Fix periodic BC description

docs/02.simulation-setup/03.flow-solver/01.boundary-conditions/05.periodic.md

@@ -41,26 +41,27 @@
 - **Required.**
 - **Accepted types:** Pairs of Surface objects
 - **Example:** `front_plane AND back_plane`
->**Note:** Flow360 handles the interpolation between periodic surfaces automatically, regardless of mesh topology.
+>**Note:** The mesh must be conformal at periodic interfaces, meaning nodes on paired surfaces must match exactly after applying the periodic transformation.
 
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-### Mesh Preparation for Periodic Boundaries
+**Mesh Preparation for Periodic Boundaries**
+
+- **Conformal mesh requirement:**
+  - The mesh must be conformal at periodic interfaces
+  - Nodes on paired surfaces must match exactly after applying the periodic transformation (translation or rotation)
+  - Ensure your mesh generator creates matching node distributions on both periodic surfaces
 
 - **For Translational Periodicity:**
-  - Periodic surfaces should have similar (but not necessarily identical) mesh resolution
-  - Flow360 automatically handles the interpolation between periodic surfaces
-  - No exact node matching is required between periodic pairs
+  - Ensure identical node positions on both surfaces, offset only by the translation vector
 
 - **For Rotational Periodicity:**
-  - Similar mesh resolution is recommended along the periodic interfaces
-  - Flow360 automatically handles the rotational transformation
-  - For best results, maintain similar element sizes across interfaces
+  - Ensure nodes on both surfaces match exactly when one surface is rotated to align with the other
 
-### Common Applications
+**Common Applications**
 
 - **Translational Periodicity:**
   - Airfoil or blade cascades
@@ -95,11 +96,10 @@
   > - The problem has no major asymmetries that would invalidate the periodic assumption
 
 - **What mesh requirements must be met for periodic boundaries?**  
-  > Flow360 is very flexible with periodic interfaces:
-  > - Non-matching meshes are fully supported through automatic interpolation
-  > - No specific node count or distribution requirements exist
-  > - Similar mesh resolution is recommended (but not required) at interfaces for best accuracy
-  > - Both conformal and non-conformal interfaces are supported
+  > Periodic boundaries require conformal meshes:
+  > - Nodes on paired surfaces must match exactly after applying the periodic transformation
+  > - The node count must be identical on both periodic surfaces
+  > - Ensure your mesh generator creates matching node distributions when generating periodic boundaries
 
 - **Can I use periodicity with other boundary conditions?**  
   > Yes, periodic boundaries are often combined with: