Types of Fluid Flow
Steady Flow means constant pattern of flow velocity everywhere in the fluid.
Irrotational flow means without turbulence, whirlpools, vortices, etc. (smooth and streamlined)
Nonviscous flow means no fluid frictional resistance to flow (like water, not molasses).
Incompressible flow means effectively infinite bulk modulus everywhere -- uniform density for case of uniform composition (like liquids, not gases).
Notes:
Steady flow isn’t the same thing as constant velocity: If you were to move along with a small piece of fluid in steady flow, you might find its motion to be complex and non-uniform. On the other hand, the instantaneous velocity of the fluid that passes any fixed point in the fluid will be constant. It is the flow pattern that is fixed.
For example, if you were in a barrel moving with the water going over Niagara falls, your velocity would be far from uniform; yet, at any given point in the falls, the speed and direction of the water passing that point doesn’t change much with time, because the flow is pretty steady.