Fluid Mechanics
Steady fluid flow occurs, when the derivative of flow variables satisfy the following condition.

∂/∂t = 0
∂/∂s = constant
∂/∂t = constant
∂/∂s = 0

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Fluid Mechanics
A stream line is

Fixed in space in steady flow
A line connecting the mid points of flow cross-section
Always the path of particle
Drawn normal to the velocity vector at every point

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