INSTANTANEOUS velocity vector at that point.There is no flow across streamlines.\(\frac{{{\rm{dx}}}}{{\rm{u}}} = \frac{{{\rm{dy}}}}{{\rm{v}}} \) is the differential equation of a streamline for 2D flow, with slope \(\frac{{{\rm{dy}}}}{{\rm{dx}}} = \frac{{{\rm{v}}}}{{\rm{u}}} \)\(\frac{{{\rm{dx}}}}{{\rm{u}}} = \frac{{{\rm{dy}}}}{{\rm{v}}} = \frac{{{\rm{dz}}}}{{\rm{w}}}\) is the differential equation of a streamline for 3D flow, where u, v and w are velocities in directions X, y, and z, respectively.Streamline flow is also called laminar flow.This type of flow is more viscous than turbulent flow.Streamline never intersects each other because if they intersect then there will be two tangents for two curves that mean there will be two velocity vector but it is not possible as at a GIVEN instant or at a given point there will be a unique velocity vector only.Path line is the actual path traversed by a given fluid particle.Streak line is the locus of particles that have earlier passed through a prescribed point.For STEADY flow, streamlines, path lines and streak lines are identical becauseFor a steady flow, the velocity vector at any point is INVARIANT with time.The path line of the particles with different identities passing through a point will not differ.The path line could coincide with one another in a single curve which will indicate the streak line too.