Reply To: species gradient calculation UDF error — can’t find the function

[Rob]

Check you're retaining all data, using the TUI command. Fluent doesn't keep all of the gradient values between iterations for RAM/efficiency reasons. 

/solve/set/advanced/retain-temporary-solver-mem   

Also, try running an iteration/timestep before running the UDF commands, not all data is available following initialisation, and you are messing with scalar gradients!