Streamline

A streamline is a curve everywhere tangent to a fluid’s instantaneous velocity field. In steady flow, streamlines coincide with particle paths; in unsteady flow, they generally do not.

Velocity field: A function that assigns a velocity vector to each point in a region of space and time. A streamline is constructed by following the local direction of this field.
Pathline: The trajectory traced by an individual fluid particle over time. Unlike a streamline, it follows one particle rather than the instantaneous velocity directions.
Computational fluid dynamics: The numerical study of fluid flow using discretized equations and computer simulations. Simulation software commonly displays calculated velocity fields as streamline plots.
Ordinary differential equation: An equation relating a function of one variable to its derivatives. Streamline curves are obtained by solving differential equations for their tangent directions.
Stagnation point: A point in a flow where the fluid velocity is zero. Streamlines approach or depart from this point in characteristic patterns.
Stream function: A scalar function whose constant-value curves represent streamlines in certain two-dimensional flows. Its contour lines provide an alternative way to describe streamline geometry.
Streakline: The curve formed by all fluid particles that have passed through a fixed point. It can differ from a streamline when the flow is unsteady.
Flow visualization: Experimental methods for making fluid motion visible through tracers, dyes, or optical measurements. Visible tracer patterns can be compared with predicted streamline shapes.
Tangent vector: A vector that gives the direction of a curve at a particular point. The streamline condition requires its tangent to align with the local velocity.
Flow separation: The detachment of a fluid boundary layer from a surface, often producing a wake. Streamline plots help locate where attached flow turns away from a surface.

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