Streamlines are a snapshot, not a route

A streamline is a curve whose direction at every point shows where the fluid is heading at that instant. It is not the path of any single drop; it is the field. Because most diagrams show steady flow, the field and the paths coincide, so streamlines are the natural way to draw moving fluid without tracing individual particles.

Draw them evenly across the incoming flow and let them deform only where the geometry forces a change. A streamline that bends for no reason is a lie about the flow.

Spacing is speed

The most useful thing a streamline drawing can say is which regions are fast and which are slow. Where lines crowd together, the same volume of fluid is being squeezed through a narrower gap, so it moves faster; where they spread apart, it slows. This is why a pipe that narrows should show the lines bunching up, and why they fan out again downstream.

Streamlines over a plate, with a growing boundary layer boundary layer grows downstream faster flow · denser lines Near the wall friction slows the fluid to almost nothing; the gap from wall to main flow is the boundary layer.
Figure 1 — Lines crowd where the flow is fast and the boundary layer thickens the further downstream it travels.

Let the wall slow the fluid

No matter how fast the free stream moves, the fluid in contact with a solid surface is held almost still by friction. The region over which the velocity climbs back from zero at the wall to the free-stream speed is the boundary layer. It starts thin at a sharp leading edge and thickens downstream as more and more fluid is dragged to a crawl.

Draw it as a soft zone rather than a hard line: a band of slowing streamlines hugging the surface. A dashed edge line marks the nominal top of the layer without claiming a precision the model does not have.

Show separation when it happens

If the flow has to climb an adverse pressure hill — behind a blunt body, or on the rear of a wing — the boundary layer loses energy and peels away from the surface. Once detached, it becomes a wake of slow, swirling fluid that the streamlines must go around. Drawing this honestly is the difference between a diagram that explains drag and one that quietly hides it.

Drawing fluid in SVG

  • Use paths, not perfect arcs. Gentle cubic curves read as fluid; rigid circles read as a machine part.
  • Keep counts low. Five to nine streamlines carry the field; thirty turn it into shading.
  • Animate the main flow only. A slow dash along the centre lines, with the boundary layer left still, keeps the contrast.
  • Reserve saturation for the special case. A single highlighted streamline or a coloured boundary layer is enough emphasis.
Bottom line: streamlines show direction, spacing shows speed, and a soft band along the wall shows the boundary layer. Together they turn an arrow into an explanation of a flow field.