# Jet Propulsion/Streamline

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In fluid dynamics, a streamline is a line which is everywhere tangent to the velocity of the flow.
This implies that if **u** is the velocity vector at a point and **dr** is the vector in the direction of the streamline, then their cross product is zero.

**u** x **dr** = 0.

This translates to,

where **u** = u**i** + v**j**+ w**k**
and **dr** = dx**i** + dy**j** + dz**k**.