FTLE Operator

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The analysis of time-dependent dynamical systems can be done with the Lagrangian Coherent Structures (LCS) Operator. On this page, we show how to use the LCS operator in both VisIt's command-line interface (CLI) and VisIt's graphical user interface (GUI). In both cases, we demonstrate FTLE usage on a known, analytic problem. This problem comes from an excellent tutorial on Lagrangian Coherent Structures.

The steps for the CLI example and the GUI example are independent; readers can focus on whichever section(s) interests them.


[edit] Data generation

We reproduce the simple (time-independent) example from section 7 of the Lagrangian Coherent Structures | LCS tutorial, with Epsilon=0.

An input grid is required. We use a trivial 2D Cartesian grid in ASCII VTK format. Simply copy the 7 lines below in a text file called ftle_input.vtk:

# vtk DataFile Version 3.0
vtk output
SPACING 0.02 0.02 1
ORIGIN 0 0 0

This VTK file represents a grid with Spatial Extents [0, 2, 0, 1] to fit the example's requirements. We will define the vector field on this mesh differently in the CLI and in the GUI.

[edit] CLI generation of vector field

In the CLI, we create a vector field with two components, u and v, defining the following expressions:

DefineVectorExpression("velocity", "{u,v}")
DefineScalarExpression("u", "-3.141592*1*sin(3.141592*coord(mesh)[0])*cos(3.141592*coord(mesh)[1])")
DefineScalarExpression("v", "3.141592*1*cos(3.141592*coord(mesh)[0])*sin(3.141592*coord(mesh)[1])")
DefineScalarExpression("vel_mag", "magnitude(velocity)")

[edit] GUI generation of vector field

  1. In the main GUI, bring up the Expressions window via Controls->Expressions (left image)
  2. In the Expressions window, make an expressions (right image):
    • Name: velocity
    • Type: Vector Mesh Variable
    • Definition: {-3.141592*1*sin(3.141592*coord(mesh)[0])*cos(3.141592*coord(mesh)[1]), 3.141592*1*cos(3.141592*coord(mesh)[0])*sin(3.141592*coord(mesh)[1])}

[edit] CLI example

We plot both the velocity glyphs and the LCS output with the code:

AddPlot("Vector", "velocity", 0, 0)
AddPlot("Pseudocolor", "velocity")
AddOperator("LCS", 0)
opAtts = LCSAttributes()
opAtts.sourceType = opAtts.NativeMesh  # NativeMesh, RegularGrid
opAtts.integrationDirection = opAtts.Forward  # Forward, Backward
# Set the view
view = View2DAttributes()
view.windowCoords = (0, 2, 0, 1)
view.viewportCoords = (0.2, 0.95, 0.15, 0.95)
view.fullFrameActivationMode = view.Auto  # On, Off, Auto
view.fullFrameAutoThreshold = 100
view.xScale = view.LINEAR  # LINEAR, LOG
view.yScale = view.LINEAR  # LINEAR, LOG
view.windowValid = 1

[edit] Results

Velocity Vectors FTLE output as a scalar field

[edit] GUI Example

The FTLE operator creates a new scalar field from a vector field. This is different from many operators in VisIt; most operators transform data and do not create new fields on the data set.

The scalar field automatically appears in the drop down menu. Make a Pseudocolor plot of operators/FTLE/velocity:

This variable requires the FTLE operator to be added. Note that VisIt adds the operator automatically:

Click draw and you will see the FTLE plot:

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