> ## Documentation Index > Fetch the complete documentation index at: https://docs.ntop.com/llms.txt > Use this file to discover all available pages before exploring further. # 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. ## 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.* ![](https://files.learn.ntop.com/help-articles/structures/50598721473811.png)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! ## Download the example file * [Circumferential Force](https://files.learn.ntop.com/Support%20Article%20Example%20Files/Knowledge%20Base/Simulation/Circumferential%20Force.ntop)