Streamlines

Advanced ▸ Streamlines… traces the curves that are everywhere tangent to a solved nodal vector field — VELOCITY, DISPLACEMENT, any 2- or 3-component field on the nodes — draws them over the mesh coloured by speed, and exports them as line cells.

Choosing the field and the seeds

The field must live on the nodes. A cell field is piecewise constant and has no gradient to follow, so it is refused with a pointer to Average field. Where the lines start is one of:

• Points — one x y z per line, or switch on Pick seeds and click nodes in the scene; each click adds that node's coordinates.
• Line — a count of equidistant seeds from From to To, both ends included.
• Plane — a lattice origin + i/(nu−1)·U + j/(nv−1)·V; a count of 1 puts that row at the middle. Use clip plane fills the three vectors from the focused pane's clip plane — its slider position and span — so the seeds fall exactly on the section you are looking at.
• SubModelPart — the nodes of the part and its children, which is the natural way to seed from an inlet.

Direction is forward, backward or both; both gives two lines per seed, one each way.

While the panel is open, orange markers preview where the current draft would seed (the first 1000 at most) — a part previews as nothing, since its nodes are already visible in the mesh.

How a line is traced

The integrator is a classical fourth-order Runge–Kutta in arc length, so a step is a distance rather than a time and does not shrink where the flow is slow. The step is a fraction (default 0.25) of the size of the cell the line is in. The field is interpolated barycentrically inside the containing cell, found with a locator built on the mesh's own cells; it is never extrapolated outside them.

Three caps bound a line: Max steps (default 2000), Max length (default five bounding-box diagonals) and a seed limit of 1000 — a request for more seeds is refused rather than silently truncated.

The trace runs in a worker thread, so the interface stays responsive while it works. The Trace button counts seeds as they complete, a newer Trace supersedes the running one (a timeline step re-traces the same seeds on the new frame), and Cancel stops after the current seed — the lines traced so far are kept and reported as a partial result. Cancellation takes effect between seeds: one very long line finishes before the run stops.

Why a line ended

Every line records one of these, and the summary counts them:

Code	Reason
0	reached the maximum length
1	reached the maximum number of steps
2	left the domain
3	reached a stagnation point — speed below 10⁻⁶ of the field's largest magnitude
4	met a node that carries no value

A seed that produces no segment at all — outside the mesh, or at zero speed — is listed with its reason in the summary and in the MCP reply. It is never dropped without a word.

How the lines look

Style switches the overlay between Lines (a pixel width) and Tubes — a surface around each line, sized as a share of the bounding-box diagonal with a ring resolution — both coloured by speed exactly like the lines. Styling is view-only: it never reaches a file, and Export… below still writes line cells. A tube layer over the two-million-ring-vertex budget falls back to lines and says so on the panel.

What this is not

These are steady streamlines of the frame on screen: the field frozen at one instant. They are not the paths particles follow through a changing flow (pathlines), which need time interpolation. Edit operations applied in the sidebar are honoured, because the trace uses the mesh as it is on screen. Stepping the timeline re-traces the same seeds on the new frame.

Exporting

Export… writes one node per vertex and one Line2D2N element per segment. Nodal fields: STREAM_SPEED, STREAM_VELOCITY, STREAM_ARCLENGTH. Per-segment fields: STREAM_LINE, STREAM_SEED (0-based index into the seed list), STREAM_DIRECTION (+1 forward, −1 backward) and STREAM_TERMINATION (the codes above). .vtu keeps all of them; a .vtp carries the nodal ones only.

From an agent or a script

The same core runs headlessly as the mesh_derive MCP tool with kind: "streamlines": give exactly one of seedPoints, seedLine, seedPlane or seedPart, and timeStep to choose a step of a multi-step file. The reply carries the termination counts and the rejected seeds, and a long trace reports per-seed progress as log lines. A clip-plane lattice needs no special tool — read the plane's position and span and pass them as an explicit seedPlane. See MCP Server.