Defining Circumferential and Radial Force Fields on a Cylinder

This article explains how to define, apply, and verify circumferential (tangential) and radial vector fields on a cylindrical surface in nTop. This workflow is essential for simulating rotating loads, pressure vessels, or centrifugal forces.

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Applies to

• Vector field definition and evaluation
• Applying surface forces to cylindrical geometry
• Distinguishing between circumferential and radial force directions
• Structural simulation preparation workflows

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Procedure

1. Create the reference geometry:

• Add a Circle block with the following settings:
  • Center point: 0, 0, 0
  • Radius: 25 mm
  • Normal: 0, 0, 1

2. Define the force magnitude:

• Create a Scalar Variable named F and set it to 1 N.

3. Construct the Circumferential Force field:

• Add a Vector Field from Components block and name it Circumferential Force. Use the following math expressions:
  • X: -(F * y / 25 mm) // Tip: Use the negative of the F variable we created in the above step to avoid multiplying by -1
  • Y: F * x / 25 mm
  • Z: 0

The Principle: The cross-axis relationship (X component driven by Y coordinate) creates rotation around the cylinder axis rather than pointing toward or away from it. 4. Generate evaluation points:

• Use a Equidistant Points on Curve block:
  • Curve: Your Circle from Step 1.
  • Increment: Length/8 (to create 8 equidistant points).
• We will use a Sub List here to exclude the start point, which is counted twice.

5. Evaluate the field:

• Add an Evaluate Field block:
  • Vector Field: Circumferential Force
  • Points: The point list from Step 4.

6. Verify the results:

• Inspect the evaluated output. For a pure circumferential field, the sum of all X and Y components should be zero, indicating no net translation, only torque.

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Switching to a Radial Field

To convert the field from circumferential to radial, swap the coordinate inputs so each component is driven by its own axis.

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Testing and Verification

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Visual Check — Viewport Streamlines

Preview the vector field in the nTop viewport from the Top View (Ctrl+5).

• Correct: Streamlines appear as smooth, concentric arcs flowing uniformly around the circle. You can also extract the Normals at the different points to see what direction it is pointing at.
• Incorrect: If streamlines show a “saddle pattern” (curving inward toward the axes), you have likely mapped same-axis components (Radial) instead of cross-axis components (Circumferential).

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Net Force and Torque Check

• Sum Fx: Should equal 0.00 N (No net force in X).
• Sum Fy: Should equal 0.00 N (No net force in Y).
• Total Torque: Calculated as $\sum (x \cdot Fy - y \cdot Fx)$ should equal 200 N·mm for F = 1\text{ N} across 8 nodes at r = 25\text{ mm}.

And that’s it! You’ve successfully applied a circumferential force field to a cylindrical surface with verified zero net force and correct tangential direction at all evaluated points. Are you still having issues? Contact the support team, and we’ll be happy to help!

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Download the example file

• Circumferential Force