## Streamlines

The field lines of the velocity vector field at a particular instant of time are called streamlines. The streamlines are visualised by taking photographs of the fluid. If the velocity field changes with time, then the streamlines will change also, but remains the same for steady flows.

Streamlines have the following properties:

At any instant, no field crosses a streamline. This follows because at any point on a streamline, the velocity vector is perpendicular tothe normal vector to the streamline. Hencea measure of the flow rate of the fluid across a streamline, is zero

At any instant of time, distinct streamlines cannot cross. Suppose two streamlines cross at a point A. At each point of a streamline the velocity vector is parallel to the streamline tangent, so the velocity vector at A has two different directions, so the velocity field is not uniquely defined, obviously a contradiction.

If the streamline has equationand velocitythenorWe can then find the streamlines by integration.

Example: Find the streamlines for the vector field

The streamlines are parabolae. Ifandthen

Example: Find the streamline for the velocity vector field

which integrates to give