2D Flow Field Example

Location: examples/rappture2/flow2d/

Simulates 2D lid-driven cavity flow and visualizes the velocity field as colored arrows.

Inputs

• Grid Size – N x N grid resolution

• Kinematic Viscosity – fluid viscosity (affects Reynolds number)

• Lid Velocity – speed of the moving lid

Key concepts

Vector field output (3-component per point):

# Interleaved vx, vy, vz values
vec_text = "\n".join(
    f"{vx[k]} {vy[k]} 0.0" for k in range(npoints)
)
rx['output.field(velocity).component.mesh'] = 'output.mesh(grid)'
rx['output.field(velocity).component.values'] = vec_text
rx['output.field(velocity).component.extents'] = '3'

• extents=3 tells the renderer this is a 3-component vector field.

• rappture2web renders vector fields as colored arrows.

• Use about.group to overlay vector and scalar fields on the same view.

tool.xml

<?xml version="1.0"?>
<run xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:noNamespaceSchemaLocation="../../../rappture2web/contract.xsd">
<tool>
  <title>2D Flow Visualizer</title>
  <about>Simulates 2D lid-driven cavity flow using a simple iterative solver and visualises the resulting velocity field as arrows coloured by magnitude.</about>
  <command>python3 @tool/flow2d.py @driver</command>
  <contract>
    <input>
      <integer id="npts">
        <about>
          <label>Grid Size</label>
          <description>Number of grid points along each axis (N×N grid).</description>
        </about>
        <min>8</min>
        <max>40</max>
        <default>20</default>
      </integer>
      <number id="viscosity">
        <about>
          <label>Kinematic Viscosity</label>
          <description>Fluid viscosity (affects Reynolds number).</description>
        </about>
        <units>m2/s</units>
        <min>0.001m2/s</min>
        <max>1.0m2/s</max>
        <default>0.1m2/s</default>
      </number>
      <number id="lid_vel">
        <about>
          <label>Lid Velocity</label>
          <description>Velocity of the moving lid at the top of the cavity.</description>
        </about>
        <units>m/s</units>
        <min>0.1m/s</min>
        <max>2.0m/s</max>
        <default>1.0m/s</default>
      </number>
    </input>
    <output>
      <mesh id="grid"><about><label>Cavity Grid</label><description>Hidden mesh used by the cavity flow fields.</description></about></mesh>
      <field id="velocity"><about><label>Velocity Field</label><description>Vector velocity field for the cavity flow.</description></about></field>
      <field id="magnitude"><about><label>Velocity Magnitude</label><description>Scalar velocity magnitude field.</description></about></field>
    </output>
  </contract>
</tool>
<input>
  <integer id="npts">
    <about>
      <label>Grid Size</label>
      <description>Number of grid points along each axis (N×N grid).</description>
    </about>
    <min>8</min>
    <max>40</max>
    <default>20</default>
  </integer>
  <number id="viscosity">
    <about>
      <label>Kinematic Viscosity</label>
      <description>Fluid viscosity (affects Reynolds number).</description>
    </about>
    <units>m2/s</units>
    <min>0.001m2/s</min>
    <max>1.0m2/s</max>
    <default>0.1m2/s</default>
    <preset>
      <value>0.01m2/s</value>
      <label>Low (Re≈100)</label>
    </preset>
    <preset>
      <value>0.1m2/s</value>
      <label>Medium (Re≈10)</label>
    </preset>
    <preset>
      <value>0.5m2/s</value>
      <label>High viscosity</label>
    </preset>
  </number>
  <number id="lid_vel">
    <about>
      <label>Lid Velocity</label>
      <description>Velocity of the moving lid at the top of the cavity.</description>
    </about>
    <units>m/s</units>
    <min>0.1m/s</min>
    <max>2.0m/s</max>
    <default>1.0m/s</default>
  </number>
</input>
</run>

Running

rappture2web examples/rappture2/flow2d/