Reply To: BounceBack boundary
The specifics depend on which cells you assign the post processor to. If you show me your entire post processor and setter code this should become clear (In any case I recommend using the new style operators with apply method templates instead of the deprecated processSubDomain style (unclear what you are using, just to make sure))
The common approach in OpenLB is to templatize the boundary operators on the discrete normal directions. Of course you can also read this information from cell-local fields instead – where do you get your unknownIndexes array from?