Dear atanuchaudhury,
thank you for posting.
Please have a look at our doxygen and user guide documentations to learn more about the correct usage of functors.
Unfortunately, we cannot offer full support on debugging your complete application.
You might want to consider taking part in our upcoming spring school for this.
More information is given here: https://www.openlb.net/spring-school-2025/
BR
Stephan