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.
Applies to
- Vector field definition and evaluation
- Applying surface forces to cylindrical geometry
- Distinguishing between circumferential and radial force directions
- Structural simulation preparation workflows
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.
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.
| Field Type | Fx | Fy | Fz |
|---|---|---|---|
| Circumferential (CCW) | -F * y / 25 |
F * x / 25 |
0 |
| Radial Outward | F * x / 25 |
F * y / 25 |
0 |
| Radial Inward | -F * x / 25 |
-F * y / 25 |
0 |
Testing and Verification
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).
| Circumferential CCW | Radial |
|---|---|
 |
 |
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!