny grain of dust goes with the fluid. Although this test object may go in a circular way, in irrotational flow the test object will not rotate about an axis through its own center of volume. For a loose analogy, the rotation of a Ferries wheel is rotation; that of its passengers is irrotational.
According to Reynolds, the critical velocity (vc) of a liquid flowing through a long narrow tube is
(a) Directly related to the coefficient of viscosity of the liquid.
(b) Inversely related to the density of the liquid and
(c) Inversely related to the diameter (D) of the tube.
That is 
... (i)
where R is the Reynold number.
If R occurs between 2000 and 3000, the flow is not stable and can modify from streamline flow to turbulent flow.
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