Function fieldAverage is included in file controlDict and it is a part of libfieldFunctionObjects.so library. Definition of these functions are created in file fieldAvg.fun. This function samples temporal average of fields  and  over a fixed time interval given by window parameter. Averaging function for rotor region is listed below:

    fieldAverageRotor
    {
        // Type of functionObject
        type            fieldAverage;

        // Where to load it from (if not already in solver)
        functionObjectLibs ("libfieldFunctionObjects.so");

        // Function object enabled flag
        enabled         true;

        // When to output the average fields
        outputControl   outputTime;

        // Region option
        region rotor;
    
        // Reset on output
        resetOnOutput true;

        // Fields to be averaged - runTime modifiable
        fields
        (
            U
            {
                mean        on;
                prime2Mean  off;
                base        time;
                window      100; // optional averaging window
            }

            p
            {
                mean        on;
                prime2Mean  off;
                base        time;
                window      100; // optional averaging window
            }
            
            p_rgh
            {
                mean        on;
                prime2Mean  off;
                base        time;
                window      100; // optional averaging window
            }
        );
    }