When running Fluent parallel, ensure that you pass the return value to the host process that it can be used for reporting. If you use Message or Message0 for debugging your UDF (user-defined function), you should get the expected value because most calculations are done on the node processes. But monitoring is done by the Cortex process which gets its data from the host only. If you don’t pass the data to the host process, you get whatever is stored in memory. It might be zero but it can be any arbitrary number. You can find more information about message passing with node_to_host_* macros in the ANSYS Fluent Customization Manual, chapter “Communicating Between the Host and Node Processes”. For a detailed example that uses REPORT_DEFINITION_FN, refer to solution 2053120: How can an input parameter be used for report expressions in ANSYS Fluent?