Parameters from poly-line

Draw a line or a circle across your data and extract a profile of any derived parameter along it — a velocity profile across a jet, or a tangential-velocity profile around a vortex core, for example.

Open via Extractions → Parameters from poly-line (Ctrl+P).

PIVlab Extract parameters from poly-line panel
The Extract parameters from poly-line panel.

Drawing a line or circle

Choose a Type first:

TypeHow to draw it
polylineLeft-click to add each point; right-click to finish.
circleClick twice with the left mouse button: first click sets the centre, second click sets the radius.
circle series (tangent vel. only)Several concentric circles at once — restricted to extracting tangential velocity, useful for a velocity-vs-radius profile around a vortex core.

Click Draw! to start, then draw in the image. Use Save coords / Load coords to reuse the same line or circle across sessions.

A circle series — many concentric circles — drawn around a vortex on the velocity field, for extracting a tangential-velocity profile versus radius
A circle series centred on a vortex: many concentric circles at once, for extracting a tangential-velocity profile as a function of radius.

Extracting data

  1. Choose what to extract from Data to extract — the same derived-parameter list as Deriving spatial parameters, plus Tangent velocity and Normal velocity, which project the velocity onto the direction along or across your line or circle.
  2. Click Extract data to plot the profile along your line or circle.
  3. Click Clear data to remove it and start over.
Tangent vs. normal velocity. Tangent velocity is the velocity component parallel to the line (or circle) — useful for the swirling velocity around a vortex core, for example. Normal velocity is the component perpendicular to the line — handy for the flow rate crossing a line, such as flux through a control surface. A positive normal velocity points in the positive y-direction as set in the spatial calibration (downwards by default). Both work on a polyline or a single circle; a circle series supports tangent velocity only.
A pop-up plot of velocity magnitude versus distance along a line, with the blue extraction line drawn on the vector field behind it
Extracting Magnitude along a line: the blue line is drawn across the flow (right), and the pop-up plots the value versus distance along it, including the integral along the line.

Saving the extraction

Tick extract and save for all frames to extract across the whole session rather than just the current frame, choose a file format (Excel file or Text file), then click Export data.

Want an average over a region instead? See Parameters from area — same idea, but for a whole enclosed area rather than a line or circle.