Orienting Boundary Normals

Correct orientation of boundary normals is essential for accurate SPH simulations, as it directly influences the boundary-normal solution quantities.

In SPH flow, fluid particles consider boundary conditions only on the positive side of a boundary. For closed surfaces, the positive side is facing outwards by default. In case surfaces lack a clear inside/outside definition, for example a flat surface, you examine the normal directions in a vector scene to determine if the normals are accurate or require reversing. If required, you can reverse the orientation of the normals for individual boundaries by using part subgroups.

To orient boundary face normals:

Click (Initialize Solution).
Create a vector scene to visualize the face normals.
1. Select the [vector scene] > [vector displayer] node and set Parts to the boundaries for which you want to assess the orientation of the normals.
2. Select the [vector displayer] > Vector Field node and set Function to Normal.
3. Examine the normals.
Select the Regions > [surface region] > Physics Values > Part Orientation node and activate Reverse Orientation.
See Part Orientation.

You can reverse the orientation of individual boundaries in the same surface region by using per-part for the subgroup that contains the parts of interest.

If you want to reverse the orientation for an individual boundary do the following:
1. Right-click the Regions > [surface region] node activate Allow Per-Part Values
2. Right-click the Regions > [surface region] > Part Subgroupings > [subgrouping] node and create a new subgroup.
3. Select the [subgrouping] > [subgroup] node and set Objects to the part for which you want to reverse the orientation.
4. Select the Regions > [surface region] > Physics Values > Part Orientation node and activate Specify by Part Subgroup.
5. Select the Part Orientation > By Part Subgroup > [subgroup] node and activate Reverse Orientation.
  
  For more information, see Applying Quantities by Subgroup.

The following image shows the orientation of the boundary normal vectors of a cylinder geometry before and after reversing their orientation:


Normals Before Reversing the Orientation (Default)	Normals After Reversing the Orientation

By default, the surface normals are oriented inward for closed geometry parts, that is, the positive side of a boundary is facing outwards. SPH particles interact with a boundary only on the positive side. Therefore, for external flow around a closed geometry part, the default orientation is correct. However, for internal flow within the geometry part, the default orientation is incorrect and must be reversed.