A fluid is any substance that flows and is normally classified into:
Liquids, which can only be compressed with difficulty but will move around quite easily, and
Gases, which have no boundaries of their own hence their volume depends on the volume of the container in which they are held.
A basic concept of conventional fluid dynamics therefore is that that a fluid, whether liquid or gas, is a continuous medium and one for which any flow is continuous.
The movement of a fluid generates (and is generated by) shearing forces between layers of the fluid When fluid flows in a pipe the velocity of the fluid must be zero at the wall and must increase progressively with the distance from the wall.
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